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immunity Ability to resist attack or overcome infection by invading microbes or larger parasites. Immunity is based on the proper function¬ ing of the body’s immune system. In natural or innate immunity, immune mechanisms present at birth work against a wide variety of microbes whether or not they have been encountered before. Acquired immune responses, tailored to act against a specific microbe or its products, are stimulated by the prior presence of that microbe. Previous infection with a particular pathogen, as well as vaccines, produce this type of immunity. The mechanisms of innate immunity include physical barriers (including
the skin) and chemical barriers (such as bactericidal enzymes present in saliva). Microbes that penetrate the body’s natural barriers encounter sub¬ stances (such as interferon) that inhibit their growth or reproduction. Phagocytes (particle-engulfing cells) surround and destroy invading microbes, and natural killer cells pierce the microbe’s outer membrane. Innate immunity does not confer lasting resistance, or immunity, to the body. Acquired immunity is based on the recognition of antigen by B CELLS and T cells and is activated when innate mechanisms are insufficient to stem further invasion by pathogens. Killer or cytotoxic T cells destroy infected and foreign cells. Helper T cells induce B cells stimulated by the presence of antigen to proliferate into antibody-secreting cells, or plasma cells. Antibodies produced by plasma cells bind to antigen-bearing cells, marking them for destruction. Acquired immunity relies on the long-term survival of sensitized T and B memory cells, which can proliferate quickly upon reinfection by the same pathogen. See also immunodeficiency; immu¬ nology; leukocyte; reticuloendothelial system.
immunity In law, exemption or freedom from liability. Under interna¬ tional treaty, a diplomatic representative is exempt from local laws, both civil and criminal. In many countries, judges, legislators, and government officials, including the heads of state, enjoy limited or absolute immunity at home to protect them from personal liability for wrongful acts or omis¬ sions that arise from the performance of their duties. A public prosecutor may grant immunity from prosecution to a witness who is suspected of criminal activity in return for testimony against other suspected criminals.
immunodeficiency Defect in immunity that impairs the body’s ability to resist infection. The immune system may fail to function for many rea¬ sons. Immune disorders caused by a genetic defect are usually evident early in life. Others can be acquired at any age through infections (e.g., AIDS) or immunosuppression. Aspects of the immune response that may be affected include lymphocytes, other leukocytes, antibodies, and complement. Severe combined immunodeficiency (SCID), which arises from several different genetic defects, disrupts all of these. Depending on the cause, treatment for immunodeficiency may be administration of immunoglob¬ ulins, bone-marrow transplant, or therapy for the underlying disease.
immunology Science dealing with the body’s defenses against disease- causing microorganisms and disorders of those defenses. Starting with Edward Jenner’s use of a vaccine against smallpox in 1796, immunology has arrived at a comprehensive and sophisticated understanding of the role of microorganisms in disease and of the formation, mobilization, action, and interaction of antibodies and antigen-reactive cells. It covers treatment of allergies, immunosuppression after organ transplants to prevent rejection, and study of autoimmune diseases and immunodeficiencies. AIDS has stimu¬ lated intensive research in the last of these.
immunosuppression Suppression of immunity with drugs, usually to prevent rejection of an organ transplant. Its aim is to allow the recipient to accept the organ permanently with no unpleasant side effects. In some cases the dosage can be reduced or even stopped without causing rejec¬ tion. Other uses are in the treatment of certain autoimmune diseases and for prevention of erythroblastosis fetalis. Its main drawback is the increased risk of infection for the duration of treatment and of lymphoma in the case of long-term immunosuppression.
impact test Test of the ability of a material to withstand impact, used by engineers to predict its behaviour under actual conditions. Many mate¬ rials fail suddenly under impact, at flaws, cracks, or notches. The most common impact tests use a swinging pendulum to strike a notched bar; heights before and after impact are used to compute the energy required to fracture the bar (see strength of materials). In the Charpy test, the test piece is held horizontally between two vertical bars, much like the lin¬ tel over a door. In the Izod test, the specimen stands erect, like a fence post. See also testing machine.
impala \im-’pa-b\ Swift-running, graceful antelope (Aepyceros me lam- pus) found in large herds, usually near water, on the savannas and open woodlands of central and southern Africa. Impalas are noted for their jumping ability; when alarmed, they bound off in leaps up to 30 ft (9 m) long and 10 ft (3 m) high. Lightly
Herd of male impalas (Aepyceros melampus) in Nairobi National Park, Kenya
JAMES P. ROWAN
© 2006 Encyclopaedia Britannica, Inc.
impatiens ► Impressionism I 931
built, the impala stands 30-40 in. (75-100 cm) high at the shoulder. It has a golden to reddish brown coat, white underparts, a vertical black stripe on each thigh, and a black tuft behind each hind hoof. The male has long, lyre-shaped horns.
impatiens \im-'pa-sh3ns\ Any of about 900 species of herbaceous plants in the genus Impatiens (balsam family), so named because the seed- pod bursts when slightly touched. Garden balsam (/. balsamina ), native to the tropics of Asia, is a favourite showy annual in U.S. gardens; its flowers are irregular, single or clustered, and of almost every colour but blue. Familiar related weeds in eastern North America are spotted jew- elweed (/. biflora or I. capensis ) and pale touch-me-not. Most impatiens have weak, hollow stems and require high moisture. Close relatives are GERANIUMS and NASTURTIUMS.
impeachment Criminal proceeding instituted against a public official by a legislative body. In the U.S. the president, vice president, and other federal officers, including judges, may be impeached by the U.S. House of Representatives. The House draws up articles of impeachment that itemize the charges and their factual bases. Once approved by a majority of House members, the articles are submitted to the Senate, which holds a trial. At its conclusion, each member votes for or against conviction on each article; conviction requires a two-thirds majority. A convicted official can be removed from office. The Constitution of the United States specifies that an officer is to be impeached for “high crimes and misdemeanors”; experts agree that impeachment is permitted for noncriminal misconduct (e.g., vio¬ lation of the Constitution). Two U.S. presidents, Andrew Johnson and Bill Clinton, were impeached; both were acquitted. In 1974, articles of impeachment were drawn up against Pres. Richard Nixon, who resigned before formal proceedings could begin. In Britain, where the House of Commons prosecutes and the House of Lords judges impeachment proceed¬ ings, impeachment was formerly a means by which Parliament could get rid of unpopular ministers, usually court favourites protected by the mon¬ arch. The procedure fell into disuse in the early 19th century, when cabinet ministers became responsible to Parliament rather than to the sovereign.
Imperial Chemical Industries PLC (ICI) Major British chemical corporation. It was founded in 1926 as Imperial Chemical Industries Ltd. to amalgamate four major British chemical companies. Between World Wars I and II, ICI was a major competitor of IG Farben. Today it produces industrial chemicals, paints, and explosives; its drug, pesticide, and spe¬ cialty chemical concerns were split off into a new corporation, Zeneca Group PLC, in 1993. ICI’s headquarters are in London.