Other cells involved in allergic asthma include mast cells that also undergo degranulation to release mediators such as tryptase, histamines, tumour necrosis factor (TNF)-α and various cytokines (e.g. interleukin (IL)-4, IL-5, IL-6 and stem cell factor (SCF)) (33, 36–39). This evidence concerns the gene IL4 and allergic asthma.