any substance capable, under appropriate conditions, of inducing a
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 the portion of the protein or polysaccharide molecule known as the
determinant (q.v.) combines with antibody or a specific receptor on
a lymphocyte. Abbreviated Ag.
allogeneic antigen, one occurring in some but not all
individuals of the same species, e.g., histocompatibility antigens and
human blood group antigens.isoantigen.
Am antigens, see under
Au antigen, Australia antigen, former name for
blood group antigens, the antigens responsible for specificities
of blood groups;
those of the ABO and Lewis blood groups were the first to be
characterized. They are formed by sequential addition of monosaccharide
moieties to any of several different types of precursor substances;
addition of one moiety produces the Lewis antigen, addition of a second
produces the H
antigen, and addition of a third produces either the A or the B
antigen. Secreted blood group antigens (in individuals with the secretor
phenotype) are glycoproteins, and red cell antigens are
glycosphingolipids; the oligosaccharide chains determining blood group
specificity are the same in both.
cancer antigen 125, a surface glycoprotein associated with
müllerian epithelial tissue; elevated serum levels are often associated
with epithelial ovarian carcinomas, particularly with nonmucinous
tumors, but are also seen in some other malignant and various benign
pelvic disorders. See also
125 (CA 125) assay, under assay.
carcinoembryonic antigen, a glycoprotein secreted into the
glycocalyx coating the luminal surface of gastrointestinal epithelia.
Originally thought to be a specific antigen of the fetal digestive tract
and adenocarcinoma of the colon, CEA is now known to occur normally in
feces and pancreaticobiliary secretions and to appear in the plasma in a
diverse group of neoplastic and non-neoplastic conditions, including
cancers of the colon, pancreas, stomach, lung, and breast, alcoholic
cirrhosis and pancreatitis, inflammatory bowel disease, rectal polyps,
and cigarette smoking. The primary use of CEA is in monitoring response
to treatment of colorectal cancer.
CD antigen, any of a number of cell surface markers expressed by
leukocytes and used to distinguish cell lineages, developmental stages,
and functional subsets; such markers can be identified by specific
monoclonal antibodies and are numbered CD1, CD2, CD3, etc. Markers used
to identify T lymphocyte subsets were formerly called
See also CD
system, under system.
class I antigens,
histocompatibility antigens found on virtually every cell, human
erythrocytes being the only notable exception; they are found on
molecules consisting of two noncovalently bound chains. One, a 44-kD
polymorphic glycoprotein partially embedded in the cell membrane, is
determined by an MHC gene (HLA-A, -B, -C, -E, -F, and -G in humans); the
other, β2-microglobulin, a 12-kD nonpolymorphic protein, is
determined by a non-MHC gene. The classical class I antigens (HLA-A, -B,
and -C, called also class Ia) are recognized during graft rejection and
are also the antigens involved in
(q.v.). The nonclassical antigens (HLA-E, -F, and -G, called also class
Ib) have a different tissue distribution than do the classical antigens;
their functions are under investigation.
class II antigens, major histocompatibility antigens found only
on immunocompetent cells, primarily B lymphocytes and macrophages; they
are found on molecules consisting of two noncovalently bound chains, the
34,000-dalton α chain and 29,000-dalton β chain, both glycoproteins
partially embedded in the cell membrane and both determined by MHC
genes. The human HLA-D, -DR, -DP, and -DQ loci are all associated with
antigenic determinants on class II antigen molecules.
class III antigens, a term used to refer to nonhistocompatibility
antigens mapping in the major histocompatibility complex, e.g., the
complement components C2, C4, factor B.
common antigen, an antigenic determinant group (epitope) that is
present in two or more different antigen molecules and frequently leads
to cross-reactions among them.
common acute lymphoblastic leukemia antigen, a tumor-associated
antigen, CD10, occurring on lymphoblasts in about 80 per cent of
patients with acute lymphoblastic leukemia (ALL) and also in 40–50 per
cent of patients with blastic phase chronic myelogenous leukemia (CML).
It does not occur on normal lymphoid cells except during fetal
common leukocyte antigens,
complete antigen, an antigen which both stimulates the immune
response and reacts with the products (e.g., antibody) of that response.
conjugated antigen, antigen produced by coupling a hapten to a
protein carrier molecule through covalent bonds; when it induces
immunization, the resultant immune response is directed against both the
hapten and the carrier.
cross-reacting antigen, one that combines with antibody produced
in response to a different but related antigen, owing to similarity of
determinants.identical antigens in two bacterial strains, so that
antibody produced against one strain will react with the other.
D antigen, one of the
an antigen of the
Rh blood group, important in the development of isoimmunization in
Rh-negative persons exposed to the blood of Rh-positive persons.
delta antigen, a 32- to 37-nm RNA particle coated with hepatitis
B surface antigen.
E antigen, one of the
an antigen of the
Rh blood group system.
epithelial membrane antigen, a protein specific to the epithelial
membrane; used as an immunohistochemical marker for epithelium.
extractable nuclear antigens, ENA; protein antigens, not
containing DNA, that are extractable from cell nuclei in
phosphate-buffered saline; anti-ENA antibodies are a component of the
antinuclear antibodies occurring in systemic lupus erythematosus and
other connective tissue diseases.
febrile antigens, a standard panel of serologic antigens (Salmonella,
used in screening patients with unexplained fever.
H a. (def.
Forssman antigen, a
antigen that induces production of antisheep
found in a variety of different animals, mainly in organs and not in
erythrocytes (guinea pig, horse); sometimes it is found only in
erythrocytes (sheep), and occasionally in both organs and erythrocytes
(chicken). In the original and strict sense, the antigen is typified by
that found in the guinea pig kidney, characterized by heat stability and
solubility in alcohol; the antigenic determinant is polysaccharide in
nature. Its antibody is absorbed by tissues containing the antigen and
contains no lysin for bovine cells and little or no agglutinin for sheep
cells. The term has been broadened to refer loosely to any antigen
hemolysin, although antibodies to them may not be identical, as they
are in the case of the true Forssman antigen.
Gm antigens, see under
Goodpasture antigen, a type of
H antigen, the precursor of the A and B blood group antigens.
Normal type O individuals lack enzymes to convert H antigen to A or B
antigens. Those individuals having the rare
lack the ability to make H antigen and thus are phenotypically type O
whether or not they possess A or B genes. Called also
of the bacterial flagellar antigens important in the serological
classification of enteric bacilli, especially
Cf. O a.
H-2 antigens, the major histocompatibility antigens in mice.
hepatitis antigen, hepatitis-associated antigen, former name for
hepatitis B core antigen, a core protein antigen of the hepatitis
B virus present inside complete virions (Dane particles) and in the
nuclei of infected hepatocytes, indicating the presence of replicating
hepatitis B virus; the antigen is not present in the blood of infected
individuals, but anti-HBc antibodies appear during the acute infection;
they do not protect against reinfection.
hepatitis B e antigen, an antigen of
hepatitis B virus
sometimes present in the blood during acute infection, usually
disappearing afterward but sometimes persisting in chronic disease.
Anti-HBe antibodies appear transiently during convalescence and do not
protect against reinfection.
hepatitis B surface antigen, a coat protein antigen of the
hepatitis B virus present on complete virions (Dane particles) and
smaller spherical and filamentous particles circulating in the blood of
individuals with active or chronic infections, being first detectable
several weeks prior to clinical disease and peaking with the appearance
of symptoms. Anti-HBs antibodies appear in the blood in late
convalescence and are protective against reinfection. Originally called
because it was first found in an Australian aborigine; also formerly
hepatitis-associated a. (HAA)
hepatitis (SH) a. See
vaccine inactivated and
vaccine (recombinant), under vaccine.
heterologous antigen, an antigen that reacts with an antibody
that is not the one (the homologous antigen) that induced its formation.
heterophil antigen, heterophile antigen, any of a group of
antigens occurring in several species and having a species
distribution that does not correspond to phylogenetic relationships,
such as the
Forssman antigen. Called also
high frequency antigens, high incidence antigens,
histocompatibility antigens, systems of allelic alloantigens that
can stimulate an immune response that leads to transplant rejection when
the donor and recipient are mismatched. Called also
human leukocyte a's.
histocompatibility antigens, major, those in the major
histocompatibility complex; HLA antigens in humans and H-2 antigens in
histocompatibility antigens, minor, systems of allelic
alloantigens that can cause transplant rejection, but with a long delay
(up to 100 days); about 15–30 such systems have been found in mice.
homologous antigen, the antigen that induces the formation of an
Hu antigen, a family of four RNA-binding proteins (HuD, HuC/ple21,
Hel-N1, and Hel-N2) that are expressed in neurons and are believed to
play an important role in the development and maintenance of the nervous
system; they are also expressed in the cells of
small cell lung
neuroblastoma, and antibodies to them are associated with neurologic
human leukocyte antigens, histocompatibility antigens governed by
genes of the HLA complex (the human major histocompatibility complex), a
region on the short arm of chromosome 6 containing several genetic loci,
each having multiple alleles. Loci are designated by letters; the
classical loci are HLA-A, -B, -C, -E, -F, -G, -DP, -DQ, and -DR (there
are at least three subloci in the D region). Alleles at each locus are
designated by numbers, e.g., HLA-A1, provisional designations being
indicated by “w” (for “workshop”), e.g., HLA-DRw10. The A, B, C, and DR
antigens are defined and typed by serologic reactions. The D antigens
are defined and typed by one-way mixed lymphocyte culture (MLC) using
panels of HLA-D-homozygous typing cells. The SB (for “secondary B cell”)
antigens are defined and typed by primed lymphocyte typing. See
class II, and
class III a's.
H-Y antigen, a minor histocompatibility antigen present in all
tissues of normal males and coded for by a structural gene on the short
arm of the Y chromosome; it is thought to promote the differentiation of
indifferent gonads into testes, thus determining male sex.
I antigen, see
syndrome, under syndrome.
i antigen, see
syndrome, under syndrome.
Ia antigens, class II histocompatibility antigens found on the
surface of mouse B cells, macrophages, and accessory cells. They are
also found on granulocyte precursors but disappear during maturation. Ia
antigens are governed by the Ia genes of the
Inv group antigens, see
K antigen, a surface antigen found on the capsule of bacteria,
external to the cell wall, such as the
antigen or some found on pneumococci. Called also
Km antigens, see under
Kveim antigen, a saline suspension of human sarcoid tissue
prepared from the spleen or lymph nodes of a patient with active
leukocyte common antigens, a group of glycoproteins,
antigenically similar but of different molecular weights, found on B
cells, T cells, thymocytes, and leukopoietic cells. Called also
leukocyte function–associated antigen 1, a β2 integrin
expressed on most lymphocytes, granulocytes, and monocytes that mediates
leukocyte adhesion; it also plays a role in antibody-dependent cellular
leukocyte function–associated antigen 2, a cell membrane
glycoprotein, perhaps related to the immunoglobulins, expressed on
thymocytes and NK cells that mediates leukocyte adhesion.
leukocyte function–associated antigen 3, a cell surface
glycoprotein expressed on a wide variety of cells that serves as a
ligand for LFA-2.
leu-M1 antigen, an antigen present on granulocytes and
Reed-Sternberg cells in Hodgkin disease, except in the
lymphocyte-predominant diffuse subtype.
low frequency antigens, low incidence antigens,
Ly antigens, cell-surface markers differentiating subpopulations
of murine T lymphocytes: Ly 1, Ly 2, and Ly 3. Most thymocytes and
undifferentiated peripheral T cells are Ly 1+2+3+;
helper cells are Ly 1+2−3−; cytotoxic T
cells and suppressor cells are Ly 1−2+3+.
Lyb antigens, cell-surface markers on murine B lymphocytes: Lyb
1,2,3,4, and 5. Lyb 1,2, and 4 are found on all B cells, Lyb 3 and 5 on
a subset of mature B cells.
lymphocyte-defined (LD) antigens, major histocompatibility
antigens defined and typed by the mixed lymphocyte reaction (MLR), e.g.,
mumps skin test antigen, preparation of killed
used in the mumps
skin test (q.v.).
nuclear antigens, the components of cell nuclei with which
antibodies (q.v.) react.
O antigen, the lipopolysaccharide-protein somatic antigens of
gram-negative bacteria, important in the serological classification of
enteric bacilli. See
oncofetal antigen, an antigenic gene product that is expressed
during fetal development, partially or completely repressed in adult
tissues, and derepressed in some tissues that have undergone neoplastic
transformation; oncofetal antigens, e.g., alpha-fetoprotein,
carcinoembryonic antigen, and pancreatic oncofetal antigen, are thus
useful tumor markers.
organ-specific 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. Called also
Oz antigen, an antigenic marker on the lambda chain of human
immunoglobulins, equivalent to Km allotypes on kappa light chains.
Together with Kern markers, they delineate three types of human lambda
pancreatic oncofetal antigen, a glycoprotein, mol. wt. 800,000,
found in fetal and neoplastic pancreatic tissue but not in that of
normal adults; it also occurs at lower levels in the serum of patients
with cancer at other sites and some normal adults.
pan–T-cell antigen, one present on several different types of
Pl(A1) antigen, the most commonly expressed antigen of platelets;
patients not expressing this isoantigen are at risk for
transfusion-induced hematologic disorders of platelets, such as
platelet antigen, any of several isoantigens expressed by
pollen antigen, see under
Pr antigen, see
syndrome, under syndrome.
antigens that occur in only a few kindreds (low
frequency blood groups). Called also
low frequency a's.HLA
antigens found only on the gene product of a single allele.a tumor
antigen expressed only on a particular type of chemically induced tumor.
Cf. public a's.
proliferating cell nuclear antigen, a 36-kD trimeric nuclear
acidic protein that acts as an auxiliary factor in
and repair; its levels in the body correlate with the rates of DNA
synthesis and it is commonly used as a marker for proliferating cells.
prostate-specific antigen, a serine endopeptidase secreted by the
epithelial cells of the
serum levels are elevated in
hyperplasia and prostate cancer. Measurement of PSA serum levels is
used as a screening test for prostate cancer.
prostate-specific membrane antigen, a substance often expressed
by the most aggressive clones of prostate cancer cells; monoclonal
antibody tests for PSMA appear to be more sensitive than those using
prostate-specific antigen alone in finding circulating prostate cancer
cells and may be useful in identifying patients with a high risk of
antigens that occur in the general population at high frequencies;
frequency blood group, under blood group. Called also
a's.HLA antigens occurring on the products of several allelic genes.
Cf. private a's.
recall antigen, an antigen to which an individual has previously
been sensitized and which is subsequently administered as a challenging
dose to elicit a hypersensitivity reaction.
Rh antigen, see under
RNP antigen, one of the extractable nuclear antigens.
SD antigens, serologically defined a's.
sequestered antigens, the cellular constituents of tissue (e.g.,
lens of the eye) sequestered anatomically from the lymphoreticular
system during embryonic development and thus thought not to be
recognized as “self.” Should such tissue be exposed to the
lymphoreticular system during adult life, an autoimmune response would
sero-defined (SD) antigens, serologically defined (SD) antigens,
major histocompatibility antigens defined by serologic reactions, e.g.,
HLA-A, HLA-B, and HLA-C antigens.
serum hepatitis antigen, SH antigen, former name for
shock antigen, an antigen capable of eliciting anaphylactic shock
in a sensitized animal.
skin test antigen, the antigen used in a
(see under test).
Sm antigen, an uncharacterized nuclear antigen that is a
nonhistone acidic protein not complexed with DNA or RNA; anti-Sm
antibodies make up a part of the antinuclear antibodies in about
one-third of patients with systemic lupus erythematosus, but do not
occur in other connective tissue diseases, except mixed connective
somatic antigens, antigens, usually cell surface antigens, of the
body of a bacterial cell, in contrast to flagellar or capsular antigens.
See O a.
species-specific antigens, antigens restricted to a single
species and found in all members of the species.
SS-A antigen, a ribonucleoprotein extractable nuclear antigen;
anti–SS-A antibody, under antibody.
SS-B antigen, a ribonucleoprotein extractable nuclear antigen;
anti–SS-B antibody, under antibody.
T antigen, any of several antigens, coded for by the viral
genome, associated with transformation of infected cells by certain DNA
tumor viruses, such as SV 40. Called also
antigen present on human erythrocytes that is exposed by treatment with
neuraminidase or contact with certain bacteria. See
Thy 1 a.
Tac antigen, the receptor for interleukin 2.
T-dependent antigen, one that requires the presence of
helper T cells
to stimulate antibody production by
B cells; most
antigens are T-dependent.
theta antigen, Thy 1 antigen, a cell-surface marker occurring on
all murine T lymphocytes.
T-independent antigen, an antigen that can trigger
B cells to
produce antibodies without the participation of
T cells; most
are polymers with a simple repeating pattern and are B cell mitogens;
only IgM is produced and few
TL antigen, a differentiation antigen, first discovered on thymic
leukemia cells, that occurs on thymocytes but not peripheral T cells in
some strains of mice.
T a. (def.
tumor-associated antigen, one that is associated with tumor
cells; it may also be found under other conditions, as on normal cells
during fetal life (oncofetal
antigen) or on normal adult cells specific to certain organs (organ-specific
antigen), or may occur normally on many cells but at a lower level.
tumor rejection antigen,
tumor-specific antigen, any cell-surface antigen of a tumor that
does not occur on normal cells of the same origin.
tumor-specific transplantation antigen, any of the cell surface
histocompatibility antigens of a given tumor that evoke a specific
immune response on transplantation to a syngeneic host.
VDRL antigen, an alcohol solution containing 0.03 per cent
cardiolipin, 0.99 per cent cholesterol, and enough lecithin to produce
standard reactivity. See
very late activation (VLA) antigen,
Vi antigen, a K antigen of the bacteria causing
enterica subsp. enterica serovar Typhi), originally thought to be
responsible for virulence.
xenogeneic antigen, an antigen common to members of one species
but not to members of other species; called also