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Background: Patient-reported outcome measures (PROMs) help to assess disease control and quality of life (QoL) in chronic spontaneous urticaria (CSU) and recurrent angioedema (RA). This study aimed to assess the correlation between two different concepts: disease control and QoL, using disease-specific PROMs., Methods: We analyzed data from 445 CSU and 330 RA patients who completed both a disease control and QoL PROM as part of the clinical routine. We included the UCT and CU-Q2oL for CSU and AECT and AE-QoL for RA., Results: In CSU and RA, disease control scores positively correlated with QoL scores (Spearman's rho correlation coefficient (CR) -0.757, -0.735; p < 0.001) with better disease control corresponding to better quality of life. However, 5.9% of CSU patients and 28% of RA patients with complete disease control had impaired QoL. In CSU, QoL was impaired in 69.2% of patients based on the CU-Q2oL and in 62.7% of patients based on a single numeric question from the UCT, with a mismatch in 89/445 patients. In RA, QoL was impaired in 58.5% using the AE-QoL and in 52.7% using a single numeric question from the AECT30mo, with a mismatch in 69/330 patients. Different domains of the QoL PROMs showed different degrees of influence on disease control, with "Itching/Embarrassment" showing the strongest correlation with the UCT (CR -0.804; p < 0.001) and "Functioning" with the AECT3mo (CR -0.824; p < 0.001)., Conclusion: Although most patients with controlled disease have better quality of life, unexpectedly, quality of life remains impaired in up to one-fourth of patients with completely controlled CSU and RA. Reasons behind this should be investigated in further studies., (© 2025 The Author(s). Clinical and Translational Allergy published by John Wiley & Sons Ltd on behalf of European Academy of Allergy and Clinical Immunology.)

Background: Mastocytosis is characterized by expanding neoplastic mast cells in organs such as the skin, bone marrow, and gastrointestinal tract. The release of mast cell mediators triggers cutaneous, gastrointestinal, and other symptoms. Currently, no validated mastocytosis-specific patient-reported outcome measure (PROM) exists to assess disease control., Objective: Here, we developed a disease-specific instrument, the Mastocytosis Control Test (MCT), for evaluating and monitoring disease control in patients with nonadvanced disease., Methods: Six potential MCT items were generated using a combined approach consisting of a literature review, patient interviews, and expert input. Item selection and reduction were performed by impact analysis and interitem correlation, followed by expert reviews and cognitive debriefings. In a validation study, the resulting MCT was tested for validity and reliability by assessing internal consistency, test-retest reliability, convergent validity, known-groups validity, and receiver operating characteristics (ROC) analysis., Results: Ten patients participated in the item generation phase and 101 in the item reduction and validation phase. The final MCT consisted of 5 items. Our methods showed a valid total score, high internal consistency, and test-retest reliability. Convergent and known-groups validity demonstrated a strong correlation with related anchors. The ROC curve analysis suggested a cutoff value of ≥13 points to identify patients with good disease control., Conclusions: The MCT is a disease-specific, valid, and reliable PROM for adult patients with nonadvanced disease. It may measure and monitor disease control in routine daily practice and clinical trials., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Mastocytosis is a clonal myeloid disorder defined by an increase and accumulation of mast cells (MCs) in one or multiple organ systems. The complex pathology of mastocytosis results in variable clinical presentations, courses, and outcomes. The World Health Organization (WHO) divides the disease into cutaneous mastocytosis (CM), several forms of systemic mastocytosis (SM), and MC sarcoma. In most patients with SM, a somatic KIT mutation, usually D816V, is identified. Patients diagnosed with CM or nonadvanced SM, including indolent SM, have a near-normal life expectancy, whereas those with advanced SM, including aggressive SM and MC leukemia, have limited life expectancy. Since 2001, a multidisciplinary consensus group consisting of experts from the European Competence Network on Mastocytosis and the American Initiative in Mast Cell Diseases has supported the field by developing diagnostic criteria for mastocytosis. These criteria served as the basis for the WHO classification of mastocytosis over 2 decades. More recently, an International Consensus Classification group proposed slightly modified diagnostic criteria and a slightly revised classification. In this article, these changes are discussed. Furthermore, we propose harmonization among the proposals of the American Initiative in Mast Cell Diseases/European Competence Network on Mastocytosis consensus group, WHO, and the International Consensus Classification Group. Such harmonization will facilitate comparisons of retrospective study results and the conduct of prospective trials., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Background: CRUSE® is an app that allows patients with chronic spontaneous urticaria (CSU) to monitor their daily disease activity through the use of visual analogue scales (VASs). We aimed to determine the concurrent validity, reliability, responsiveness and minimal important difference (MID) of CRUSE® VASs., Methods: We evaluated the properties of three daily VASs: VAS for how much patients were affected by their CSU ('VAS urticaria'), VAS for the impact of urticaria on work/school productivity ('VAS productivity') and the VAS of EQ-5D. Concurrent validity was assessed by measuring the association between each VAS and the Urticaria Activity Score (UAS). Intra-rater reliability was determined based on the data of users providing multiple daily questionnaires within the same day. Test-retest reliability and responsiveness (ability to change), respectively, were tested in clinically stable and clinically unstable users. MIDs were determined using distribution-based methods., Results: We included 5938 patients (67,380 days). Concurrent validity was high, with VAS urticaria being more strongly associated with the UAS score than the remaining VASs. Intra-rater reliability was also high, with intraclass correlation coefficients (ICC) being above 0.950 for all VASs. Moderate-high test-retest reliability and responsiveness were observed, with reliability ICC being highest for VAS EQ-5D and responsiveness being highest for VAS urticaria. The MID for VAS urticaria was 17 (out of 100) units, compared to 15 units for VAS productivity and 11 units for VAS EQ-5D., Conclusion: Daily VASs for CSU available in the CRUSE® app display high concurrent validity and intra-rater reliability and moderate-high test-retest reliability and responsiveness., (© 2024 European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd.)

Chronic spontaneous urticaria (CSU) is a debilitating, inflammatory skin condition characterized by infiltrating immune cells. Available treatments are limited to improving the signs and symptoms. There is an unmet need to develop therapies that target disease-driving pathways upstream of mast cell activation to inhibit or delay the progression of CSU and associated comorbidities. Here, we aim to define disease modification due to a treatment intervention and criteria that disease-modifying treatments (DMTs) must meet in CSU. We have defined disease modification in CSU as a favorable treatment-induced change in the underlying pathophysiology and, therefore, the disease course, which is clinically beneficial and enduring. A DMT must fulfil the following criteria: (1) prevents or delays the progression of CSU, (2) induces long-term, therapy-free clinical remission, which is the sustained absence of CSU signs and symptoms without the need for treatment, and (3) affects the underlying mechanism of CSU, as demonstrated by an effect on disease-driving signals and/or a biomarker. DMTs in CSU should slow disease progression, achieve long-lasting disease remission, target disease-driving mechanisms, reduce mast cell-activating IgE autoantibodies, target cytokine profile polarization, and normalize the gut microbiome and barrier. Treating CSU at the immune system level could provide valuable alternatives to pharmacotherapy in CSU management. Specific DMTs in CSU are yet to be developed, but some show potential benefits, such as inhibitors of Bruton's Tyrosine Kinase, IL-4 and IL-13. Future therapies could prevent CSU signs and symptoms, achieve long-term clinical benefits after discontinuing treatment, and prevent associated concomitant disorders., (© 2024 The Author(s). Allergy published by European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd.)

Background: Angioedema (AE) manifests with intermittent, localized, self-limiting swelling of the subcutaneous and/or submucosal tissue. AE is heterogeneous, can be hereditary or acquired, may occur only once or be recurrent, may exhibit wheals or not, and may be due to mast cell mediators, bradykinin, or other mechanisms. Several different taxonomic systems are currently used, making it difficult to compare the results of studies, develop multicenter collaboration, and harmonize AE treatment., Objective: We developed a consensus on the definition, acronyms, nomenclature, and classification of AE (DANCE)., Methods: The initiative involved 91 experts from 35 countries and was endorsed by 53 scientific and medical societies, and patient organizations. A consensus was reached by online discussion and voting using the Delphi process over a period of 16 months (June 2021 to November 2022)., Results: The DANCE initiative resulted in an international consensus on the definition, classification, and terminology of AE. The new consensus classification features 5 types and endotypes of AE and a harmonized vocabulary of abbreviations/acronyms., Conclusion: The DANCE classification complements current clinical guidelines and expert consensus recommendations on the diagnostic assessment and treatment of AE. DANCE does not replace current clinical guidelines, and expert consensus algorithms and should not be misconstrued in a way that affects reimbursement of medicines prescribed by physicians using sound clinical judgment. We anticipate that this new AE taxonomy and nomenclature will harmonize and facilitate AE research and clinical studies, thereby improving patient care., (Copyright © 2024. Published by Elsevier Inc.)

Background: Chronic spontaneous urticaria (CSU) can present with non-skin related symptoms (NSRS), including recurrent unexplained fever, joint, bone, or muscle pain (JBMP), and malaise, which also occur in other conditions that manifest with wheals (eg, urticarial vasculitis or autoinflammatory disorders) or without wheals (eg, infection)., Objective: We sought to determine the rate of patients with CSU affected by fever, JBMP, and malaise, their trigger factors, links with clinical and laboratory characteristics, and their impact on everyday life and treatment responses., Methods: We analyzed baseline data from the Chronic Urticaria Registry of 2,521 patients with CSU who were aged 16 years or older., Results: One third of CSU patients (31.2%; 786 of 2,521) had one or more NSRS, including recurrent fever (5.3%), JBMP (19.1%), and/or malaise (18.6%). In a multivariable analysis, having one or more of these NSRS correlated with food and infection as trigger factors of urticaria (adjusted odds ratio [aOR] = 1.7 and 1.5), wheals of 24 hours or greater duration (aOR = 2.5), sleep disturbance (aOR = 2.4), anxiety (aOR = 2.8), comorbid atopic dermatitis (aOR = 2.1), gastrointestinal disease (aOR = 1.8), elevated leukocytes (aOR = 1.7) and erythrocyte sedimentation rate (aOR = 1.5). In a bivariate analysis, these NSRS were additionally associated with higher disease activity (weekly Urticaria Activity Score, median: 21 vs 14; P = .009), longer disease duration (years, median: 2 vs 1; P = .001), the presence of angioedema (74.6% vs 58.7%; P < .001), worse quality of life (Chronic Urticaria Quality of Life Questionnaire, median: 42 vs 29; P < .001) and more frequent poor control of CSU (78% vs 69%; P < .001)., Conclusions: The presence of NSRS in a subpopulation of patients with CSU points to the need for better control of the disease, exclusion of comorbid conditions, and/or exclusion of urticarial vasculitis and urticarial autoinflammatory diseases., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

The pathophysiology of mast cell (MC)-driven disorders is diverse, ranging from localized reactions to systemic disorders caused by abnormal accumulation and activation in multiorgan systems. Prompt and accurate diagnosis is critically important, both for informing treatment and objective assessment of treatment outcomes. As new therapeutics are being developed to deplete MCs or silence them (eg, by engaging inhibitory receptors that block activation), new biomarkers are needed that can distinguish between MC activation versus burden. Serum tryptase is the gold standard for assessing both MC burden and activation; however, commercial tryptase assays have limitations related to timing of release, lack of discernment between inactive (α) and active (β) forms of tryptase, and interpatient variability of baseline levels. Alternative approaches to measuring MC activation include urinary MC mediators, flow cytometry-based assays or gene expression profiling. Additional markers of MC activation are needed for use in clinical diagnostics, to help selection of treatment of MC diseases, and for assessing outcomes of therapy. We review the spectrum of disorders with known or suspected MC contribution, describe the utility and limitations of current MC markers and assays, and discuss the need for new markers that can differentiate between MC activation and burden., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Stress exposure worsens allergic inflammatory diseases substantially. Mast cells (MCs) play a key role in peripheral immune responses to neuroendocrine stress mediators such as nerve growth factor (NGF) and substance P (SP). Mast cell proteases (MCPs) and cholinergic factors (Chrna7, SLURP1) were recently described to modulate MC stress response. We studied MCPs and Chrna7/SLURP1 and their interplay in a mouse model for noise induced stress (NiS) and atopic dermatitis-like allergic inflammation (AlD) and in cultured MC lacking Chrna7. We found that the cholinergic stress axis interacts with neuroendocrine stress mediators and stress-mediator cleaving enzymes in AlD. SP-cleaving mMCP4+ MC were upregulated in AlD and further upregulated by stress in NiS+AlD. Anti-NGF neutralizing antibody treatment blocked the stress-induced upregulation in vivo, and mMCP4+ MCs correlated with measures of AlD disease activity. Finally, high mMCP4 production in response to SP depended on Chrna7/SLURP1 in cultured MCs. In conclusion, mMCP4 and its upstream regulation by Chrna7/SLURP1 are interesting novel targets for the treatment of allergic inflammation and its aggravation by stress.

Inner ear disorders have a variety of causes, and many factors can contribute to the exacerbation of cochlear and vestibular pathology. This systematic review aimed to analyze clinical data on the coexistence and potential causal interaction between allergic diseases and inner ear conditions. A search of PubMed and Web of Science identified 724 articles, of which 21 were selected for full-text analysis based on inclusion and exclusion criteria. The epidemiologic evidence found overwhelmingly supports an association between allergic disease and particular inner ear disorders represented by a high prevalence of allergic reactions in some patients with Ménière's disease (MD), idiopathic sudden sensorineural hearing loss (ISSHL), and acute low-tone hearing loss (ALHL). In addition, patients with MD, ISSHL, and ALHL had higher levels of total serum IgE than healthy subjects. Finally, in some cases, changes in cochlear potential may have been induced by antigen exposure, while desensitization alleviated allergy and inner ear-related symptoms. The exact mechanism of interaction between the auditory/vestibular and immune systems is not fully understood, and further clinical and basic research is needed to understand the relationship between the two systems fully., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Zeng, Domarecka, Kong, Olze, Scheffel, Moñino-Romero, Siebenhaar and Szczepek.)

Chronic urticaria (CU) is a mast cell (MC)-dependent disease with limited therapeutic options. Current management strategies are directed at inhibiting IgE-mediated activation of MCs and antagonizing effects of released mediators. Due to the complexity and heterogeneity of CU and other MC diseases and mechanisms of MC activation-including multiple activating receptors and ligands, diverse signaling pathways, and a menagerie of mediators-strategies of MC depletion or MC silencing (i.e., inhibition of MC activation via binding of inhibitory receptors) have been developed to overcome limitations of singularly targeted agents. MC silencers, such as agonist monoclonal antibodies that engage inhibitory receptors (e.g., sialic acid-binding immunoglobulin-like lectin8 -[Siglec-8] [lirentelimab/AK002], Siglec-6 [AK006], and CD200R [LY3454738]), have reached preclinical and clinical stages of development. In this review, we (1) describe the role of MCs in the pathogenesis of CU, highlighting similarities with other MC diseases in disease mechanisms and response to treatment; (2) explore current therapeutic strategies, categorized by nonspecific immunosuppression, targeted inhibition of MC activation or mediators, and targeted modulation of MC activity; and (3) introduce the concept of MC silencing as an emerging strategy that could selectively block activation of MCs without eliciting or exacerbating on- or off-target, immunosuppressive adverse effects., (© 2023 The Authors. Allergy published by European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd.)

Mastocytosis is a rare disease characterized by clonal expansion and accumulation of mast cells (MC) in various organs. Mastocytosis results from an activating mutation of the KIT surface receptor leading to an increased proliferation of MC. There are significant differences between children and adult patients with mastocytosis. Children mainly present the cutaneous form, whereas adults more often exhibit the systemic form of mastocytosis. Patients with mastocytosis may be asymptomatic or affected by a variety of symptoms. Treatment is primarily supportive and aims at symptom control. New approved targeted therapies such as midostaurin and avapritinib changed the treatment paradigm in advanced forms of the disease, and next-generation inhibitors currently in clinical trials are expected in the near future., (© 2023. The Author(s), under exclusive licence to Springer Medizin Verlag GmbH, ein Teil von Springer Nature.)

Objective: Mastocytosis is a complex disorder affecting various organs. The diagnostic workup can be challenging and requires a multidisciplinary approach including the use of uncommon tests. To assess mastocytosis management around the globe, we conducted the first worldwide online survey for physicians., Methods: A 21-item questionnaire was sent out to the members of the World Allergy Organization (WAO), the Global Allergy and Asthma European Network (GA 2 LEN), the Urticaria (UCARE) and Angioedema (ACARE) Centers of Reference and Excellence, the German Society of Allergology and Clinical Immunology (DGAKI), and the European Mast Cell and Basophil Research Network (EMBRN) in April-June 2021., Results: Across 628 respondents from 79 countries 87.7% and 9.7% of physicians were allergists/clinical immunologists and/or dermatologists. Participating physicians were from all regions of the world (Europe, EU: 41.6%; North America, NA: 24.8%; Latin America, LA: 14.5%; Asia-Pacific, AP: 12.6%; and Africa/Middle East, AME: 6.5%). Only 2.2% of respondents worked at Specialized Mastocytosis Centers (SMCs) in North America or European Union. Physicians reported caring for 4 patients with mastocytosis per year, with higher numbers in European Union and Asia Pacific (5/year) compared to Latin America (2/year). Dermatologists and physicians who work at SMCs reported higher patient numbers (15/year and 80/year, respectively). Suspicion of mastocytosis in the allergology and dermatology community is commonly driven by anaphylaxis (82.9%), mastocytosis skin lesions (82.1%), or elevated tryptase levels (76.6%). Osteoporosis and gastrointestinal symptoms less often prompted suspicion of mastocytosis (21.4% and 49.9%, respectively). World Health Organisation (WHO)-diagnostic criteria and classification, regardless of the region, are only used by about 50% of physicians, with higher rates for SMCs (83.3%). Serum tryptase, bone marrow biopsy, and KIT D816V mutation analysis are included in the diagnostic workup by 90.9%, 61.5%, and 58.4% of physicians, respectively. The biggest challenges for the management of mastocytosis are the lack of more effective treatment options (51.1%), missing multidisciplinary networks (47.1%), and the lack of experience of specialists from other disciplines (39.0%)., Conclusions: The diagnostic workup for mastocytosis differs from consensus recommendations and varies between regions. This may be improved by establishing active multidisciplinary networks, increasing access to diagnostic procedures, consistently applying WHO criteria, and developing new Mastocytosis Centers of Reference and Excellence., (© 2023 The Authors.)

Mastocytosis is a heterogeneous disease with a wide spectrum of signs, symptoms, and concomitant disorders, such as skin lesions, anaphylaxis, osteoporosis, gastrointestinal involvement, and organomegaly. Disease specificity for frequently reported symptoms, such as fatigue, headache, anxiety, and brain fog, is poorly defined and need to be addressed in further studies. Patients with CM and non-AdvSM are mostly affected by mast cell mediator-related symptoms, whereas in AdvSM symptoms also result from organ damage, which makes their assessment challenging. In this paper we discuss approaches currently used to measure symptom burden and QoL impairment in relation to the clinical phenotype., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Physiological levels of basal serum tryptase vary among healthy individuals, depending on the numbers of mast cells, basal secretion rate, copy numbers of the TPSAB1 gene encoding alpha tryptase, and renal function. Recently, there has been a growing debate about the normal range of tryptase because individuals with the hereditary alpha tryptasemia (HαT) trait may or may not be symptomatic, and if symptomatic, uncertainty exists as to whether this trait directly causes clinical phenotypes or aggravates certain conditions. In fact, most HαT-positive cases are regarded as asymptomatic concerning mast cell activation. To address this point, experts of the European Competence Network on Mastocytosis (ECNM) and the American Initiative in Mast Cell Diseases met at the 2022 Annual ECNM meeting and discussed the physiological tryptase range. Based on this discussion, our faculty concluded that the normal serum tryptase range should be defined in asymptomatic controls, inclusive of individuals with HαT, and based on 2 SDs covering the 95% confidence interval. By applying this definition in a literature screen, the normal basal tryptase in asymptomatic controls (HαT-positive persons included) ranges between 1 and 15 ng/mL. This definition should avoid overinterpretation, unnecessary referrals, and unnecessary anxiety or anticipatory fear of illness in healthy individuals., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Background: Two endotypes of chronic spontaneous urticaria (CSU) associated with mast cell-activating autoantibodies are described, namely autoallergic chronic spontaneous urticaria (aaCSU; with immunoglobulin E [IgE]-anti-autoallergens) and autoimmune chronic spontaneous urticaria (aiCSU; with IgG-anti-high-affinity receptor for the Fc region of immunoglobulin E [FcεRI]/IgE)., Objective: To investigate the rates of CSU patients with aaCSU and aiCSU., Methods: We analyzed 111 CSU patients for aaCSU (ie, IgE to thyroid peroxidase, interleukin 24) and for aiCSU (ie, a positive autologous serum skin and Basophil Activation Test plus immunoglobulin G [IgG]-anti-FcεRI/IgE). Clinical and laboratory parameters were compared in patients with aaCSU, aiCSU, and both., Results: Across 111 patients with CSU, 64 (58%) had aaCSU and 9 (8%) had aiCSU. Eight of the 9 aiCSU patients had aaCSU, but only 8 of 64 patients with aaCSU had aiCSU. In total, 7% (8 of 111) of patients had both aiCSU and aaCSU, 41% (46 of 111) had neither, and 16% (18 of 111) tested negative for all markers of aaCSU and aiCSU assessed. Patients with aaCSU or aiCSU are different from those without: patients with stand-alone aaCSU tend to be younger than non-aaCSU patients, aiCSU, and aaCSU/aiCSU overlapping subpopulations. In contrast, patients with aiCSU, with or without aaCSU coexistence, are more often female, have higher levels of thyroid peroxidase autoantibodies (both IgG and IgE), and show more severe quality of life impairment., Conclusions: Our novel finding that aiCSU coexisting with aaCSU needs to be confirmed in bigger cohorts and multicenter studies. Autoimmunity driven by autoreactive IgE and/or IgG in CSU needs further investigation for better understanding of the pathophysiology., (Copyright © 2023 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

Efficient anti-viral responses of CD8 + T cells require signals that promote their effector cell differentiation, that are mainly provided by dendritic cells (DCs). Mast cells (MCs) are key drivers of DC maturation, but also influence their migration and antigen presenting properties and therefore indirectly mediate CD8 + T cell activation. MCs initiate innate immune responses at pathogen entry sites, promote the development of adaptive immune responses after infection, and release mediators including chemokines that recruit and activate immune cells including T cells during viral infections. However, whether MCs can directly activate virus-specific CD8 + T cells remains largely unknown. Here, we used an in vitro viral infection model with lymphocytic choriomeningitis virus (LCMV)-infected MCs or DCs co-cultured with either LCMV-specific CD8 + T cells or with WT (unspecific) CD8 + T cells. Similar to LCMV-infected DCs, LCMV-infected MCs clustered with virus-specific CD8 + T cells and induced their activation and production of antiviral cytokines. In addition, the co-stimulatory molecules CD86 and OX40L, but not CD80, were upregulated on MCs and an increased production of IL-6 and type I interferons after LCMV infection was shown. Our findings suggest that MCs can promote CD8 + T cell activation during viral infections. MC-mediated CD8 + T cell activation might be especially important within infected tissues where direct cellular interaction can take place. A better understanding of anti-viral functions of MCs may help developing new strategies to better treat viral infections., (© 2023 The Authors. Scandinavian Journal of Immunology published by John Wiley & Sons Ltd on behalf of The Scandinavian Foundation for Immunology.)

Background: Chronic spontaneous urticaria (CSU) is believed to be Autoimmune (aiCSU) (type IIb CSU) in at least 8% of patients, associated with mast cell-activating IgG autoantibodies. Basophil tests such as the basophil activation test (BAT) and basophil histamine release assay (BHRA) are considered the best single tests for an aiCSU diagnosis. To date, the strength of associations among a positive BAT and/or BHRA (BAT/BHRA + ) and CSU features, patient demographics, and response to treatment remains poorly characterized., Objective: To evaluate the strength of current evidence on basophil tests as parameters for CSU characteristics., Methods: We performed a systematic literature search and review to assess the relationship between BAT/BHRA + and clinical and laboratory parameters of CSU. Of 1,058 records found in the search, 94 studies were reviewed by experts in urticaria and 42 were included in the analysis., Results: In CSU patients, BAT/BHRA + showed a strong level of evidence for an association with high disease activity and low levels of total IgE. A weak level of evidence was shown for the association of BAT/BHRA + and the presence of angioedema, and basopenia., Conclusions: Our results suggest that aiCSU defined by BAT/BHRA + is more active or severe and is linked to other aiCSU markers such as low total IgE/basopenia. Basophil tests should be standardized and implemented in routine clinical care to improve the diagnosis and treatment of patients with aiCSU., (Copyright © 2023 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

Beall's list is widely used to identify potentially predatory journals. With this study, we aim to investigate the impact of Beall's list on the perception of listed journals as well as on the publication and citation behavior of the scientific community. We performed comprehensive bibliometric analyses of data extracted from the ISSN database, PubMed, PubMed Central (PMC), Crossref, Scopus and Web of Science. Citation analysis was performed by data extracted from the Crossref Cited-by database. At the time of analysis, Beall's list consisted of 1,289 standalone journals and 1,162 publishers, which corresponds to 21,735 individual journals. Of these, 3,206 (38.8%) were located in the United States, 2,484 in India (30.0%), and 585 in United Kingdom (7.1%). The majority of journals were listed in the ISSN database (n = 8,266), Crossref (n = 5,155), PubMed (n = 1,139), Scopus (n = 570), DOAJ (n = 224), PMC (n = 135) or Web of Science (n = 50). The number of articles published by journals on Beall's list as well as on the DOAJ continuously increased from 2011 to 2017. In 2018, the number of articles published by journals on Beall's list decreased. Journals on Beall's list were more often cited when listed in Web of Science (CI 95% 5.5 to 21.5; OR = 10.7) and PMC (CI 95% 6.3 to 14.1; OR = 9.4). It seems that the importance of Beall's list for the scientific community is overestimated. In contrast, journals are more likely to be selected for publication or citation when indexed by commonly used and renowned databases. Thus, the providers of these databases must be aware of their impact and verify that good publication practice standards are being applied by the journals listed., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Richtig et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

BACKGROUND: Indolent systemic mastocytosis (ISM) is a clonal mast-cell disease driven by the KIT D816V mutation. We assessed the efficacy and safety of avapritinib versus placebo, both with best supportive care, in patients with ISM. METHODS: We randomized patients with moderate to severe ISM (total symptom score [TSS] of ≥28; scores range from 0 to 110, with higher numbers indicating more severe symptoms) two to one to avapritinib 25 mg once daily (n=141) or placebo (n=71). The primary end point was mean change in TSS based on the 14-day average of patient-reported severity of 11 symptoms. Secondary end points included reductions in serum tryptase and blood KIT D816V variant allele fraction (≥50%), reductions in TSS (≥50% and ≥30%), reduction in bone marrow mast cells (≥50%), and quality of life measures. RESULTS: From baseline to week 24, avapritinib-treated patients had a decrease of 15.6 points (95% CI, −18.6 to −12.6) in TSS compared to a decrease of 9.2 points (−13.1 to −5.2) in the placebo group; P<0.003. From baseline to Week 24, 76/141 patients (54%; 45% to 62%) in the avapritinib group compared to 0/71 patients in the placebo group achieved a ≥50% reduction in serum tryptase level; P<0.001. Edema and increases in alkaline phosphatase were more common with avapritinib than placebo; there were few treatment discontinuations because of adverse events. CONCLUSIONS: In this trial, avapritinib was superior to placebo in reducing uncontrolled symptoms and mast-cell burden in patients with ISM. The long-term safety and efficacy of this approach for patients with ISM remain the focus of the ongoing trial. (Funded by Blueprint Medicines Corporation; ClinicalTrials.gov number, NCT03731260.)

In 2002, the European Competence Network on Mastocytosis (ECNM) was launched as a multidisciplinary collaborative initiative to increase the awareness and to improve diagnosis and management of patients with mast cell (MC) disorders. The ECNM consists of a net of specialized centers, expert physicians, and scientists who dedicate their work to MC diseases. One essential aim of the ECNM is to timely distribute all available information about the disease to patients, doctors, and scientists. In the past 20 years, the ECNM has expanded substantially and contributed successfully to the development of new diagnostic concepts, and to the classification, prognostication, and treatments of patients with mastocytosis and MC activation disorders. The ECNM also organized annual meetings and several working conferences, thereby supporting the development of the World Health Organization classification between 2002 and 2022. In addition, the ECNM established a robust and rapidly expanding patient registry and supported the development of new prognostic scoring systems and new treatment approaches. In all projects, ECNM representatives collaborated closely with their U.S. colleagues, various patient organizations, and other scientific networks. Finally, ECNM members have started several collaborations with industrial partners, leading to the preclinical development and clinical testing of KIT-targeting drugs in systemic mastocytosis, and some of these drugs received licensing approval in recent years. All these networking activities and collaborations have strengthened the ECNM and supported our efforts to increase awareness of MC disorders and to improve diagnosis, prognostication, and therapy in patients., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

In chronic spontaneous urticaria (CSU), wheals, angioedema, or both appear spontaneously for > 6 weeks. Current recommended treatment options for urticaria target mast cell mediators such as histamine, or activators, such as autoantibodies. The goal of CSU treatment is to treat the disease until it is gone as effectively and safely as possible. As no cure is available for CSU as of now, the treatment is aimed at continuously suppressing disease activity, with complete control of the disease and a normalization of quality of life. To achieve this, pharmacological treatment should be continued until no longer needed. Treatment of CSU should follow the basic principles of treating as much as needed and as little as possible taking into consideration that the activity of the disease may vary. Since CSU is a disease with spontaneous remission, it is hard to tell, in patients with complete control and no signs or symptoms, when medication is no longer needed. The current international guideline for urticaria suggests that the treatment can be stepped down once a patient is free of signs and symptoms. Other reasons for stepping down the treatment of CSU patients include safety concerns or issues, pregnancy or wanting to become pregnant, and economic factors. As of now, it is unclear over which period, with what intervals and with which dosages CSU treatment should be stepped down. Guidance on this is needed for all recommended therapies: (i) standard-dosed second-generation H1-antihistamine (sgAH), (ii) higher than standard-dosed sgAH, (iii) standard-dosed omalizumab, (iv) higher than standard-dosed omalizumab, and (v) cyclosporine. However, there is a lack of controlled trials on the step down and discontinuation of these treatments. Here, we aim to provide a summary of what is known and what needs to be investigated in further studies, based on our own experience and real-world evidence., (© 2023. The Author(s).)

Background: The Indolent Systemic Mastocytosis Symptom Assessment Form (ISM-SAF) (©Blueprint Medicines Corporation), a 12-item daily diary that assesses 11 signs and symptoms of indolent systemic mastocytosis (ISM) and smoldering systemic mastocytosis (SSM), was psychometrically evaluated among patients with ISM. Additionally, thresholds of the ISM-SAF total symptom score (TSS) to distinguish patients with moderate to severe symptoms from those with mild symptoms were evaluated., Methods: The ISM-SAF was completed daily as an electronic diary in a prospective, observational study utilizing an online survey of patients with ISM in the United States. Descriptive statistics, psychometric analyses, and analyses to estimate ISM-SAF TSS clinical cutoff values were conducted., Results: A total of 103 patients (81.6% female; mean age = 50.2 [± 12.6]) with a self-reported diagnosis of ISM or SSM (58 of whom also had a medically documented diagnosis) contributed to the analyses. Psychometric analysis supported the trustworthiness of the biweekly TSS, which was reliable (α > 0.8, ICC > 0.9), construct-valid, and able to distinguish among clinically distinct groups as specified by the Patient Global Impression of Severity, 12-item Short-Form Health Survey, and Mastocytosis Quality of Life Questionnaire (p < 0.01). A biweekly ISM-SAF TSS from 21 to 28 begins to distinguish the moderately to severely symptomatic ISM/SSM patients from mildly symptomatic patients., Conclusion: The biweekly TSS of ISM-SAF was reliable, construct-valid, and able to distinguish among clinically distinct groups. A cut-off value of 28 is a conservative threshold that can be used for screening purposes in future clinical studies to identify patients with at least a moderate severity of ISM symptoms., (© 2023. The Author(s).)

Background: Idiopathic mast cell activation syndrome (MCAS) is characterized by three diagnostic criteria: (1) episodic mast cell (MC)-driven signs/symptoms of at least two organ systems in the absence of clonal MC expansion and definite triggers, (2) episodic increase in tryptase, and (3) response to MC-targeted treatment. Many patients believe they have MCAS, but how often this is the case remains unknown., Methods: We prospectively investigated patients with suspected MCAS (n = 100) for the diagnostic criteria including baseline tryptase, KIT D816V mutation, and patient-reported outcome measures (PROMs) over the course of 12 weeks. Comorbid depression and anxiety were explored with the Hospital Anxiety and Depression Scale (HADS)., Results: In 53% of our patients (80% females), suspicion of MCAS was based on self-evaluation. In total, patients reported 87 different symptoms, mostly fatigue (n = 57), musculoskeletal pain/weakness (n = 49), and abdominal pain (n = 43), with overall high disease activity and impact. Two of 79 patients had increased tryptase (by >20% +2 ng/ml) following an episode. Only 5%, with any of the PROMs used, showed complete response to MC-targeted treatment. Depression and anxiety disorders were frequent comorbidities (n = 23 each), and 65 patients had pathological HADS values, which were linked to high disease impact and poor symptom control., Conclusion: Mast cell activation syndrome was confirmed in only 2% of patients, which implies that it is not MC activation that drives signs and symptoms in most patients with suspected MCAS. There is a high need for comprehensive research efforts aimed at the identification of the true underlying pathomechanism(s) in patients with suspected MCAS., (© 2022 The Authors. Allergy published by European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd.)

Indolent systemic mastocytosis (ISM) has a favorable prognosis and normal life expectancy. However, many patients suffer from mast cell (MC) mediator-related symptoms, which significantly affect quality of life (QoL). Cutaneous, gastrointestinal, and neurological complaints, musculoskeletal pain, and the presence of skin lesions, anaphylaxis, and osteoporosis are the main symptoms and signs in ISM and must be assessed in all patients before and during treatment. Validated mastocytosis-specific patient-reported outcome measures (PROMs) should be used for this purpose. Serum tryptase and KIT D816V allele burden are recommended as secondary outcome parameters, noting that they do not reflect the severity of signs, symptoms, and related QoL impairment, but indirectly express MC burden. Changes from baseline of 90%, 60%, and 30% indicate complete response >90%, major response 60% to 90%, partial response 30% to 60%, and no response <30% to treatment. To conclude, we recommend the use of PROMs as primary outcome parameters to define treatment response in patients with ISM in clinical trials and in everyday clinical practice., (Copyright © 2022. Published by Elsevier Inc.)

Mast cell activation (MCA) is common and occurs in a number of pathologic conditions, including IgE-dependent and independent allergic reactions, atopic disorders, autoimmune processes, and mastocytosis. In a subset of patients, no underlying disease and no known trigger of MCA are found. When the symptoms are severe, systemic, and recurrent, and accompanied by a diagnostic increase in the serum tryptase level or other mast cell mediators, an MCA syndrome (MCAS) may be diagnosed. In these patients, the symptoms typically respond to drugs suppressing MCA, mediator production in mast cells, or mediator effects. In each case, diagnostic consensus criteria must be fulfilled to diagnose MCAS. In other patients, MCA may be local, less severe, or less acute, or may be suspected but not confirmed, so that the diagnostic criteria of MCAS are not fulfilled. In these patients, it may be difficult to prove MCA, for example, by measuring multiple mast cell mediators or basophil activation, the latter as a surrogate of IgE-dependent hypersensitivity. However, validated diagnostic criteria for implicating suspected MCA behind such conditions are lacking, even if some of these conditions have recently been assigned to an International Classification of Diseases-10-Clinical Modification code (ICD-10-CM). In this article, we discuss diagnostic features and criteria and propose a ICD-10-CM-adjusted classification for disorders associated with MCA, herein referred to as MCA disorders (MCADs), with special emphasis on the delineation between confirmed MCAS, MCAD not fulfilling MCAS criteria, and suspected MCAD that is not present. In addition, we discuss the discrimination between overt MCAD and predisposing conditions, such as atopic states, mastocytosis, and hereditary alpha tryptasemia., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Advanced systemic mastocytosis (AdvSM) is characterized by the presence of KIT D816V and other somatic mutations (eg, in SRSF2, ASXL1, and RUNX1) in 95% and 60% to 70% of patients, respectively. The biological and clinical consequences of AdvSM include multilineage involvement (eg, associated hematologic neoplasm) in 60% to 80% of patients, variable infiltration and damage (C-findings) of predominantly bone marrow and visceral organs through affected mast cell (MC) and non-MC lineages, and elevated levels of serum tryptase. Recently, the treatment landscape has substantially changed with the introduction of the multikinase/KIT inhibitor midostaurin and the selective KIT D816V inhibitor avapritinib. In this review, we discuss the evolution of AdvSM response criteria that have been developed to better capture clinical benefit (eg, improved responses and progression-free and overall survival). We propose refined response criteria from European Competence Network on Mastocytosis and American Initiative in Mast Cell Diseases investigators that use a tiered approach to segregate the effects of histopathologic (eg, bone marrow MC burden, tryptase), molecular (eg, KIT D816V variant allele frequency), clinical (eg, C-findings), and symptom response on long-term outcomes. These response criteria require evaluation in future prospective clinical trials of selective KIT inhibitors and other novel agents., (Copyright © 2022. Published by Elsevier Inc.)

Background: Since 2010, patients and physicians have collaborated to understand unmet needs of patients with mast cell diseases, incorporating mastocytosis and mast cell activation disorders, which include mast cell activation syndromes., Objective: This Open Innovation in Science project aims to expand understanding of the needs of patients affected by mast cell diseases, and encourage global communication among patient advocacy groups, physicians, researchers, industry, and government. A major aim is to support the scientific community's efforts to improve diagnosis, management, therapy, and patients' quality of life by addressing unmet needs., Methods: In collaboration with mast cell disease specialists, 13 patient advocacy groups from 12 countries and regions developed lists of top patient needs. A core team of leaders from patient advocacy groups collected and analyzed the data and proposed possible actions to address patient needs., Results: Findings identified similarities and differences among participating countries in unmet needs between patients with mastocytosis and those with mast cell activation syndromes. Issues emphasized struggles relating to the nature and rarity of mast cell diseases, their impact on quality of life, the diagnostic process, access to appropriate care, more effective treatment, and the need for research., Conclusions: Solutions vary across countries because situations differ, in particular regarding the existence of and access to centers of excellence and reference centers. Multifaceted mast cell activation syndrome barriers necessitate innovative approaches to improve access to appropriate care. The outcomes of this project should greatly support scientists and clinicians in their efforts to improve diagnosis, management, and treatment of patients with mastocytosis and mast cell activation disorders., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Background: Recently, the expression of the mast cell (MC) receptor Mas-related G protein-coupled receptor X2 (MRGPRX2) has been detected in lesional skin of adult patients with cutaneous mastocytosis. As of yet, little is known about the clinical relevance of MRGPRX2 and its agonists in patients with mastocytosis, including indolent systemic mastocytosis (ISM)., Methods: MRGPRX2 and MRGPRX2 agonists, cortistatin (CST), and major basic protein (MBP) were analyzed in lesional and non-lesional skin of patients with ISM and skin of healthy controls by immunohistochemistry. Co-localization of MRGPRX2 and MRGPRX2-mRNA with the MC marker tryptase was assessed by immunofluorescence microscopy and in situ hybridization, respectively. We assessed clinical, demographic, and laboratory data, including mastocytosis activity score (MAS), serum tryptase, and KIT D816V allele burden., Results: The number of MRGPRX2-expressing (MRGPRX2+) cells, MRGPRX2-mRNA+ MCs, and CST-expressing (CST+) and MBP-expressing (MBP+) cells was significantly higher in lesional skin as compared to non-lesional skin and/or skin of healthy controls (all p < 0.05). Increased numbers of MRGPRX2+ cells, MRGPRX2-mRNA+ MCs, and CST+ and MBP+ cells were not associated with clinical and laboratory features of ISM, including disease burden, symptom severity, evidence of anaphylaxis, and tryptase levels., Conclusions: Skin lesions of patients with ISM showed high numbers of MRGPRX2+ cells, although they were not linked to symptom severity. Clinical relevance of the MRGPRX2-mediated pathway of MC activation in ISM remains unclear and should be investigated in further studies., Competing Interests: HB received honoraria (advisory board, speaker) from AbbVie, Novartis, and Sanofi-Aventis, outside of submitted work. JS has no relevant conflict of interest in relation to this work. Outside of it, JS is or recently was advisor for Boehringer Ingelheim. BY is employed by Allakos Inc. MM has no relevant conflict of interest in relation to this work. Outside of it, MM is or recently was a speaker and/or advisor or received institutional research funding from Astria, Allakos, Alnylam, Amgen, Aralez, ArgenX, AstraZeneca, BioCryst, Blueprint, Celldex, Centogene, CSL Behring, Dyax, FAES, Genentech, GIInnovation, GSK, Innate Pharma, Kalvista, Kyowa Kirin, Leo Pharma, Lilly, Menarini, Moxie, Novartis, Pfizer, Pharming, Pharvaris, Roche, Sanofi/Regeneron, Shire/Takeda, Third Harmonic Bio, UCB, and Uriach. FS is or recently was a speaker and/or advisor for and/or has received research funding from Allakos, Blueprint, Celldex, CogentBio, Genentech, Novartis, Moxie, Sanofi/Regeneron, and Uriach. PK has no relevant conflict of interest in relation to this work. Outside of it, PK is or recently was a speaker and/or advisor for Novartis and Roche. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Pyatilova, Ashry, Luo, He, Bonnekoh, Jiao, Moñino-Romero, Hu, Scheffel, Frischbutter, Hermans, Youngblood, Maurer, Siebenhaar and Kolkhir.)

Mast cells are highly granular tissue-resident cells and key drivers of inflammation, particularly in allergies as well as in other inflammatory diseases. Most mast cell research was initially conducted in rodents but has increasingly shifted to the human system, with the advancement of research technologies and methodologies. Today we can analyze primary human cells including rare subpopulations, we can produce and maintain mast cells isolated from human tissues, and there are several human mast cell lines. These tools have substantially facilitated our understanding of their role and function in different organs in both health and disease. We can now define more clearly where human mast cells originate from, how they develop, which mediators they store, produce de novo, and release, how they are activated and by which receptors, and which neighboring cells they interact with and by which mechanisms. Considerable progress has also been made regarding the potential contribution of mast cells to disease, which, in turn, has led to the development of novel approaches for preventing key pathogenic effects of mast cells, heralding the era of mast cell-targeted therapeutics. In this review, we present and discuss a selection of some of the most significant advancements and remaining gaps in our understanding of human mast cells during the last 25 years, with a focus on clinical relevance., (Copyright © 2022 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

Background: Chronic urticaria (CU) is a debilitating mast cell-driven disease, often refractory to standard therapy (ie, antihistamines). Lirentelimab, an anti-sialic acid-binding immunoglobulin-like lectin 8 mAb, selectively inhibits mast cells and depletes eosinophils., Objective: We sought to determine safety and efficacy of lirentelimab in patients with CU., Methods: This phase 2a study enrolled patients with CU refractory to up to 4-fold H1-antihistamine doses. Patients received 6 monthly intravenous doses of lirentelimab (0.3, 1, and up to 3 mg/kg). Primary efficacy end point was change in Urticaria Control Test score at week 22. Urticaria Activity Score weekly average (UAS7) was assessed in patients with chronic spontaneous urticaria (CSU), and Cholinergic UAS7 was used for patients with cholinergic urticaria (CholU)., Results: A total of 45 patients were enrolled in 4 cohorts (n = 13 omalizumab-naive CSU, n = 11 omalizumab-refractory CSU, n = 11 CholU, n = 10 symptomatic dermographism). Urticaria Control Test scores increased with lirentelimab across cohorts, with mean changes at week 22 of 11.1 ± 4.1, 4.8 ± 7.0, 6.5 ± 6.2, and 3.4 ± 4.1 and complete response rates (Urticaria Control Test score ≥ 12) of 92%, 36%, 82%, and 40%, respectively. In omalizumab-naive and omalizumab-refractory patients with CSU, disease activity decreased at week 22 (mean UAS7 change, -73% and -47%, respectively), with UAS7 response rates (≥50% reduction) of 77% and 45%, respectively. In patients with symptomatic dermographism, 50% (5 of 10) and 40% (4 of 10) had complete itch and hive resolution by FricTest, respectively, and 100% (7 of 7) evaluable patients with CholU had negative responses to Pulse-Controlled Ergometry exercise test. Most common adverse events included infusion-related reactions (43%; all mild/moderate and transient), nasopharyngitis (21%), and headache (19%). No treatment-related serious adverse events occurred., Conclusions: Lirentelimab demonstrated activity across 3 forms of antihistamine-refractory CU., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Mast cells have crucial roles in allergic and other inflammatory diseases. Preclinical approaches provide circumstantial evidence for mast cell involvement in many diseases, but these studies have major limitations - for example, there is still a lack of suitable mouse models for some mast cell-driven diseases such as urticaria. Some approaches for studying mast cells are invasive or can induce severe reactions, and very few mediators or receptors are specific for mast cells. Recently, several drugs that target human mast cells have been developed. These include monoclonal antibodies and small molecules that can specifically inhibit mast cell degranulation via key receptors (such as FcεRI), that block specific signal transduction pathways involved in mast cell activation (for example, BTK), that silence mast cells via inhibitory receptors (such as Siglec-8) or that reduce mast cell numbers and prevent their differentiation by acting on the mast/stem cell growth factor receptor KIT. In this Review, we discuss the existing and emerging therapies that target mast cells, and we consider how these treatments can help us to understand mast cell functions in disease., (© 2021. Springer Nature Limited.)

Background: Mast cells (MCs) are considered the main effectors in allergic reactions and well known for their contribution to the pathogenesis of various inflammatory diseases, urticaria, and mastocytosis. To study their functions in vitro, human primary MCs are isolated directly from several tissues or differentiated from hematopoietic progenitors. However, these techniques bear several disadvantages and challenges including low proliferation capacity, donor-dependent heterogeneity, and the lack of a continuous cell source., Objective: To address this, we developed a novel strategy for the rapid and efficient differentiation of MCs from human-induced pluripotent stem cells (hiPSCs)., Methods: A 4-step protocol for the generation of hiPSC-derived MCs, based on the use of 3 hiPSC lines, was established and validated by comparison with human skin MCs and peripheral hematopoietic stem cell-derived MCs., Results: hiPSC-MCs share phenotypic and functional characteristics of human skin MCs and peripheral hematopoietic stem cell-derived MCs. They display stable expression of the MC-associated receptors CD117, FcεRIα, and Mas-related G protein-coupled receptor X2 and degranulate in response to IgE/anti-IgE and substance P., Conclusions: This novel hiPSC-based approach provides a sustainable and homogeneous source for a rapid and highly productive generation of phenotypically mature, functional MCs, and its principle allows for the investigation of disease- and patient-specific MC populations., (Copyright © 2021 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

This update and revision of the international guideline for urticaria was developed following the methods recommended by Cochrane and the Grading of Recommendations Assessment, Development and Evaluation (GRADE) working group. It is a joint initiative of the Dermatology Section of the European Academy of Allergology and Clinical Immunology (EAACI), the Global Allergy and Asthma European Network (GA²LEN) and its Urticaria and Angioedema Centers of Reference and Excellence (UCAREs and ACAREs), the European Dermatology Forum (EDF; EuroGuiDerm), and the Asia Pacific Association of Allergy, Asthma and Clinical Immunology with the participation of 64 delegates of 50 national and international societies and from 31 countries. The consensus conference was held on 3 December 2020. This guideline was acknowledged and accepted by the European Union of Medical Specialists (UEMS). Urticaria is a frequent, mast cell-driven disease that presents with wheals, angioedema, or both. The lifetime prevalence for acute urticaria is approximately 20%. Chronic spontaneous or inducible urticaria is disabling, impairs quality of life, and affects performance at work and school. This updated version of the international guideline for urticaria covers the definition and classification of urticaria and outlines expert-guided and evidence-based diagnostic and therapeutic approaches for the different subtypes of urticaria., (© 2021 GA²LEN. Allergy published by European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd.)

Mast cells (MCs) play important roles in normal immune responses and pathological states. The location of MCs on the boundaries between tissues and the external environment, including gut mucosal surfaces, lungs, skin, and around blood vessels, suggests a multitude of immunological functions. Thus, MCs are pivotal for host defense against different antigens, including allergens and microbial pathogens. MCs can produce and respond to physiological mediators and chemokines to modulate inflammation. As long-lived, tissue-resident cells, MCs indeed mediate acute inflammatory responses such as those evident in allergic reactions. Furthermore, MCs participate in innate and adaptive immune responses to bacteria, viruses, fungi, and parasites. The control of MC activation or stabilization is a powerful tool in regulating tissue homeostasis and pathogen clearance. Moreover, MCs contribute to maintaining the homeostatic equilibrium between host and resident microbiota, and they engage in crosstalk between the resident and recruited hematopoietic cells. In this review, we provide a comprehensive overview of the functions of MCs in health and disease. Further, we discuss how mouse models of MC deficiency have become useful tools for establishing MCs as a potential cellular target for treating inflammatory disorders.