[anti- + -gen]
Any substance capable of eliciting an immune response or of binding with an antibody. Cellular antigens are proteins or oligosaccharides that mark and identify the cell surface as self or nonself. Cell surface antigens can stimulate the production of antibodies by B lymphocytes and cytotoxic responses by white blood cells, e.g., granulocytes, monocytes, and lymphocytes.
Antigens on the body's own cells are called autoantigens. Antigens on all other cells are called foreign antigens. Matching certain types of tissue antigens is important for the success of an organ transplant. Inflammation occurs when neutrophils, monocytes, and macrophages encounter an antigen from any source during bodily injury. The antigen may be foreign or an autoantigen that has been damaged and therefore appears to be foreign. Reactions to antigens by T and B cells are part of the specific immune response.
SEE: autoantigen; SEE: cytokine; SEE: histocompatibility locus antigen

antigenic (ant″i-jen′ik), adj.
antigenically (ant″i-jen′ĭ-k(ă-)lē), adv.
antigenicity (ant″i-jĕ-nis′it-ē), n.

allogeneic antigen

An antigen that occurs in some individuals of the same species. Examples are the human blood group antigens.

alpha-fetoprotein antigen

SEE: alpha-fetoprotein.

Australia antigen

Australian antigen A term formerly used for hepatitis B surface antigen.

bladder tumor antigen

ABBR: BTA A protein released into the urine by malignant cells in the bladder, studied as a possible marker of cancer of the urinary bladder. Because of the low prevalence of bladder cancer and the low positive predictive value of the test, the test is not currently recommended for use.

cancer antigen

ABBR: CA A protein or carbohydrate that is either expressed by cancerous cells but not by healthy cells or is expressed by cancerous cells in much greater concentrations than by healthy cells. Cancer antigens are used in clinical medicine to screen body fluids for tumors or to follow the response of tumors to treatment. Since they stimulate the immune response, they are also used in the manufacture of antitumor vaccines. SEE TABLE: Cancer Antigens Used as Tumor Markers
Cancer Antigens Used as Tumor Markers

Antigen Name or DesignationAbbreviationThe Tumor It Detects or Monitors
Alpha-fetoproteinAFPNonseminomatous germ cell tumor
CA 15-3Breast cancer
CA 19-9Pancreatic cancer
CA 27-29Breast cancer
CA 50Gastrointestinal tract tumors
CA 125Ovarian/peritoneal cancer
CalcitoninMedullary cancer of the thyroid gland
Carcinoembryonic antigenCEAGastrointestinal tract tumors and tumors of solid internal organs
CD 20Non-Hodgkin lymphoma
Cytokeratin fragment 21-1Lung cancer
Estrogen receptorERBreast cancer
HE4Ovarian cancer
Human chorionic gonadotropinHCGNonseminomatous germ cell tumors; choriocarcinoma
Human epidermal growth factor receptor 2HER2/neuBreast and gastric cancers; adenocarcinoma of the gastroesophageal junction
ImmunoglobulinsMultiple myeloma; Waldenstrom macroglobulinemia
Microglobulin-beta 2 subunitb2-MMultiple myeloma
Neuron-specific enolaseNSEBroad variety of cancers, including small-cell carcinoma of lung
NY-BR-40 and othersBreast cancer
Prostate specific antigenPSAProstate cancer
ThyroglobulinThyroid cancer
Urinary tumor associated antigenUTAAMelanoma

carcinoembryonic antigen

ABBR: CEA A molecular marker found on normal fetal cells and in the bloodstream of patients with cancers of the colon, breast, lung, and other organs. Assays for CEA are used both to monitor the effectiveness of treatments for cancer and to provide prognostic information to patients.

CD antigen

SEE: cluster of differentiation

class I antigen

Any of the major histocompatibility molecules present on almost all cells except human red blood cells. These antigens are important in the rejection of grafts and transplanted organs.

class II antigen

Any of the major histocompatibility molecules present on immunocompetent cells.

cross-reacting antigen

An antigen having the ability to react with more than one specific antibody.

D antigen

The protein marker in the Rh group of antigens that stimulates the greatest immune response.
SEE: Rh blood group.

glutamate dehydrogenase antigen

ABBR: GDH antigen An antigen found in the cell wall of Clostridium difficile a common cause of infectious diarrhea. It is released by the bacterium into stool. Its presence in stool is an indication of active infection.

H antigen

A flagellar protein present on the surface of some enteric bacilli such as Escherichia coli. The antigen is important in classifying these bacilli.

hepatitis antigen

The original term for the Australian antigen, now called hepatitis B surface antigen (HBsAg). Its discovery made possible the differentiation of hepatitis B from other forms of viral hepatitis.

hepatitis B core antigen

ABBR: HBcAg A protein marker found on the core of the hepatitis B virus (HBV). HBV antigen does not circulate in the blood but is found only in liver cells infected by HBV. HBcAg stimulates the production of a protective antibody, immunoglobulin M (IgM-anti-HBc), which appears in the blood shortly before the onset of symptoms. Tests for this antibody are used with other blood tests in the diagnosis of acute and chronic hepatitis B infection. During the convalescent stage of hepatitis B infection, IgM anti-HBc is replaced by another antibody, IgG anti-HBc, which remains in the blood for years.
SEE: hepatitis B e antigen; SEE: hepatitis B surface antigen

hepatitis B e antigen

ABBR: HBeAg A polypeptide from the hepatitis B viral core that circulates in the blood of infected people and indicates that the patient is highly infectious. It is released when viral DNA is actively replicating.

hepatitis B surface antigen

ABBR: HBsAg The glycoprotein found on the surface of the hepatitis B viral envelope. It is the first marker of infection with the hepatitis B virus. If HBsAg is still found in blood samples 6 months after infection with the virus, chronic and potentially contagious infection with hepatitis B is present.
SEE: hepatitis B core antigen; SEE: hepatitis B e antigen

hepatitis C core antigen

A protein released by the hepatitis C virus (HCV) into the bloodstream of infected patients. Because hepatitis C core antigen is detectable in the blood before HCV antibodies are produced, it can be used as a marker of early infection, e.g., in donated blood or plasma. It can also measure the response of HCV infection to treatment protocols; antigen levels drop with effective treatment.

high-frequency antigen

In transfusion medicine, any red blood cell surface antigen that does not belong in any known blood group system but can cause antibody-mediated transfusion reactions, lung injury, or hemolytic disease in the newborn.

histo-blood group antigen

Any of the major antigens on blood cells implicated in transfusion reactions.

histocompatibility locus antigen

ABBR: HLA Any of the multiple antigens present on all nucleated cells in the body that identify the cells as self. Immune cells compare these antigens to foreign antigens, which do not match the self and therefore trigger an immune response. These markers determine the compatibility of tissue for transplantation.
They are derived from genes at seven sites (loci) on chromosome 6, in an area called the major histocompatibility complex (MHC); each histocompatibility antigen is divided into one of two MHC classes.

In humans, the proteins created in the MHC are called human leukocyte antigens (HLA) because these markers were originally found on lymphocytes. Each gene in the MHC has several forms or alleles. Therefore, the number of different histocompatibility antigens is very large, necessitating the identification and matching of HLAs in donors and recipients involved in tissue and organ transplantation. (The identification of HLAs is called tissue typing.)

The identification of HLA sites on chromosome 6 has enabled researchers to correlate the presence of specific histocompatibility and certain autoimmune diseases, e.g., insulin-dependent diabetes mellitus, multiple sclerosis, some forms of myasthenia gravis, rheumatoid arthritis, and ankylosing spondylitis.

SYN: SEE: human leukocyte antigen
SEE: major histocompatibility complex

human leukocyte antigen

SEE: Histocompatibility locus antigen

H-Y antigen

A histocompatibility antigen located on the cell membrane. It has a primary role in determining the sexual differentiation of the male embryo.

K antigen

A capsular antigen present on the surface of some enteric bacilli. The antigen is important in classifying these bacilli.

leukocyte common antigen

A protein found in the cell membranes of various white blood cells, including basophils, granulocytes, lymphocytes, macrophages, plasma cells, and thymocytes, among others.

lymphogranuloma venereum antigen

An antigen used in a skin test for lymphogranuloma venereum.

mumps skin test antigen

A standardized suspension of sterile formaldehyde-inactivated mumps virus. It is used in diagnosing mumps.

nuclear antigen

An antigen present in the cells of patients with certain types of connective tissue disorders. Corticosteroids can be very helpful in treating patients with high concentrations of nuclear antigen.

O antigen

A surface antigen of some enteric bacilli. The antigen is important in classifying these bacilli.

oncofetal antigen

An antigen that is normally expressed in the fetus and may reappear in the adult in association with certain tumors. Examples include alpha-fetoprotein and carcinoembryonic antigens.
SYN: SEE: oncofetal protein

onconeural antigen

An antigen found on the surface of cancer cells that closely resembles antigens found on nerve cells. Antibodies formed by immune cells against onconeural antigens cause paraneoplastic syndromes.

p24 antigen

The core protein of HIV. The presence of p24 antigen in the blood is a marker of uncontrolled HIV replication. p24 antigenemia is encountered in the acute retroviral syndrome before host immune response and in advanced AIDS when the immune system has been destroyed. When p24 antigen is detected in the blood, the HIV viral load is high and the person is highly infectious.

proliferating cell nuclear antigen

ABBR: PCNA A protein complex released by cells actively synthesizing DNA. In the blood, PCNAs can be used as markers of disease activity in autoimmune and inflammatory illnesses, malignancies, and other conditions marked by rapid cell replication.

prostate-specific antigen

ABBR: PSA A nonspecific marker of abnormalities in the prostate gland, including prostatic infection, inflammation, and prostate cancer. PSA circulates in the blood and can be detected by blood tests. PSA levels have been used to screen for prostate cancer, but are neither sensitive nor specific for detecting the disease.
Descriptive text is not available for this imageElevations in PSA levels may prompt invasive testing (such as with biopsies) that may detect indolent cancers as well as aggressive ones. Side effects of prostate biopsy include urinary incontinence and erectile dysfunction.

SEE: prostate cancer.

protective antigen

The protein made by Bacillus anthracis, which binds to cell membranes and allows the lethal components of anthrax toxin to enter and kill cells.

soluble antigen

An antigen dissolved in a liquid. A soluble antigen is recognized by B lymphocytes but cannot be detected by T lymphocytes until it has been processed by an antigen-presenting cell.
SEE: T cell.

soluble liver antigen

A liver antigen detectable in the serum of some patients with autoimmune hepatitis but not usually in patients with chronic viral hepatitis or hepatitis caused by exposure to toxic drugs.

squamous cell carcinoma antigen

ABBR: SCCAg A tumor marker found in many patients with cancers of the breast, cervix, head and neck, liver, lung, and skin. Tests to detect SCCAg help determine the prognosis and management of these malignancies.

T-dependent antigen

An antigen that can stimulate an antibody response only in the presence of helper T cells.

thymus-dependent antigen

Any of the foreign antigens that require B lymphocyte stimulation by T cells before production of antibodies and memory cells can occur.

thymus-independent antigen

Any of the foreign antigens capable of stimulating B cell activation and the production of antibodies without T cell interaction. Most of these antibodies fall into the IgM class. A few memory cells are created.

T-independent antigen

ABBR: TI Either of two types of antigens that stimulate B-cell production of antibodies without the presence of T cells. TI-1 antigens, e.g., lipopolysaccharides from gram-negative organisms, stimulate production of both specific (monoclonal) and nonspecific (polyclonal) antibodies and promote the release of cytokines from macrophages that enhance the immune response. TI-2 antigens, which result in monoclonal antibody production, may require the presence of cytokines.
SEE: B cell; SEE: T cell

transplantation antigen

The commonly used term for any of the histocompatibility antigens that cause the immune system of one individual to reject transplanted tissue.

tumor-specific antigen

An antigen produced by certain tumors. It appears on the tumor cells but not on normal cells derived from the same tissue.