Immunology => Antibody
Antibody, any of perhaps a million kinds of normally occurring protein molecules that are produced in the body of cells called lymphocytes and that act primarily as a defence against invasion by foreign substances. An important component of the immune system, antibodies are found in the blood of all vertebrates, in the fraction of the blood called gamma globulin.
The synthesis, or manufacture, of antibodies is initiated when a foreign substance, referred to as an antigen, enters the body. Lymphocytes respond to the foreign substance by making an antibody with a molecular arrangement that fits the shape of molecules on the surface of the substance so that the antibody combines with it. Common antigens are the protein components of bacteria and viruses. These antigens may enter the body during infection or may be deliberately introduced by vaccination in order to stimulate the production of antibodies. The binding of antibodies to the surfaces of bacteria, viruses, or toxins can neutralize and eliminate these harmful substances in any or all of three ways: (1) by directly inactivating them, (2) by enabling other blood cells to engulf and destroy them , and/or (3) by weakening their surfaces and rendering them vulnerable to destruction by other blood proteins (collectively called complement).
Animals do not have antibodies to substances to which they have not been exposed, but one animal is able to produce enough different kinds of antibodies to fit the molecular arrangement of any foreign substance it is likely to encounter.
In diseases such as multiple sclerosis and systemic lupus erythematosus, the body mistakenly makes antibodies against normal tissue components (see Autoimmune Diseases). Sometimes viruses may disturb the immune mechanism.
The five known classes of antibody are distinguished by the letters M, G, E, A, and D; all are preceded by the abbreviation Ig for immunoglobulin, another name for antibody. IgM is the first antibody made by newborns and the first made during an infection. IgG is the predominant antibody in serum; it is made on the second exposure to an antigen. IgE is associated with allergy. IgA is found in saliva and mother's milk. The role of IgD is not known.
A high concentration of one type of antibody is found in the blood of people with a form of malignant tumour called multiple myeloma. In the 1970s scientists learned how to fuse these myeloma cells with lymphocytes from tissues that had been exposed to an antigen. The resulting hybrid cells ("hybridomas") were able to produce large amounts of antibody of one specific arrangement ("clones"), called monoclonal antibodies. By selecting the appropriate hybridoma, scientists can obtain pure antibody that combines with any chosen foreign substance. The use of monoclonal antibodies has become a valuable tool in biology and medicine because pure lines of antibody can combine with and therefore mark, or identify, the component substances of cells and tissues. Under investigation is the use of monoclonal antibodies for immunization, for typing tissue used in transplants (as blood is now typed in transfusions), and for targeting drugs to specific sites in the body.
See also Lymphatic System.