Antigen: From 5 Different Sources
A substance that can trigger an immune response, resulting in production of an antibody as part of the body’s defence against infection and disease. Many antigens are foreign proteins (not found naturally in the body) such as parts of microorganisms and toxins or tissues from another person that have been used in organ transplants. Sometimes, harmless substances (pollen, for example) are misidentified by the immune system as potentially harmful antigens, which results in an allergic response (see allergy).
Whenever the body identi?es a substance entering it as foreign or potentially dangerous, the immune system (see IMMUNITY) produces an an antibody (see ANTIBODIES) to combat it. Antigens are normally proteins, but simple substances – for instance, metals – may become antigenic by combining with and changing the body’s own proteins. Such a product is called a hapten.
A substance (often a protein or carbohydrate on the surface of an infectious agent) foreign to the body that stimulates the formation of specific antibodies to combat its presence. Any protein (including toxins) encountered that may cause the body to produce antibodies against it.
A substance, usually a protein, that induces the formation of defending antibodies. Example: bacterial toxins, Juniper pollen (in allergies). Auto-immune disorders can occur when antibodies are formed against normal proteins created within the body.
n. any substance that may be specifically bound by an *antibody molecule. In order to generate antibodies specific for small molecules, the latter are attached to a larger molecule before immunization. The small molecule is called a hapten; the larger molecule is called a carrier. —antigenic adj.
See HLA SYSTEM.... human leucocyte antigen (hla)
An ENZYME produced by glandular tissue in the PROSTATE GLAND. When the gland enlarges (see PROSTATE, DISEASES OF), greater amounts of PSA are secreted, raising the concentration of the enzyme in the blood. This is especially so in cancer of the prostate, and testing the level of PSA is an indicator that the disease may be present. There is much controversy about the use of PSA as a screening test. Its proponents claim that its use reduces deaths from prostate cancer; its opponents suggest that it does this only by bringing to light many cases that needed no treatment and would not have caused death in any case. Further, if the level of PSA is very high, the disease is already advanced; where the result is equivocal it is uncertain whether the bene?ts of treatment outweigh the risks.... prostate specific antigen (psa)
Substances, usually harmful, that when entering the body stimulate the immune system to produce antibodies. Invaders may be polio virus, typhoid bacillus, or chain-like streptococcal bacteria – the usual pus-forming type found in infected wounds, or toxins they release into the blood stream. ... antigens
(HLA) A type of protein belonging to the group known as histocompatibility antigens, which play a role in the immune system.... human leukocyte antigen
(APC) a cell, such as a *dendritic cell or a *macrophage, that processes antigen for presentation to a T lymphocytes (see helper T cell).... antigen-presenting cell
another name for the *hepatitis B antigen, which was first discovered in the blood of an Australian aborigine. This disease is caused by a virus of which the Australia antigen forms part.... australia antigen
(CEA) a protein produced in the fetus but not in normal adult life. It may be produced by carcinomas, particularly of the colon, and is a rather insensitive marker of malignancy. It is an example of an *oncofetal antigen that is used as a *tumour marker, particularly in the follow-up of colorectal cancer.... carcino-embryonic antigen
see HLA system.... human leucocyte antigen system
a group of antigens that may or may not be present on the surface of red blood cells, forming the basis of a *blood group. This group is important in blood transfusion reactions. [Mrs Kell (20th century), patient in whom they were first demonstrated]... kell antigens
a protein normally produced only by fetal tissue but often produced by certain tumours. An example is *carcino-embryonic antigen (CEA), which has been used as a *tumour marker, especially in colorectal carcinomas.... oncofetal antigen
a protein produced by cancer cells. Its presence in the blood can be revealed by means of a simple blood test, aiding the diagnosis of malignant melanoma and other cancers at their earliest – and most treatable – stages of development.... tumour-associated antigen
(PSA) a protease enzyme produced by the glandular epithelium of the prostate. Its effect is to liquefy the semen within the ejaculate. Overall PSA has a half-life of 2–3 days. Increased quantities are secreted when the gland becomes enlarged or inflamed, and levels of PSA in the blood are significantly elevated in cancer of the prostate. Although there is no clear ‘cut-off’ level for normality, over 4 ng/ml in the blood is associated with a 20% risk of prostate cancer, even in patients with normal-feeling prostates on rectal examination. Age-specific PSA reference ranges are often used. Newer PSA assays can measure free PSA and compare it to the total PSA in the blood. Low free:total PSA ratios indicate a greater risk of prostate cancer and improve the discrimination between cancer and benign disease in men with a PSA in the range 4–10 ng/ml. PSA levels tend to be much higher in advanced prostate cancer and the rate of fall on treatment (e.g. after radical prostatectomy or radiotherapy) is a good prognostic indicator of response.... prostate specific antigen
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