That first exposure causes the immune system to produce an antibody called IgE (immunoglobulin E). Then IgE antibodies attach to certain cells, called mast cells, in tissue throughout the body. There they stay like wary soldiers waiting for war. With a second exposure, even months later, some of the allergen binds with the IgE on the mast cell. This time, the mast cell releases a flood of irritating chemicals, which cause inflammation and itching.