ObjectiveTo investigate the pathogenesis of severe and critical H1N1 influenza A.MethodsBefore and after medical treatment, changes in peripheral blood cells of patients with severe or critical H1N1 influenza A were analyzed, and changes in costimulatory molecules and apoptosis on lymphocytes of patients with severe or critical H1N1 influenza A were dynamically observed via flow cytometry and monoclonal antibodies.ResultsFor patients with severe or critical H1N1 influenza A, the count of leukocytes and lymphocytes before treatment were（2.55±0.87）×109/L and （1.02±0.54）×109/L, respectively, after treatment were （6.95±4.36）×109/ L and （2.13±0.78）×109/L, respectively, there was significant  difference(t=3.28,P=0.01;t=9.52,P=0.00); count of erythrocyte before and after treatment were （4.10±0.45）×1012/L and（4.14±0.39）×1012/L, respectively,  there was no statistical difference (t=0.35,P=0.73). During the recovery phase, lymphocyte count  increased gradually, the expression of  costimulatory molecule CD28 and CD152, and the apoptosis signal CD95 of T lymphocytes decreased. ConclusionH1N1 influenza virus A can repress the activation and proliferation of T lymphocytes, and induce overactivation apoptosis of T lymphocytes,  cause the decrease in lymphocyte and leukocyte count, which may play an important role in the pathogenesis of severe and critical H1N1 influenza A.