CD4 and asthma: Th9 cells mediate type II immune responses by activating mast cells and eosinophils, and are associated with allergic reactions and asthma.[30] By contrast, type I immunity stimulates macrophages to produce IFN‐γ, which activates NK cells and CD8 T cells to kill cells infected with viruses.[30] We speculate that Id1 deficiency reduces IL‐9 production, which causes the characteristics of Th9 cells to change from type II to type I. One possible reason for this change is that IL‐9 antagonizes type I immune responses since each CD4 T cell subset cross regulates each other.