The body's immune system is designed to produce various chemical factors to fight foreign substances, such as bacteria, viruses and other proteins that the immune system perceives as threatening. An allergic response occurs when the body's immune system overresponds, or is hypersensitive to, specific particles known as allergens. Common allergens include plant pollens, molds, dust mites, animal dander, industrial chemicals, foods, medicines and insect venom.
Among the important components of the immune system are the antibodies, which are produced by lymph tissue. A key player in the allergic response is the antibody known as immunoglobulin E (IgE). IgE is overproduced in certain people, usually those with inherited susceptibility. During an allergic attack, these antibodies attach to cells known as mast cells, which are generally concentrated in the lungs, skin and mucous membranes. Once IgE binds to mast cells, these cells are programmed to release a number of chemicals. One of these chemicals, histamine, opens the blood vessels and causes skin redness and swollen membranes. More mucus or tears may be produced than is normal. Histamine causes many of the symptoms associated with allergies. A common seasonal allergy is hay fever.