Various conditions caused by inappropriate or exaggerated reactions of the immune system (known as hypersensitivity reactions) to a variety of substances. Many common illnesses, such as asthma and allergic rhinitis (hay fever), are caused by allergic reactions to substances that in the majority of people cause no symptoms. Allergic reactions occur only on 2nd or subsequent exposure to the allergen, once 1st contact has sensitized the body. The function of the immune system is to recognize antigens (foreign proteins) on the surfaces of microorganisms and to form antibodies (also called immunoglobulins) and sensitized lymphocytes (white blood cells). When the immune system next encounters the same antigens, the antibodies and sensitized lymphocytes interact with them, leading to destruction of the microorganisms. A similar immune response occurs in allergies, except that the immune system forms antibodies or sensitized lymphocytes against harmless substances because these allergens are misidentified as potentially harmful antigens. The inappropriate or exaggerated reactions seen in allergies are termed hypersensitivity reactions and can have any of four different mechanisms (termed Types I to IV hypersensitivity reactions). Most well known allergies are caused by Type I (also known as anaphylactic or immediate) hypersensitivity in which allergens cause immediate symptoms by provoking the immune system to produce specific antibodies, belonging to a type called immunoglobulin E (IgE), which coat cells (called mast cells or basophils). When the allergen is encountered for the second time, it binds to the IgE antibodies and causes the granules in mast cells to release various chemicals, which are responsible for the symptoms of the allergy. Among the chemicals released is histamine, which causes widened blood vessels, leakage of fluid into tissues, and muscle spasm. Symptoms can include itching, swelling, sneezing, and wheezing. Particular conditions associated with Type I reactions include asthma, hay fever, urticaria (nettle rash), angioedema, anaphylactic shock (a severe, generalized allergic reaction), possibly atopic eczema, and many food allergies. Types II to IV hypersensitivity reactions are less often implicated in allergies. However, contact dermatitis, in which the skin reacts to substances such as nickel, is due to a Type IV hypersensitivity reaction. It is not known why certain individuals and not others get allergies, but about 1 in 8 people seem to have an inherited predisposition to them (see atopy). Whenever possible, the most effective treatment for allergy of any kind is avoidance of the relevant allergen. Drug treatment for allergic reactions includes the use of antihistamine drugs, which relieve the symptoms. Some antihistamine drugs have a sedative effect, which is useful in treating itching at night due to eczema. Many antihistamines do not cause drowsiness, making them more suitable for daytime use. Other drugs, such as sodium cromoglicate and corticosteroid drugs, can be used regularly to prevent symptoms from developing. Hyposensitization can be valuable for a minority of people who suffer allergic reactions to specific allergens such as bee stings. Treatment involves gradually increasing doses of the allergen, but it must be carried out under close supervision because a severe allergic reaction can result.


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