IL-9 also plays a pathogenic role in allergic asthma by controlling mast cells (survival and proliferation, increases expression of proteases and FcεRI expression [32,36,37,38]), epithelial cells (increase mucus production) and promotes airway eosinophilia (eosinophil survival, differentiation and IL-5 Receptor expression [39,40]). Here, IL9 is linked to allergic asthma.