Allergic reactions are also called hypersensitive reactions. Both terms refer to an abnormal immune response to substances that should not trigger reactions. These substances, known as allergens, are harmless to most people and include pollen, certain foods, dust, or latex.
For allergic people, these allergens cause allergic reactions upon contact, inhalation, or consumption. These reactions can be as mild as itching or as serious as breathlessness and possibly death. Allergic reactions may occur immediately or take up to 24 hours to appear.
There are several types of allergic reactions. These are type I, type II, type III, and type IV.
Type I, II, and III are mediated by antibody actions and are called immediate allergic reactions because they occur within 24 hours of exposure to an allergen. Type IV reactions are called delayed allergic reactions because they take more than 24 hours. They are not mediated by antibody actions but by T cell lymphocytes and cell-mediated immune responses.