IL21 and infection: Bronchial asthma accompanied by recurrent infection may have a bearing on abnormal expression of peripheral blood immunoregulatory T lymphocytes, in which CD4+, Th2, Th17, IL-4 and IL-17 significantly increased, while CD8+, Treg, Th1 and IL-21 significantly decreased, CD4+/CD8+ ratio increased, and Th1/Th2 ratio decreased.