Mast cells act on body's defense mechanism. When some allergens like different pollutants, smog, smoke, pollen grains etc. come in contact with mast cell, they stimulate the mast cells. The allergic reaction is triggered by cross linking of $\operatorname{Ig} \mathrm{E}$ molecules on the surface of mast cells by allergen. And ultimately mast cells release excessive amounts of inflammatory chemicals which cause allergic reactions.