Allergies, autoimmune diseases, and immunodeficiency diseases are different kinds of disorders of the immune system. Allergies are abnormal sensitivities to a substance that is harmless to many other people. Common allergens include pollen, certain foods, cosmetics, medications, fungal spores, and insect venom. The antibody immunoglobulin E (IgE) is responsible for most allergic reactions. When exposed to an allergen, IgE antibodies attach themselves to mast cells or basophils. Mast cells are normal body cells that produce histamines and other chemicals. When exposed to the same allergen at a later time, the individual may experience an allergic response when the allergen binds to the antibodies attached to mast cells, causing the cells to release histamine and other inflammatory chemicals. While most allergic reactions are expressed as a runny nose, difficulty in breathing, skin rashes and eruptions, or intestinal discomfort, a severe allergic reaction results in anaphylactic shock.