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1 Allergy Article by: A. L. de Weck, Institut für Klinische Immunologic Inselspital, Universität Bern, Switzerland. Access to this content is available to Ward's World readers for free from McGraw Hill's AccessScience, an award-winning, digital STEM resource that provides immediate, authoritative answers to students' thirst for scientific knowledge on topics such as climate change, virology, pollution, and more. Ward's World and McGraw Hill have partnered to offer educators a no-obligation, free trial subscription to this product. Request your free trial today and discover how valuable AccessScience can be for you and your students. Altered reactivity in humans and other animals to allergens (substances foreign to the body that trigger an allergic response) induced by exposure through injection, inhalation, ingestion, or skin contact. An allergy (or allergic reaction; Fig. 1) is an antigen–antibody reaction that is marked by an exaggerated physiologic response to a substance (an allergen) that causes no symptoms in nonsensitive individuals. An allergen is a type of antigen (a substance that causes the immune system to produce specific antibodies against it) that induces an allergic state in animals, including humans. Allergens Allergies are triggered by various substances, including pollens (Fig. 2), animal proteins, fungal molds, foods, insect venoms, foreign serum proteins, industrial chemicals, and drugs. Most natural allergens are proteins or polysaccharides of moderate molecular size (molecular weights of 10,000 to 200,000 daltons). Chemicals or drugs of lower molecular weight (haptens) need to bind to the body's own proteins (carriers) in order to become fully effective allergens. For the development of the hypersensitivity state underlying clinical allergies, repeated contact with the allergen is required. Duration of the sensitization period is usually dependent on the sensitizing strength of the allergen and the intensity of exposure. Some allergens (for example, saliva, urine, and hair proteins of domestic animals) are more sensitizing than others. In most instances, repeated contact with minute amounts of allergen is required; for example, several annual seasonal expo- sures to grass pollens or ragweed pollen usually occur before an overt manifestation of hay fever. On the other hand, allergy to cow milk proteins in infants can develop within a few weeks. When previous contacts with allergens have not been apparent (for example, antibiotics in food), an allergy may become clini- cally manifest even upon the first conscious encounter with the offending substance. + ward ' s science Content • Allergens • Mechanisms • IgE antibodies • IgG or IgM antibodies • Sensitized lymphocytes • Clinical forms and types • Allergic rhinitis • Bronchial asthma • Food allergies • Occupational allergies • Skin allergies • Anaphylaxis • Diagnosis • Therapy Key Concepts • An allergy is a reaction in humans and other animals to allergens (substances foreign to the body that cause an allergic response). • Repeated contact with an allergen is required for the development of hypersensitivity to that allergen. • The three main types of immune responses to allergens are the production of immunoglobulin E (IgE) antibodies, IgG or IgM antibodies, and sensitized lymphocytes. • The most common symptoms of allergies are allergic rhinitis, bronchial asthma, hives, atopic eczema and other eczematous skin lesions, and anaphylactic shock. • The diagnosis of allergies often consists of skin tests, in which a small amount of allergen is applied on or injected into the skin. Blood also can be analyzed for immunoglobulin antibodies by serological assays. • The most efficient treatment is elimination of an identified allergen from the affected person's environment and avoidance of further exposure. Drugs can relieve inflammatory symptoms.