Rashes appear when you get into contact with an allergen due to the body’s overreaction to the substance. The immune system treats the allergen like a “wanted” person and sends its “police force” called IgE. The IgE then gathers its forces of mast cells and basophils which bring with them their “weapons” that are histamine. Histamine are the “bombs” that cause the symptoms of rashes. Read more about allergic skin rashes in this article.