CD4 and asthma: Although these processes comprise a plethora of cells such as T cells, B cells, mast cells, and macrophages on the one hand and smooth muscle cells, fibroblasts, and airway epithelial cells on the other hand, over the last 15 years a subpopulation of CD4+ T cells emerged as key players in asthma pathogenesis—the T helper 2 (TH2) cell.