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Immune system.

Humans are protected in various degrees against disease-causing microorganisms and cancer cells by a surveillance mechanism referred to as the immune system. Together, different parts of this system provide protection or immunity by establishing barriers to invasion by microorganisms or other infectious-disease agents or by selectively neutralizing or eliminating materials recognized by the immune system as being foreign to the body.

Immunological responses may be either non-specific or specific. These responses serve three functions: 1) defence against invasion by microorganisms; 2) maintenance of a stable internal environment or homeostasis within the body; and 3) surveillance for or recognition of abnormal cell types and their destruction.

The bloodstream carries the cells and molecules of the immune system to most tissues of the body. Components of the lymphatic system include lymphatic

vessels, lymph fluid, lymph nodes, and lymphocytes. The lymphatic vesselspass through the lymph nodes,which are accumulations of lymphocytes and macrophages, held together by a fibrous connective tissue capsule. Macrophagesarise from blood-circulating monocytes. They can accomplish several immunologically related functions, including clearing and breaking down foreign substances within the body (phagocytosis),and preparing foreign substances for antibody formation. Non-specific defences protect against any intruder. Such defences include unbroken skin, mucous membranes of the respiratory and genitourinary systems, nasal hairs, coughing, and sneezing reflexes, tears, etc. In the event the mechanical and chemical barriers are overcome by foreign invaders, the body has other important defences available, including phagocytosis. In this process the ingestion and eventual digestion of foreign matter by circulating granulocytes, monocytes, and macrophages take place. Unfortunately, some disease agents can escape this destructive defence mechanism.

Inflammation is considered to be the body’s second line of defence against infection. A large number of phagocytic cells accumulate at the site of inflammation. The typical signs of inflammation are heat, pain, redness, swelling, and pus formation. They represent the body’s first step in the repair of injured tissue.

One of the most important defence mechanisms is fever,a frequently found sign of many disease states and a part of the local inflammatory reaction. Temperatures of 38.5°-39°C are known to be helpful to a person’s recovery, since destruction of disease agents occurs more rapidly because of an increase in antibody production and more effective phagocytic activity. In addition to phagocytosis, inflammation and fever, the body is capable of launching various types of chemical substances to interfere with the actions of pathogens and, in some cases, even to destroy them. Examples of these body bullets include the proteins known as complementand the interferons.

The immune system has two major types of specific responses to foreign cells and substances. These are humoral immunityand cell-mediated immunity.Both types of responses depend on lymphocytes, which originate from different body locations early in the development of the individual. Any foreign particle may act as an antigen (immunogen), that is, a substance that provokes a response by the immune system. This response comes from two types of lymphocytes that circulate in the lymphatic system and the blood. One type, the T cells (T lymphocytes), mature in the thymus gland. They are capable of attacking a foreign cell directly, producing cell-mediated immunity. The B cells (B lymphocytes) mature in lymphoid tissue. When they meet a foreign antigen, they multiply rapidly and produce antibodies (immunoglobulins) that inactivate the antigen. Antibodies remain in the blood, often providing long-term immunity to the specific organism against which they were formed. Antibody-based immunity is referred to as humoral immunity. Immunoglobulinsare produced by the body in response to: 1) infectious disease agents; 2) preparations used for immunization such as vaccines containing killed, weakened or attenuated microorganisms; 3) other foreign substances such as pollens, which cause allergies.

Passive immunityinvolves the transfer of antibodies to an individual, either naturally through the placenta or mother’s milk, or artificially by the administration of an immune serum. However, the antibody levels eventually drop, and the child must be exposed to the antigens and develop his or her own antibodies for permanent immunity. Active immunityinvolves the individual’s own response to a disease organism, either through natural contact with the organism or by the administration of an artificially prepared vaccine. Vaccinations are given in a set schedule during childhood, and periodic boosters are required several years later to ensure continued immunity against the viral agent.

The immunodeficiency diseasesinclude a wide range of disorders and are marked by the inability of the immune system to function normally. The exact effects and symptoms of such conditions range from major, life-threatening abnormalities to minor deficiency conditions and vary according to the immune response affected. Two general categories of these disorders are recognized: congenital (primary)immunodeficiencies and acquired (secondary)immunodeficiencies. Primary disorders develop from an inherited failure of one or more immune system components to develop. The causes of secondary deficiencies include malnutrition, malignancies, exposure to radiation, burns, drugs and infections that interfere with the lymphatic system.

Date: 2016-03-03; view: 1131