Allergens cause the production of immunoglobulin E (IgE), an antibody that all of us have in small amounts. Allergic persons, however, produce IgE in abnormally quantities. Normally, this antibody is important in protecting us from parasites, but not from other allergens. During the sensitization period in allergy, IgE is overproduced. It coats certain potentially explosive cells that contain chemicals including histamine. These chemicals, in turn, cause inflammation and the typical allergic symptoms. Allergy symptoms are caused by histamine that initiates an inflammatory response to protect the body. Histamine expands the blood vessels so that the mucous membrane will swell. It is released from the mast cells. Allergic reactions can be grouped into two classes. The most common and best understood by researchers is described above: it involves IgE antibodies and is known as IgE-mediated allergic reaction. Symptoms usually develop within 15 minutes of being exposed to the allergen. Other reactions are known as non-IgE mediated and typically cause symptoms to appear more slowly, sometimes several hours after exposure. These reactions do not necessarily involve antibodies but instead, cell reactions of the immune system. They are much less common and are generally less well understood.