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see immunityimmunity,
ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
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A substance that initiates and mediates the formation of the corresponding immune body, termed antibody. Antigens can also react with formed antibodies. Antigen-antibody reactions serve as host defenses against microorganisms and other foreign bodies, or are used in laboratory tests for detecting the presence of either antigen or antibody. See Antibody, Antigen-antibody reaction

A protein immunogen (any substance capable of inducing an immune response) is usually composed of a large number of antigenic determinants. Thus, immunizing an animal with a protein results in the formation of a number of antibody molecules with different specificities. The antigenicity of a protein is determined by its sequence of amino acids as well as by its conformation. Antigens may be introduced into an animal by ingestion, inhalation, sometimes by contact with skin, or more regularly by injection into the bloodstream, skin, peritoneum, or other body part.

With a few exceptions, such as the autoantigens and the isoantigens of the blood groups, antigens produce antibody only in species other than the ones from which they are derived. All complete proteins are antigenic, as are many bacterial and other polysaccharides, some nucleic acids, and some lipids. Antigenicity may be modified or abolished by chemical treatments, including degradation or enzymatic digestion; it may be notably increased by the incorporation of antigen into oils or other adjuvants. See Isoantigen

Bacteria, viruses, protozoans, and other microorganisms are important sources of antigens. These may be proteins or polysaccharides derived from the outer surfaces of the cell (capsular antigens), from the cell interior (the somatic or O antigens), or from the flagella (the flagellar or H antigens). Other antigens either are excreted by the cell or are released into the medium during cell death and disruption; these include many enzymes and toxins, of which diphtheria, tetanus, and botulinus toxins are important examples. The presence of antibody to one of these constituent antigens in human or animal sera is presumptive evidence of past or present contact with specific microorganisms, and this finds application in clinical diagnosis and epidemiological surveys. See Botulism, Diphtheria, Toxin

Microbial antigens prepared to induce protective antibodies are termed vaccines. They may consist of either attenuated living or killed whole cells, or extracts of these. Since whole microorganisms are complex structures, vaccines may contain 10 or more distinct antigens, of which generally not more than one or two engender a protective antibody. Examples of these are smallpox vaccine, a living attenuated virus; typhoid vaccine, killed bacterial cells; and diphtheria toxoid, detoxified culture fluid. Several independent vaccines may be mixed to give a combined vaccine, and thus reduce the number of injections necessary for immunization, but such mixing can result in a lesser response to each component of the mixture. See Vaccination

Allergens are antigens that induce allergic states in humans or animals. Examples are preparations from poison ivy, cottonseed, or horse dander, or simple chemicals such as formaldehyde or picryl chloride. See Hypersensitivity, Immunology


A substance which reacts with the products of specific humoral or cellular immunity, even those induced by related heterologous immunogens.


a substance that stimulates the production of antibodies
References in periodicals archive ?
Immunohistochemical localization of human immunodeiciency virus p24 antigen in placental tissue.
Initially their donations tested negative for HIV antibody and HIV p24 antigens, but subsequently became positive in follow-up tests.
After the last wash, supernatant was tested for p24 antigen (0 PID--post infection day).
Controversy has swirled around the FDA's decision because the federal agency in June rejected its own advisory committee's recommendation not to use the p24 antigen test for screening blood.
Second generation tests can detect IgG antibodies, third generation can detect both IgM and IgG antibodies, and fourth generation can detect both HIV antibodies and the p24 antigen.
Alere Determine HIV 1-2 Ag/Ab Combo was FDA-approved in August 2013 as the first fourth-generation, rapid point-of-care test that detects both HIV-1/2 antibodies and free HIV-1 p24 antigen.
Unfortunately, current p24 antigen assays cannot detect p24 antigen in the early stages of HIV infection (12).
The alternative would be a p24 antigen test in the case of indeterminate results.
If this does not confirm the result of the first test, it will be followed with a P24 antigen test to retest the antigen component.
Primary human immunodeficiency virus type 1 (HIV-1) infection is diagnosed through detection of a high plasma viral RNA level or the presence of plasma p24 antigen accompanied by a negative HIV-1 antibody test.
The most common tests used to diagnose PHI are the quantitative HIV RNA (viral load), which uses polymerase chain reaction to detect the presence of HIV, and the p24 antigen test, which detects a viral protein and indicates ongoing viral replication.
Within two weeks, levels of the p24 antigen -- a gauge of AIDS virus replication -- dropped significantly in patients receiving high or low doses of the drug, while CD4 lymphocytes -- infection-fighting white blood cells usually ravaged by the AIDS virus -- increased significantly.