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an antigen that does not require T-helper cell activation for the host's B cells to be stimulated. Repeating polymers such as polysaccharides are examples of T-independent antigens.
an antigen that induces antibody (immunoglobulin M) production without direct cooperation from T cells.
any substance which is capable, under appropriate conditions, of inducing a specific immune response and of reacting with the products of that response; that is, with specific antibody or specifically sensitized T lymphocytes, or both. Antigens may be soluble substances, such as toxins and foreign proteins, or particulate, such as bacteria and tissue cells; however, only a small portion of the protein or polysaccharide molecule known as the antigenic determinant or epitope is recognized by the specific receptor on a lymphocyte. Similarly the antibody or effector lymphocyte produced by the response combines only with the one antigenic determinant. A bacterial cell or large protein will have many hundreds of antigenic determinants, some of which are more important than others in protective immunity. Abbreviated Ag.
one occurring in some but not all individuals of the same species, e.g. histocompatibility antigens and blood group antigens; formerly called isoantigen.
see antibody-antigen reaction.
blood group a's
present on the surface of erythrocytes which vary between individuals of the same species and are used as the basis for blood typing.
a link between antigen-specific receptors of two antibodies.
K, L and V antigens (below).
carcinoembryonic antigen (CEA)
see oncofetal antigen (below).
an antigenic determinant present in two or more different antigen molecules and the basis for cross-reactions among them.
an antigen which both stimulates the immune response and reacts with the products, e.g. antibody, of that response, cf. hapten.
1. one that combines with antibody produced in response to a different but related antigen, owing to similarity of antigenic determinants.
2. identical antigens in two bacterial strains, so that antibody produced against one strain will react with the other.
dog erythrocyte antigen (DEA)
the antigens found on dog erythrocytes and used to distinguish different blood groups in the species. See Table 7.
those found in pollens, fungi, house dust, foods and animal dander.
see antigenic determinant.
feline oncornavirus cell membrane antigen (FOCMA)
tumor-specific antigen present on the membrane of cells in cats infected with feline leukemia virus.
H antigen (below).
1. some components of flea saliva, as well as whole flea extracts, are antigenic and certain individuals may become hypersensitive to flea bites; the most common hypersensitivity in dogs.
2. extracts, usually of whole fleas, but sometimes of flea saliva, are used for intradermal skin testing and desensitization procedures.
heterophil antigen occurring in various unrelated species, mainly in the organs but not in the erythrocytes (guinea pig, horse), but sometimes only in the erythrocytes (sheep), and occasionally in both (chicken). Antibody to Forssman antigen is usually recognized by agglutination of sheep red blood cells.
group specific (gs) antigen
common to a certain group of organisms, e.g. streptococci, oncornaviruses.
[Ger.] Hauch (film) the antigen that occurs in the bacterial flagella.
see xenogeneic antigen (below).
heterophil antigen, heterogenetic antigen
one capable of stimulating the production of antibodies that react with tissues from other animals or even plants.
one not normally exposed to circulating lymphocytes, e.g. within central nervous tissue, testicular tissue and certain intracellular components, so they do not normally evoke an immune response.
see histocompatibility antigen.
a histocompatibility antigen of the cell membrane, determined by a locus on the Y chromosome; it is a mediator of testicular organization (hence, sexual differentiation) in the male.
histocompatibility antigens governed by the I region of the major histocompatibility complex (MHC), located principally on B lymphocytes, although T lymphocytes, skin and certain macrophages may also contain Ia antigens.
an antigen carried by an individual, or members of the same inbred strain, which is capable of eliciting an immune response in genetically different individuals of the same species, but not in individuals bearing it.
bacterial capsular antigens.
a capsular antigen of Escherichia coli.
antigenic cell-surface markers of subpopulations of T lymphocytes, classified as Ly 1, 2 and 3; they are associated with helper and suppressor activities of T lymphocytes.
lymphocyte-defined (LD) a's
class II antigens found in lymphocytes, macrophages, epidermal cells and sperm. Important in graft rejection.
a type-specific antigen that appears to be located primarily in the cell wall and is associated with virulence of Streptococcus pyogenes.
Marek's tumor-specific antigen (MATSA)
found on the surface of cells infected by Marek's disease herpesvirus.
an antigen prepared from dead, dried and triturated tubercle bacilli by means of acetone and methyl alcohol; used in serum tests for tuberculosis in humans.
the components of cell nuclei with which antinuclear antibodies react.
[Ger.] ohne Hauch (without film) the antigen that occurs in the cell wall of bacteria.
a gene product that is expressed during fetal development, but repressed in specialized tissues of the adult and that is also produced by certain cancers. In the neoplastic transformation, the cells dedifferentiate and these genes can be derepressed so that the embryonic antigens reappear. Examples are alpha-fetoprotein and carcinoembryonic antigen.
any antigen that occurs exclusively in a particular organ and serves to distinguish it from other organs. Two types of organ specificity have been proposed: (1) first-order or tissue specificity is attributed to the presence of an antigen characteristic of a particular organ in a single species; (2) second-order organ specificity is attributed to an antigen characteristic of the same organ in many, even unrelated species.
the essential polypeptides of the pollen of plants extracted with a suitable menstruum, used in diagnosis, prophylaxis and desensitization in hay fever.
the presentation of peptide derivatives of antigens on the surface of antigen presenting cells (APCs), which include macrophages, dendritic cells and B lymphocytes, in association with class II major histocompatibility complex (MHC) antigens as required for recognition by T lymphocytes. Also includes antigen presentation in association with MHC class I by cells that are targets for lysis by cytotoxic T lymphocytes.
cells (macrophages, Langerhans cells, dendritic cells, and B lymphocytes) that process and present antigen to T lymphocytes.
antigens of the low-frequency blood groups, so-called because they are found only in members of a single kindred.
an antigen to which an individual has previously been sensitized and which is subsequently administered as a challenging dose to elicit a hypersensitivity reaction.
immunoglobulin molecules on the cell membranes of B lymphocytes and a structurally related, but quite distinct molecule on the surface of T lymphocytes which recognize particular antigenic determinants of an antigen.
see recognition (2).
certain antigens, e.g. the lens of the eye and thyroid proteins, that are sequestered anatomically from the immune system during embryonic development and thus thought not to be recognized as 'self'. Should such antigens be exposed to the immune system during adult life, an autoimmune response would be elicited.
serologically defined (SD) antigen
class I antigen of the major histocompatibility complex, identifiable by the use of specific antisera.
chemically synthesized or produced by recombinant DNA technology, the synthesis of polymers, based on sequences found in microbial antigens, has been used in the production of vaccines.
the immune response of most antigens requires T helper (Th) lymphocytes; lymphokines produced by T lymphocytes determine the characteristics of antibodies produced, which may change during the immune response.
an antigen that requires T lymphocyte participation before an immune response can occur. Most antigens are of this type.
an antigen that elicits an antibody response without the participation of T lymphocytes. Usually large carbohydrate molecules with repeating epitopes are of this type.
tumor-specific antigen (TSA)
antigens found only in tumor cells.
V antigen, Vi antigen
an antigen contained in the capsule of a bacterium and thought to contribute to its virulence.
an antigen common to members of one species but not to members of other species; called also heterogeneic antigen.