Accessory cell:
Cell required for, but not actually mediating, a specific immune response.
Often used to describe antigen-presenting cells (APC; see below).
Affinity:
A measure of the binding constant of a single antigen combining site with a
monovalent antigenic determinant.
Agglutination:
The aggregation of particulate antigen by antibodies. Agglutination applies
to red blood cells as well as to bacteria and inert particles covered with antigen.
Allelic:
Relating to one of a series of two or more alternate forms of a gene that occupy the
same position or locus on a specific chromosome.
Allelic exclusion:
The ability of heterozygous lymphoid cells to produce only one allelic
form of antigen-specific receptor when they have the genetic endowment to produce both.
Genes other than those for the specific receptors are usually expressed codominantly.
Allergen:
An antigen responsible for producing allergic reactions by inducing IgE
formation.
Allergy:
A term covering immune reactions to non-pathogenic antigens, which lead to
inflammation and deleterious effects in the host.
Allogeneic:
Having a genetic dissimilarity within the same species.
Allograft:
A tissue transplant (graft) between two genetically nonidentical members of a
species.
Allotypes:
Antigenic determinants that are present in allelic (alternate) forms. When used
in association with immunoglobulin, allotypes describe allelic variants of
immunoglobulins detected by antibodies raised between members of the same species.
Alternate (Alternative)
pathway: The mechanism of complement activation that does not
involve activation of the C1, C4, C2 pathway by antigen- antibody complexes, and begins
with the activation of C3.
Anaphylatoxin:
Substance capable of releasing histamine from mast cells.
Anaphylaxis:
Immediate hypersensitivity response to antigenic challenge, mediated by IgE
and mast cells. It is a life-threatening allergic reaction, caused by the release of
pharmacologically active agents.
Antibody:
Serum protein formed in response to immunization; antibodies are generally
defined in terms of their specific binding to the immunizing antigen.
Antibody-dependent,
cell-mediated cytotoxicity (ADCC): A phenomenon in which
target cells, coated with antibody, are destroyed by specialized killer cells (NK cells and
macrophages), which bear receptors for the Fc portion of the coating antibody (Fc
receptors). These receptors allow the killer cells to bind to the anti-body-coated target.
Antigen:
Any foreign material that is specifically bound by specific antibody or specific
lymphocytes; also used loosely to describe materials used for immunization. Antigens may
also be immunogens if they are able to trigger an immune response, or haptens if not.
Antigen-binding
site: The part of an immunoglobulin molecule that binds antigen
specifically.
Antigen-presenting
cell (APC): A specialized type of cell, bearing cell surface class II
MHC (major histocompatibility complex) molecules, involved in processing and
presentation of antigen to inducer, or helper , T cells. Examples: macrophage, dendritic
cells.
Antigen receptor:
The specific antigen-binding receptor on T or B lymphocytes; these
receptors are transcribed and translated from rearrangements of V genes.
Antigenic determinant:
A single antigenic site or epitope on a complex antigenic
molecule or particle.
Antigen processing:
Large molecules are broken down (processed) within macrophages
into peptides and presented within the groove of MHC molecules.
Atopy: A
term used by allergists to describe IgE-mediated anaphylactic responses in
humans, usually genetically determined.
Autograft:
A tissue transplant from one area to another on a single individual.
Autoimmunity
(autoallergy): An immune response to "self" tissues or components. Such
an immune response may have pathological consequences leading to autoimmune diseases.
Avidity:
The summation of multiple affinities, for example when a polyvalent antibody
binds to a polyvalent antigen.
B lymphocyte
(B cell): The precursors of antibody-forming plasma cells; these cells
carry immunoglobulin and class II MHC (major histocompatibility complex) antigens on
their surfaces.
Basophil:
A polymorphonuclear leukocyte., whose basophils granules contain heparin,
histamine and other vasoactive amines. Within tissues, these cells are known as mast cells
q.v.
Bence-Jones
protein: Dimers of immunoglobulin light chains in the urine of patients with
multiple myeloma.
Blocking antibody:
A functional term for an antibody molecule capable of blocking the
interaction of antigen with other antibodies or with cells.
Bursa of Fabricius:
An outpouching of the cloaca in birds; site of development of B cells
in birds.
Carcinoembryonic
antigen (CEA): Antigen present during embryonic development
which normally disappears but reappears in malignant tissue.
Carrier:
A large immunogenic molecule or particle to which an antigenic determinant is
attached, allowing the determinant to become immunogenic.
Cell-mediated
cytotoxicity (CMC): Killing (lysis) of a target cell by an effector
lymphocyte.
Cell-mediated
immunity (CMI): Immune reaction mediated by T cells; in contrast to
humoral immunity, which is antibody mediated. Also referred to as delayed-type
hypersensitivity.
Chemotaxis:
Migration of cells along a concentration gradient of an attractant.
Class I, II
and III MHC molecules: Proteins encoded by genes in the major
histocompatibility complex (q.v.). Class I molecules are designated HLA-A, B, or C.
Class II molecules are designated DP, DQ or DR.
Class switch:
See isotype switch.
Classical pathway:
The mechanism of complement activation initiated by antigen-
antibody aggregates and proceeding by way of C1, C4 and C2.
Clonal deletion:
The loss of lymphocytes of a particular specificity due to contact with
either "self" or artificially introduced antigen.
Clonal selection
theory: The prevalent concept that specificity and diversity of an
immune response are the result of selection by antigen of specifically reactive clones from
a large repertoire of preformed lymphocytes, each with individual specificities.
Cluster determinant
(CD): Cluster of antigens with which antibodies react that
characterize a cell surface marker.
Combinatorial
joining: The joining of segments of DNA to generate essentially new
genetic information, as occurs with Ig genes during the development of B cells.
Combinatorial joining allows multiple opportunities for 2 sets of genes to combine in
different ways.
Complement:
A series of serum proteins involved in the mediation of immune reactions.
The complement cascade is triggered classically by the interaction of antibody with
specific antigen.
Complement components:
An enzymatic system of serum proteins triggered by the
classical and alternative pathways, and resulting in target cell lysis, phagocytosis,
opsonization and chemotaxis.
Complement receptor:
A structure found on erythrocytes, lymphocytes, neutrophils,
monocytes and macrophages that binds C3 fragments.
Constant region
(C region): The invariant carboxyl-terminal portion of an antibody
molecule, as distinct from the variable region which is at the amino- terminal of the chain.
Coombs' test:
A test named for its originator, R.R.A. Coombs, used to detect non-
agglutinating antibodies on red blood cells by addition of an anti- immunoglobulin
antibody.
Cross-reactivity:
The ability of an antibody, specific for one antigen, to react with a
second antigen; a measure of relatedness between two different antigenic substances.
Cytokines:
Soluble substances secreted by cells, which have a variety of effects on other
cells, e.g. Interleukin 1 (Il-1).
Cytotoxic (Cytolytic)
T cell: Cell that kills target cells bearing appropriate antigen
within the groove of an MHC class I molecule that is identical to that of the T cell.
D gene:
A small segment of immunoglobulin heavy-chain and T-cell receptor DNA,
coding for the third hypervariable region of most receptors.
Delayed type
hypersensitivity (DTH): A T cell-mediated reaction to antigen, which
takes 24-48 hours to develop fully, and which involves release of lymphokines and
recruitment of monocytes and macrophages. Also called c cell-mediated immunity.
Determinant:
Part of the antigen molecule which binds to an antibody- combining site or
to a receptor on T cells (see hapten and epitope).
Differentiation
antigen: A cell surface antigenic determinant found only on cells of a
certain lineage and at a particular developmental stage; used as an immunologic marker.
Domain:
A compact segment of an immunoglobulin molecule, made up of about 110 amino
acids around an S-S bond, and encoded by a unique segment of DNA, surrounded by
nontranslated sequences.
DR antigens:
MHC class II molecules found on B cells and antigen-presenting cells of
humans.
Enchancing antibodies:
Antibodies which enhance the survival of a graft or of a tumour.
Enzyme-linked
immunosorbent assay (ELISA):An assay in which an enzyme is linked
to an antibody and a coloured substrate is used to measure the activity of bound enzyme
and, hence, the amount of bound antibody.
Eosinophil:
A polymorphonuclear leukocyte with large eosinophilic (i.e. red) cytoplasmic
granules.
Eosinophil chemotactic
factor of anaphylaxis (ECF-A): A substrate released from mast
cells during anaphylaxis which attracts eosinophils.
Epitope:
An alternative term for antigenic determinant.
Equivalence
zone: In a precipitin reaction, the region in which the concentration of
antigen and antibody leads to maximal precipitation.
Exon: The
region of DNA coding for a protein or a segment of a protein.
Fab: Fragment
of antibody containing the antigen-binding site, generated by cleavage of
the antibody with the enzyme papain, which cuts at the hinge region N-terminally to the
inter-H-chain disulphide bond and generates two Fab fragments from one antibody
molecule.
F(ab')2:
A fragment of an antibody containing two antigen-binding sites generated by
cleavage of the antibody molecule with the enzyme pepsin which cuts at the hinge region C-
terminally to the inter-H-chain disulphide bond.
Fc: Fragment
of antibody without antigen-binding sites, generated by cleavage with
papain; the Fc fragment contains the C-terminal domains of the heavy immunoglobulin
chains.
Fc receptor
(FcR): A receptor on a cell surface with specific binding affinity for the Fc
portion of an antibody molecule. Fc receptors are found on many types of cells.
Fluorescent
antibody: An antibody coupled with a fluorescent dye, used with a
fluorescence microscope to detect antigen on cells, tissues, or microorganisms.
Freund's complete
adjuvant: A water-in-oil emulsion that contains an immunogen, an
emulsifying agent, and killed mycobacteria which enhance the immune response to the
immunogen; termed "incomplete" Freund's adjuvant if mycobacteria are not included.
Genotype:
All of the genes possessed by an individual; in practice it refers to the
particular alleles present at the loci in question.
Germ line:
Refers to genes in germ cells as opposed to somatic cells, that is, genes in
their unrearranged state rather than those rearranged for production of a protein.
Graft versus
host reaction (GVH): The pathologic consequences of a response initiated
by transplanted immunocompetent T lymphocytes into an allogeneic, immunologically
incompetent host. The host is unable to reject the grafted T cells and becomes their target.
HLA complex:
See 'Major histocompatibility complex'.
H-2 complex:
The major histocompatibility complex situated on chromosome 17 of the
mouse; contains subregions K, I and D.
Haplotype:
A particular combination of closely linked genes on a chromosome inherited
from one patient.
Hapten:
A compound, usually of low molecular weight, that is not itself immunogenic but
that, after conjugation to a carrier protein or cells, becomes immunogenic and induces
antibody, which can bind the hapten alone in the absence of carrier.
Heavy chain
(H chain): The larger of the two types of chains that comprise a normal
immunoglobulin or antibody molecule.
Helper T cells:
A class of T cells which help trigger B cells to make antibody against
thymus-dependent antigens. Helper T cells also help generate cytotoxic T cells.
Heterophile
antigen: A cross-reacting antigen that appears in widely ranging species
such as humans and bacteria.
Hinge region:
A flexible, open segment of an antibody molecule that allows bending of
the molecule. The hinge region is located between Fab and Fc and is susceptible to
enzymatic cleavage.
Histocompatibility:
Literally, the ability of tissues to get along; in immunology, it means
identity in all transplantation antigens. These antigens, in turn, are collectively referred to
as histocompatibility antigens.
Humoral immunity:
Any immune reaction that can be transferred with immune serum is
termed humoral immunity (as opposed to cell-mediated immunity). In general, this term
refers to resistance that results from the presence of specific antibody.
Hybridoma:
A hybrid cell that results from the fusion of an antibody-secreting cell with a
malignant cell; the progeny secrete antibody without stimulation and proliferate
continuously both in vivo and in vitro.
Hypersensitivity:
State of reactivity to antigen that is greater than normal for the antigenic
challenge; hypersensitivity is the same as allergy and denotes a deleterious outcome rather
than a protective one.
Hypervariable
regions: Portions of the light and heavy immunoglobulin chains that are
highly variable in amino acid sequence from one immunoglobulin molecule to another, and
that, together, constitute the antigen- binding site of an antibody molecule. Also, portions
of the T-cell receptor which constitute the antigen-binding site.
Ia: "Immune
response-associated" proteins, found on B cells and antigen- presenting cells
of mice; an old term now replaced with MHC (major histocompatibility complex) class II
molecules.
Idiotype:
The combined antigenic determinants (idiotopes) found on antibodies of an
individual that are directed at a particular antigen; such antigenic determinants are found
only in the variable region.
Immediate-type
hypersensitivity:Hypersensitivity tissue reaction occurring within
minutes after the interaction of antigen and antibody.
Immune adherence:
The adherence of particulate antigen coated with C3b to tissue
having cells with C3b receptors.
Immune complex:
Antigen bound to antibody.
Immune modulators:
Substances that control the expression of the immune response.
Immune response
(Ir) gene: A gene controlling an immune response to a particular
antigen; most genes of this type are in the MHC (major histocompatibility complex), and
the term is rarely used to describe other types of Ir genes outside the MHC.
Immunogen:
A substance capable of inducing an immune response (as well as reacting
with the products of an immune response). Compare with antigen.
Immunoglobulin
(Ig): A general term for all antibody molecules. Each Ig unit is made up
of two heavy chains and two light chains and has two antigen- binding sites.
Interferon:
A group of proteins having antiviral activity and capable of enhancing and
modifying the immune response.
Interleukins:
Glycoproteins secreted by a variety of leukocytes which have effects on
other leukocytes.
Internal image:
A spatial configuration of the combining site of an anti- idiotype antibody
which resembles the epitope to which the idiotype is directed.
Intron:
A segment of DNA that does not code for protein; the intervening sequence of
nucleotides between coding sequences or exons.
Isograft:
A tissue transplanted between two genetically identical individuals.
Isohemagglutinins:
Antibodies to major red blood cell antigens present normally as a
result of inapparent immunization by cross-reactive antigens in bacteria, food, etc.
Isotypes:
Classes of antibody that differ in the constant region of their heavy chain (Fc
portion); distinguishable also on the basis of reaction with antisera raised in another
species. These differences also result in different biological activities of the antibodies.
Isotype switch:
The shift of a B cell or its progeny from the secretion of antibody of one
isotype or class of antibody with the same V regions but a different heavy- chain constant
region and, hence, a different isotype (class switch).
J chain (joining
chain): A polypeptide involved in the polymerization of immunoglobulin
molecules IgM and IgA.
J gene:
A gene segment coding for the J or joining segment in immunoglobulin DNA; V
genes translocate to J segments in L chains, and to D and J segments in H chains. Also,
codes for a portion of the T-cell receptor.
K cell:
An effector lymphocyte with Fc receptors which allow it to bind to and kill
antibody-coated target cells.
Killer T cell:
A T cell with a particular immune specificity and an endogenously produced
receptor for antigen, capable of specifically killing its target cell after attachment to the
target cell by this receptor. Also called cytotoxic T cell.
Light chain
(L chain): The light chain of immunoglobulin is a structural feature that
occurs in two forms: kappa and lambda.
Linkage disequilibrium:
The frequency, in a population of linked genes, which is
governed by factors other than change.
Lymphocyte:
Small cell with virtually no cytoplasm, found in blood, in all tissue, and in
lymphoid organs, such as lymph nodes, spleen, and Peyer's patches, and bears antigen-
specific receptors.
Lymphokines:
Soluble substances secreted by lymphocytes, which have a variety of
effects on lymphocytes and other cell types.
Macrophage:
A large phagocytic cell of the mononuclear series found within tissues.
Properties include phagocytosis, and antigen presentation to T cells.
Macrophage-activating
factor (MAF):Actually several lymphokines, including
interferon, released by activated T cells, which together induce activation of macrophages,
making them more efficient in phagocytosis and cytotoxicity.
Major histocompatibility
complex (MHC): A cluster of genes on chromosome 6 in
humans, encoding cell surface molecules that are polymorphic and that code for antigens
which lead to rapid graft rejection between members of a single species which differ at
these loci. Several classes of protein such as MHC class I and II proteins are encoded in
this region. These in humans, are known as 'Human leukocyte antigens' (HLA).
Mast cell:
Tissue located cell probably derived from basophils. Possesses receptor for
Fc of IgE. Participates in 'Immediate hypersensitivity' reactions.
Memory:
In the immune system, memory denotes an active state of immunity to a specific
antigen, such that a second encounter with that antigen leads to a larger and more rapid
response.
MHC class I
molecule: A molecule encoded to genes of the MHC which participates in
antigen presentation to cytotoxic T (CD8+) cells.
MHC class II
molecule: A molecule encoded by genes of the MHC which participates in
antigen presentation to helper T (CD4+) cells.
MHC restriction:
The ability of T lymphocytes to respond only when they 'see' the
appropriate antigen in association with "self" MHC class I or class II proteins on the
antigen presenting cells.
Migration inhibition
factor (MIF): A lymphokine that inhibits the motility of
macrophages in culture.
Minor histocompatibility
antigens: These antigens, encoded outside the MHC, are
numerous, but do not generate rapid graft rejection or primary responses of T cells in
vitro. They do not serve as restricting elements in cell interactions.
Mitogen:
A substance that stimulates the proliferation of many different clones of
lymphocytes.
Mixed lymphocyte
reaction (MLR): When lymphocytes from two individuals are
cultured together, a proliferative response is generally observed, as the result of reactions
of T cells of one individual to MHC antigens on the other individual's cells.
Monoclonal:
Literally, coming from a single clone. A clone is the progeny of a single cell.
In immunology, monoclonal generally describes a preparation of antibody that is
monogenous, or cells of a single specificity.
Monocyte:
Large circulating white cell, 2-10% of total white cells, phagocytic, indented
nucleus. Migrates to tissues, where it is known as a macrophage.
Monokines:
Soluble substances secreted by monocytes, which have a variety of effects on
other cells.
Myeloma:
A tumour of plasma cells, generally secreting a single species of
immunoglobulin.
NK cell:
Naturally occurring, large, granular, lymphocyte-like killer cells that kill various
tumour cells; they may play a role in resistance to tumours. Also, they participate in
ADCC. They do not exhibit antigenic specificity, and their number does not increase by
immunization.
Null cells:
An early population of lymphocytes bearing neither T-cell nor B- cell
differentiation antigens.
Opsonin:
A substance, usually antibody or complement component, which coats a particle
such as a bacterium and enhances phagocytosis by phagocytic cells.
Opsonization:
Literally means "preparation for eating". The coating of a bacterium with
antibody and/or complement that leads to enhanced phagocytosis of the bacterium by
phagocytic cells.
Paratope:
An antibody combining site that is complementary to an epitope.
Passive immunization:
Immunization by the administration of preformed antibody into a
nonimmune individual.
Phagocytosis:
The engulfment of a particle or a microorganism by leukocytes.
Phenotype:
The physical expression of an individual's genotype.
Pinocytosis:
Ingestion of liquid or very small particles by vesicle formation in a cell.
Plasma cell:
End-stage differentiation of a B cell to an antibody-producing cell.
Polyclonal activator:
A substance that induces activation of many individual clones of
either T or B cells. See Mitogen.
Polymorphism:
Literally, "having many shapes"; in genetics polymorphism means
occurring in more than one form within a species; the existence of multiple alleles at a
particular genetic locus.
Polymorphonuclear
leukocyte: White cell, granular cytoplasm. Neutral staining
(neutrophil) - most frequent, phagocytic. Basophilic staining - basophil q.v. Eosinophilic
staining - eosinophil q.v.
Primary lymphoid
organs: Organs in which the maturation of T and B lymphocytes take
place and antigen-specific receptors are first acquired.
Primary responses:
The immune response to a first encounter with antigen. The primary
response is generally small, has a long induction phase or lag period, consists primarily of
IgM antibodies, and generates immunologic memory.
Prophylaxis:
Protection.
Radioallergosorbent
test (RAST): A solid-phase radioimmunoassay for detecting IgE
antibody specific for a particular allergen.
Radioimmunoassay
(RIA): A widely used technique for measurement of primary antigen-
antibody interactions, and for the determination of the level of important biological
substances in mixed samples. It takes advantage of the specificity of the antigen-antibody
interaction and the sensitivity that derives from measurement of radioactively labelled
materials.
Reagin:
Allergist's term for IgE antibodies.
Respiratory
burst: Oxygen dependent increase in metabolic activity within phagocytic
cells stimulated by bacteria or parasites, to be microbicidal.
Reticuloendothelial
system: A network of phagocytic cells.
Rheumatoid factor:
An autoantibody (usually IgM) which reacts with the individual's
own IgG. Present in rheumatoid arthritis.
Second set rejection:
Accelerated rejection of an allograft in an already immune
recipient.
Secondary lymphoid
organs: Organs in which antigen-driven proliferation and
differentiation of B and T lymphocytes takes place.
Secretory component:
A surface receptor on epithelial cells lining mucosal surfaces
which binds dimeric IgA and transports it through the cell into mucosal secretions.
Serum sickness:
A hypersensitivity reaction consisting of fever, rashes, joint pain and
glomerulonephritis, resulting from localization of circulating, soluble, antigen-antibody
complexes, which induce inflammatory reactions. Serum sickness was originally induced
following therapy with large doses of antibody from a foreign source - e.g. horse serum.
Slow-reacting
substance of anaphylaxis (SRS-A): A group of leukotrienes released by
mast cells during anaphylaxis which induces a prolonged constriction of smooth muscle.
This prolonged constriction is not reversible by treatment with antihistamines.
Suppression:
A mechanism for producing a specific state of immunologic
unresponsiveness by the induction of suppressor T cells. This type of unresponsiveness is
passively transferable by suppressor T cells or their soluble products.
Syngeneic:
Literally, genetically identical.
Syngraft:
Same as isograft.
T cell:
A lymphocyte which undergoes a developmental stage in the thymus.
T-dependent
antigen: An immunogen that is able to induce antibody synthesis only in the
presence of lymphokines released by helper T cells.
T-independent
antigen: An immunogen which induces antibody synthesis in the absence
of lymphokines released by T cells; the antibodies are generally only of the IgM isotype.
Titre: The
reciprocal of the last dilution of a titration giving a measurable effect; e.g. if
the last dilution giving significant agglutination is 1:128, the titre is 128.
Tolerance:
Diminished or absent capacity to make a specific response to an antigen,
usually produced as a result of contact with that antigen under nonimmunizing conditions.
Toxoid:
A nontoxic derivative of a toxin used as an immunogen for the induction of
antibodies capable of cross-reacting with the toxin.
Unresponsiveness:
Inability to respond to antigenic stimulus. Unresponsiveness may be
specific for a particular antigen (see tolerance), or broadly nonspecific as a result of
damage to the entire immune system, for example after whole body irradiation.
Vaccination:
Originally referred to immunization against smallpox with the less virulent
cowpox (vaccinia) virus; more loosely used for any immunization against a pathogen.
