Accessory cell: Term used for a cell (often an antigen presenting cell), which
plays a vital role in a specific immune response but cannot by itself mediate the
same.
Affinity: A measure of the strength of binding (the binding constant) between a
single, monovalent antigenic determinant and a single antigen combining site.
Agglutination: The aggregation of cells or particulate antigens as a result of
antibody binding to antigenic determinants on the cells or antigens.
Allele: One of two or more different forms of the same gene, which occupy the
same position (locus) on a particular chromosome.
Allelic: Relating to an allele.
Allelic exclusion: The expression of only one form of a particular antigen
receptor (e.g. TCRa/b or TCRg/d) despite having the genetic material to
produce both.
Allergen: An antigen that has the capability of inducing IgE rather than IgG or
A production in an individual, resulting in an allergic response.
Allergy: A largely IgE-mediated, inflammatory response to non-pathogenic
antigens resulting in pathological changes that may be damaging to the host.
Allogeneic: Term referring to genetically different members of the same species.
Allograft: A tissue graft between two members of the same species who are not
genetically identical.
Allotypes: Usually used in relation to antibodies, this term refers to the antigenic differences between antibodies of the same class caused by transcription of different alleles at the same locus.
Alternative pathway: Activation of complement via C3 which does not involve
the activation of C1, C4 and C2 by immune complexes.
Anaphylatoxin: A pro-inflammatory substance, which causes the release of
histamine from mast cells.
Anaphylaxis: A response to challenge by an allergen that is largely IgE and mast cell mediated. It is an extreme form of immediate hypersensitivity where a range of pharmacologically active mediators are released. In anaphylaxis these
chemicals are released in very high quantities and have rapid effects on smooth
muscle cells and vascular permeability. The results can be life-threatening.
Antibody: A globular, serum protein formed in response to stimulation with an
immunogen. They are capable of highly specific discrimination between
antigens and perform a variety of biological functions. They may also be found
on the surface of B cells as part of the B cell antigen receptor (see BCR).
Antibody-dependent cellular cytotoxicity (ADCC): Cells expressing foreign
antigen (e.g. viral antigens) become the target of antigen-specific antibodies.
These antibody-labelled target cells may then be destroyed by specialised killer
cells (including some large granular lymphocytes and macrophages) which have
receptors for the Fc part of the antibody (Fc receptors, FcR) and bind to the
target cell.
Antigen: A molecule or group of molecules that bind to specific receptors on
lymphocytes. If an antigen is capable of stimulating an immune response alone,
it is also known as an immunogen. If it cannot, it is known as a hapten.
Antigen-binding site: That part of an antibody or a T cell antigen receptor that binds to antigen.
Antigen-presenting cell (APC): Cells that express molecules coded for by the
Class II genes of the major histocompatibility complex (MHC). They are
capable of processing and presenting antigen to T cells. APC include dendritic
cells, macrophages and B lymphocytes.
Antigenic determinant: That part of an antigen which binds to antigen-binding sites on the T or B cell antigen receptors. Also known as an epitope. Complex antigens may have many different antigenic determinants or epitopes, each of which can be recognised by different T or B cells.
Antigen processing: The pathways (endogenous or exogenous) by which large
molecules are broken down within antigen presenting cells so that they can
associate with the products of the major histocompatibility complex genes and
be presented on the surface of the antigen-presenting cell.
Anti-idiotype: An antibody that recognises the antigenic nature of the variable region (or idiotype) of another antibody.
Atopy: Usually used synonymously with allergy. It is used to describe IgEmediated hypersensitivity responses.
Autograft: Transplantation of tissue from one area to another on the same
individual.
Autoimmunity: An immune response to self antigens which may be confined to a particular tissue or may be expressed systemically or throughout the body. Such a response may have a range of pathological effects resulting in autoimmune disease.
Avidity: A measure of the strength of binding between antigen and antibody
when one or both are polyvalent (i.e. have more than one binding site).
BCR (B cell antigen receptor): The complex of molecules on the surface of a B cell responsible for recognising antigen and signalling to the B cell after
binding the antigen. It comprises a membrane anchored immunoglobulin and
two molecules responsible for the signalling namely CD79a (Iga) and CD79b
(Igb)
B lymphocyte (B cell): Mature products of the lymphoid progenitor cell that
when stimulated by antigen may proliferate and differentiate into memory cells
or terminally differentiated plasma cells, which secrete antibody of the same
specificity as that on the originally activated parent cell.
Basophil: A polymorphonuclear leukocyte or granulocyte with cytoplasmic
granules that stain intensely blue with basic dyes. These granules contain
histamine, heparin and other vasoactive amines and are important in
hypersensitivity responses.
Bursa of Fabricius: The primary lymphoid tissue in birds responsible for the
development of B lymphocytes.
Carcinoembryonic antigen (CEA): An antigen expressed during embryonic
development often expressed by malignant tissues.
Carrier: A large molecule, which when attached to a smaller, non-immunogenic
molecule (hapten) allows the latter to stimulate an immune response.
Cell-mediated cytotoxicity: The killing of another cell by an effector cell (e.g.
cytotoxic T cell, natural killer cell, macrophage).
Cell-mediated immunity (CMI): All those immune responses in which antibody plays little or no part. Largely mediated by T cells, macrophages and NK cells.
Chemotaxin: A chemical capable of attracting cells through binding of specific
receptors on the cell surface and promoting their chemotaxis.
Chemotaxis: The directed migration of cells up a concentration gradient of an
attractive chemical.
Class I, II and III MHC genes: See Major histocompatibility complex.
Class switching: When B cells are stimulated during the response to a Tdependent antigen, cytokines are produced, which encourage the B cell to switch from producing antibody of one class (usually IgM in a primary response) to another (e.g. IgG or IgA in a secondary response or IgE in an allergic response).
Classical pathway: Activation of the serum complement proteins usually via
immune complexes (antibody bound to antigen) and involving the activation of
C1, C4 and C2.
Clonal deletion: The elimination of lymphocytes that recognise a particular
antigenic epitope either due to contact with self (e.g. thymic selection) or an
artificially introduced antigen (desensitisation).
Clonal selection theory: The proliferation and expansion of specific lymphocytes with receptors that recognise part of a particular antigen.
Cluster determinant (CD): The nomenclature used to identify specific antigens on the surface of cells (also known as surface markers). Such markers may have several different epitopes each recognised by a different antibody. In order to regularise the process, any antibody which recognises a particular surface marker (regardless of the epitope) is given the same CD designation e.g. anti-CD3.
Combinatorial joining: This occurs during the development of variable regions in antibody and the TCR. It involves the joining of DNA segments to create new genetic information.
Complement: A group of serum and cell surface proteins involved in
inflammation and immunity. They exist in an inactive form but may be triggered
by the classical, alternative or lectin pathways to form an enzyme cascade, the
products of which have highly pro-inflammatory and lytic activities.
Complement components: The proteins that comprise the complement cascade. They are designated as either C1, C2 etc or Factor B, Factor D etc.
Complement receptor: Molecules capable of binding C3 and its degradation
products found on the surface of a range of cells including red cells,
lymphocytes, neutrophils, monocytes and macrophages.
Constant regions (C region): The region of a molecule (e.g. antibody, TCR)
usually the carboxyl terminus, the chemical structure of which is relatively
invariant.
Cross-reactivity: When one epitope or antigenic determinant shares similarity
with another, both may bind to the same antigen receptor but with different
affinities. This is known as cross-reactivity. It is a measure of relatedness
between two antigens.
Cytokines: Soluble chemicals secreted by cells, which have a range of effects on
the cells which produced them or on other cells within the vicinity e.g. tumour
necrosis factor, interleukin-1.
Cytotoxic T cell: See Cell-mediated cytotoxicity.
D(iversity) region: A small region associated with the variable region of the
antigen binding site of both the BCR and TCR. It codes for the third hypervariable region of most receptors.
Delayed type hypersensitivity (DTH): A cell-mediated immune response that
develops over 24–48 hours that results in a variable degree of tissue damage
depending on the extremity of the response. Characterised by the infiltration of
monocytes and macrophages into the area of the lesion.
Determinant: See Antigenic determinant.
Domain: A region found in molecules coded for by members of the
immunoglobulin supergene family, which comprises approximately 110 amino
acids held together in a globular-type form by disulphide bonds.
DR antigens: See Major histocompatibility complex.
Eosinophil: A polymorphonuclear leukocyte or granulocyte with distinct
cytoplasmic granules that stain red with eosin. The granules contain important
proteins (e.g. eosinophil basic protein), which are toxic to parasitic organisms.
Epitope: See Antigenic determinant.
Exon: The region of a gene coding for a protein or part of a protein.
Fab: (Fragment antigen binding) That part of an antibody, which contains the
antigen binding site of the molecule composed of the variable regions of one
light chain and one heavy chain.
F(ab )2: A fragment of antibody formed by cleavage at the hinge region (e.g. by
pepsin) giving a fragment that contains both antigen binding sites of the
molecules.
Fc: (Fragment crystallisable) A fragment of an antibody molecule lacking the
antigen binding sites caused by papain digestion. The Fc fragment contains the
constant regions of both the heavy chains from the hinge region to the carboxyl
terminus of the molecules.
Fc receptors (FcRs): Molecules found on the surface of a range of cells which
bind to the Fc region of antibodies. Each antibody class has its own receptor i.e.
FcgR for IgG, FceR for IgE etc.
HLA (Human leukocyte antigen) complex: Cell surface and soluble antigens coded for by the genes of the major histocompatibility complex.
Hapten: See Antigen.
Heavy chains (H chains): Pairs of molecules found in antibodies, which are
larger than the other pair and dictate the class of an antibody molecule i.e. an
antibody with m (mu) heavy chains is IgM.
Helper T cells: A subpopulation of T cells, which help in the generation of
effector T and B cells usually through the production of cytokines. Previously
identified by the expression of cell surface molecule CD4 (although this
molecule is not unique to T helper cells).
Hinge region: That area of an antibody between the first and second constant
regions of the heavy chain, which confers flexibility upon the molecule. It is
highly susceptible to enzymatic cleavage.
Histocompatibility: (Histo referring to cells and tissues) Refers to the degree of
identity between two tissues with regard to their cell surface antigens coded for
by the major histocompatibility complex genes (see MHC).
Humoral immunity: Any immune response in which antibody plays the principal or sole role.
Hybridoma: A cell and its progeny that result from the fusion of a continuously
replicating (malignant) cell and an antibody-secreting cell. Hybridomas
replicate indefinitely and secrete antibody without the need for stimulation by
specific antigen.
Hypersensitivity: An immune response, which in one individual results in a
greater degree of tissue damage than would occur normally in others. This
includes allergy (type I hypersensitivity) and delayed type hypersensitivity (type
IV hypersensitivity).
Hypervariable regions: This refers to sequences in proteins such as the
immunoglobulins and the T cell antigen receptor which show a high degree
of genetic variability and are found in the antigen binding regions of these
molecules.
Idiotope: The variable region of an antibody which, when introduced into a
foreign host may act as an antigen. An idiotope is a single antigenic epitope
within that variable region.
Idiotype: The antigenic nature of all the idiotopes of an antibody.
Immediate-type hypersensitivity: A hypersensitivity response that occurs within minutes after exposure to antigen. This is usually a type I response involving IgE but may also be type II (IgG bound to a cellular antigen) or type III (IgG bound to soluble antigen).
Immune complex: Antigen bound to antibody. The antigen may be soluble,
particulate or cell-associated.
Immunogen: An antigen capable of inducing an immune response.
Immunoglobulin (Ig): A globular protein involved in the immune response. Used interchangeably with the term antibody. Each Ig unit has at least two heavy chains, two light chains and two antigen-binding sites.
Interferon: A group of proteins, members of which have a range of anti-viral
activities and variable capabilities of moderating the immune response.
Interleukins (ILs): A large group of glycoproteins secreted by a wide variety of
cells, which may affect the cells that produce them or other cells in the vicinity.
Often collectively referred to as cytokines.
Intron: That part of a genetic sequence that does not code for a protein.
Isotypes: Minor differences in the constant region of a particular class of
antibody may lead to altered epitopes, which can stimulate the production of
antibodies when it is introduced into another species. These antibodies can be
used to identify the different isotypes, which may have distinct biological
properties.
Isotype switch: See Class switch.
J chain (joining chain): A polypeptide that stabilises the polymeric IgA and IgM molecules.
J gene: Codes for the J or joining segment involved in the formation of the
variable region in the BCR and TCR.
K cell: Killer cells bind antibody-coated target cells through their FcR and
destroy them by antibody-dependent cellular cytotoxicity. K cells include some
large granular lymphocytes, macrophages and some T cells.
Killer T cells: Also called cytotoxic T cells. The cell recognises antigen on the
surface of a target cell through its antigen specific receptor (TCR). This and
subsequent events trigger the T cell to destroy the target cell. Previously
identified by the expression of CD8, it is now known that not all T cells with
cytotoxic activity express this antigen.
Light chain (L chain): The smaller of the two molecules that comprise an
antibody. Light chains may be either kappa or lambda.
Lymphocyte: A small cell (6–8 mm in diameter) found in the blood and in
specialised lymphoid tissues. They have little cytoplasm but when activated can
become enlarged (blast cells) and highly metabolically active. They are the key
cells in a specific immune response having the capability to recognise antigen
through their antigen-specific receptors.
Lymphokines: Soluble substances produced by lymphocytes which have a range of effects on the cells that produce them and other cells. Often also referred to as cytokines.
Macrophage: A large cell found in the tissues derived from the blood-borne
monocyte. It may have a range of characteristics and functions depending upon
the tissue in which it is found. It plays a key role in both the innate and specific
immune responses.
Major histocompatibility complex: Genes encoding proteins expressed on cell surfaces. Class I genes code for the human leukocyte antigens (HLA) A, B and C found on all nucleated cells. Class II genes code for the HLA DP, DQ and
DR expressed on antigen presenting cells. Class III genes code for molecules
such as some complement components and some heat shock proteins which do
not appear to truly be part of the MHC.
Mast cell: A large cell with extremely large cytoplasmic granules found
particularly in the connective and mucosal tissues. The key cells in IgEmediated
allergic responses.
Memory: Exposure to an antigen usually results in the formation of memory
cells, which upon subsequent exposure to the same antigen are able to respond
more rapidly and (in the case of B cells) produce highly specific antibodies.
MHC class I gene products: Molecules expressed on the surface of all nucleated blood cells and platelets, which participate in the recognition of virally infected cells by CD8 positive T cells. They are also involved in graft rejection by the immune system in poorly matched donors and recipients.
MHC class II gene products: Molecules expressed on the surface of cells
involved in antigen presentation to CD4 positive T cells (e.g. macrophages,
dendritic cells, B cells and activated T cells).
MHC restriction: T cells can only respond to antigen presented to them in
association with self-MHC antigens on antigen presenting cells.
Mitogen: A molecule or group of molecules, which stimulate the proliferation of
a number of different lymphocytes, regardless of their antigen specificity. The
responding cells may recognise a range of different epitopes and the response is
therefore known as polyclonal.
Mixed lymphocyte response (or reaction, MLR): Lymphocyte proliferation,
which occurs when lymphocytes from two different donors are mixed together
in vitro.
Monoclonal: A response that occurs when all the responding cells are the
progeny (daughter cells) of a single cell.
Monocyte: The blood borne form of the tissue macrophage. A large cell capable
of phagocytosis.
Monokines: Cytokines secreted by monocytes that have an effect on a range of
other cells.
NK (Natural killer) cell: A large granular lymphocyte capable of killing a limited range of tumour or virally infected cells and of a restricted type of antigen recognition.
Opsonin: A substance which enhances phagocytosis of particulate material such
as bacteria by binding to surface receptors on cells. Opsonins include
antibodies, complement components and acute phase proteins.
Opsonisation: The coating of a particulate antigen by an opsonin, which results
in enhanced phagocytosis.
Paratope: That part of an antibody, which binds to an antigenic epitope. It is
therefore complementary to the epitope.
Phagocytosis: The active formation of extrusions of the cellular membrane to
enclose material within an intracellular vesicle known as a phagosome.
Phenotype: The physical expression of the genotype.
Pinocytosis: Ingestion of extracellular fluid and soluble particles by small
invaginations of the cellular membrane to form a membrane bound intracellular
vesicle.
Plasma cell: A terminally differentiated, antigen-stimulated B cell, which
produces large quantities of antibody.
Polyclonal activator: See Mitogen.
Polymorphonuclear leukocytes: White blood cells (leukocytes) possessing nuclei with many varied shapes. They include the neutrophils, basophils and
eosinophils.
Primary lymphoid tissues (organs): Tissues or organs in which lymphocytes
differentiate and first acquire their antigen-specific receptors.
Primary responses: Those specific immune responses that occur upon first
exposure to an antigen. They are usually characterised by the production of
antigen-specific IgM. They usually have a long lag phase (before any specific
antibody is detected) and result in the formation of memory cells.
Respiratory burst: An increase in metabolic activity, which is dependent upon
oxygen and occurs in phagocytic cells as a result of phagocytosis or other
stimuli. The resulting radicals of oxygen are highly reactive and either directly,
or indirectly, microbicidal.
Reticuloendothelial cells: Now known as the mononuclear phagocyte system
comprising the network of monocytes and tissue-based macrophages.
Rheumatoid factor: An antibody, (usually IgM or IgG) that reacts with the nonvariable regions of self immunoglobulin (usually IgG). Found in sero-positive
rheumatoid arthritis.
Secondary lymphoid organs: Organised tissues in which the interaction between antigen presenting cells and antigen-specific lymphocytes is facilitated and lymphocyte activation and differentiation occurs.
Secretory component: A protein produced by mucosal epithelial cells, which
binds to dimeric IgA and facilitates its transport into the lumen of the gut.
Syngeneic: Genetically identical.
T cells: A subpopulation of lymphocytes that undergoes development and
maturation within the thymus.
T cell antigen receptor (TCR): The complex of molecules on the surface of a
T cell responsible for binding antigen and signalling the fact to the T cell. It
comprises a bimolecular complex the abTCR or the gdTCR, which binds
antigen and the CD3 molecular complex, which is responsible for the
intracellular signalling.
T-dependent antigen: An antigen that depends upon the activation of effector
T cells for the production of cytokines required for B cell activation,
proliferation and differentiation.
T-independent antigens: Antigen that can by themselves directly activate B cells leading to their proliferation and differentiation. T-independent antigens
generally only stimulate an IgM response.
Tolerance: The inability (or severely reduced ability) to respond to a specific
antigen. Usually caused by exposure to a non-immunising dose of antigen.
Vaccination: Also known as immunisation. Involves the stimulation of an
immune response to a non-toxic, non-infectious agent that will subsequently
protect the host when exposed to the toxic or infectious agent.
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