IL9 and allergic asthma: For example, during allergic asthma, IL-9 promotes asthma aggravating immune cell survival, recruitment, and proliferation, such as type 2 innate lymphoid cells (ILC2s), Th2 cells, B cells producing IgE, mast cells, and eosinophils, along with increasing mucus production, collagen deposition, and smooth muscle cell hyperplasia [229,230].