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  Immunology (Latin immunis, ‘exempt’ + ology) is the study of the immune system, which is a complex set of biological processes which act co-operatively to prevent invasion by other living organisms (pathogens) and to regulate the growth and differentiation of the cells of the body. The immune system is divided into two functional parts, called the innate and adaptive arms.

The innate system provides defence against invasion by maintaining barriers such as skin, toxic secretions, acids in the stomach and enzymes which destroy bacteria in tears, mucous, etc. Phagocytic cells are capable of enveloping and digesting foreign cells and particles, and are present in the blood and in organs such as the brain, the lungs and the skin. Killer cells circulate in the blood along with molecules which can cause clotting of blood and which can combine to puncture the membranes of foreign cells. When activated, killer cells and phagocytes produce hormones which attract more of their own kind, producing the symptoms of inflammation.

Despite this defensive barrage, foreign cells can penetrate the body and begin to multiply; at this stage the adaptive immune response becomes important. This arm of the immune response centres around a class of molecules called antibodies which act as specific receptors for molecular structures such as proteins. Antibodies can be produced against millions of structures which are foreign to the individual; these structures are called antigens and are frequently proteins found at the surface of pathogenic organisms. Antibodies are produced by specialized cells called lymphocytes which bear a specific antibody at their surface; when this antibody binds to an antigen the lymphocyte is stimulated to divide and produce large quantities of antibody which are released into the blood. The antibody serves to label foreign material in the body so that it may be recognized by cells such as the phagocyte. Antibodies can also act alone to agglutinate foreign particles, inhibiting their growth and causing them to precipitate. When an individual is exposed to an antigen which it has not encountered before, a specific antibody is produced in large quantities and the immune response can be targeted efficiently against the threat. If the same antigen is experienced again, the immune system is already primed and can respond much more rapidly; this is the basis of vaccination, in which the individual is exposed to the antigen without the pathogen, priming the immune response without the risk of causing pathology. Vaccination was pioneered by Edward Jenner (1749 - 1823) who discovered that humans could be made resistant to smallpox by inoculation with blood serum from animals infected with cowpox.

Self antigens are ignored by the immune system because they have been experienced from the outset of life and the cells which produced antibodies against self antigens have been deleted from the complement of lymphocytes at an early stage of development. When self antigens which are not normally found in the extracellular environment are encountered by lymphocytes, the immune response removes them; this is the mechanism by which abnormal body cells are controlled. Occasionally, the immune response becomes sensitized against normal self antigens and autoimmune diseases such as rheumatoid arthritis occur. The discrimination of self from non-self by the immune system is efficient and, while this is important in the control of tumour cells, it is also the reason for the rejection of transplanted organs. The immune response of organ transplant patients is suppressed with drugs to prevent rejection—and such patients must be monitored carefully to avoid the establishment of infections against which they have no defences. More commonly, immunopathology occurs when the immune response produces an inappropriately severe response to an innocuous foreign particle, such as pollen, and inflammation occurs leading to symptoms of allergy. RB

See also bacteriology; endocrinology; helminthology; homeostasis; molecular biology; natural selection; network theory; parasitology; toxicology; virology.Further reading Robert Desowitz, The Thorn in the Starfish; , Ivan Roitt, Immunology.



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