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MicrobiologyBytes - Glossary of Immunology Category: Health and Medicine > Infection & Immunity Date & country: 13/07/2008, US Words: 158

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.

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 processing
Large molecules are broken down (processed) within macrophages into peptides and presented within the groove of MHC molecules.

Antigen receptor
The specific antigen-binding receptor on T or B lymphocytes; these receptors are transcribed and translated from rearrangements of V genes.

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.

Antigenic determinant
A single antigenic site or epitope on a complex antigenic molecule or particle.

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.

Eosinophil
A polymorphonuclear leukocyte with large eosinophilic (i.e. red) cytoplasmic granules.

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.

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.

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.

HLA complex
See 'Major histocompatibility complex'.

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.

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).

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.

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.