The frequency of occurrence of atypical lymphocytes in peripheral blood smears of natalizumab‐treated patients with multiple sclerosis

Summary Introduction Aside from the extensive published data on immunophenotypic lymphocyte subsets in natalizumab‐treated patients with multiple sclerosis (MS), an impact of natalizumab on lymphocyte morphology has rarely been studied. As patients treated with immunomodulating or immunosuppressive drugs are at risk for infectious disorders such as viral infections, knowledge of drug‐derived changes in lymphocyte morphology may be beneficial in the diagnostic work‐up in such clinical situations. This study aimed to determine the frequency of occurrence of atypical lymphocytes and defined subtypes of variant lymphocytes in natalizumab‐treated patients with MS. Methods We compared eight defined morphological lymphocyte subtypes in peripheral blood smears between 14 natalizumab‐treated, 13 interferon‐treated and 10 untreated subjects with relapse‐remitting MS. Results Atypical lymphocytes were significantly enhanced in natalizumab‐treated patients compared to the interferon and control group (P