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Abstract Recently, immune checkpoint inhibitors have been administered for several cancers and have shown great efficacy. However, they are associated with various immune‐related adverse events. Here, we report a case with severe lichenoid eruption following nivolumab administration that improved after the withdrawal of concomitant medications and the patient could continue nivolumab therapy. While nivolumab has been reported to cause severe skin reactions, the current case suggests that severe cutaneous manifestations after nivolumab administration could also be caused by the acquisition of immunogenicity of concomitant medications, including the one that was safely administered before nivolumab treatment and discontinuation of them may enable the patient to continue nivolumab therapy.

Background: Natto (fermented soybeans)-induced hypersensitivity is characterized by delayed symptom onset that hampers diagnosis. We aimed to clarify the clinical utility of the basophil activation test (BAT) in the diagnosis of natto-induced hypersensitivity. Methods: Five patients with a history of anaphylaxis and chronic urticaria suspected of natto-induced hypersensitivity and seven with chronic spontaneous urticaria clinically unrelated to natto were enrolled in the patient and control groups, respectively. The BAT was performed with two incubation times, 15 min and 1 h, in combination with various concentrations of natto-mucilage extract. Results: In controls, CD203c levels in basophils remained low in the 15-min incubation but were significantly increased in the 1-h incubation. In the patient group, in the 15-min condition, basophil CD203c was significantly upregulated by natto mucilage but not by soybean vs controls (P = 0.001). Low concentrations of natto mucilage were sufficient to upregulate basophil CD203c in the anaphylaxis cases, but high concentrations were required to induce the same effect in the urticaria cases. Finally, the dose-dependent pattern of the BAT results differed significantly between the anaphylaxis and urticaria cases (P = 0.006). Thus, a strong background reaction was observed in the BAT with 1 h incubation; 15 min of incubation was sufficient to identify patients with natto-induced hypersensitivity and may distinguish the clinical phenotype of natto-induced hypersensitivity, i.e., anaphylaxis or urticaria. Conclusions: The BAT with a 15-min incubation period is useful in diagnosing natto-induced hypersensitivity.