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Background: Human inborn errors of immunity (IEI) are a group of inherited genetic disorders of the immune system. IEI Patients suffer from severe repeated infections, autoimmunity, lymphadenopathy and/or increased susceptibility to malignancies. IEI are due to absence, disproportion, or loss of function of immune cells; mostly inherited in autosomal recessive manner, hence are more common in countries with high rate of consanguinity. Definite diagnosis of IEI is achieved by genetic analysis, however it is not always available., Aim: to report on different IEI categories and impact of expanding the use of flow cytometry (FCM) in diagnosis, categorization and follow up of IEI patients in a highly consanguineous population., Methods: Retrospective chart review on different IEI categories diagnosed at the primary immunodeficiency center in Cairo University Specialized Pediatric hospital from 2011 to 2021 based on expanding the use of FCM., Results: 1510 IEI patients were diagnosed; 480 were diagnosed genetically with FMF, 11 with cystic fibrosis and 1019 patients were diagnosed with other IEI disorders. Phagocytic defects were the commonest (30%) followed by severe combined immunodeficiency (22%) and combined immunodeficiency (18.3%). FCM testing properly diagnosed and categorized 73% of the cases., Conclusion: Using multi-color FCM to evaluate immune cells populations, subpopulations, functions, and intracellular proteins expression is proved a useful cost-effective method for screening, categorization and follow up of IEI patients. FCM can improve the diagnosis of IEI significantly when tests are properly targeted and well designed. This study presents a 10-year experience in diagnosis of IEI using FCM at a tertiary referral center in a setting of limited resources and yet high prevalence of IEI., (© 2022. The Author(s).)