IMMUNOLOGY - Roitt I. - Mir 2000

Glossary of Terms

Avidity. A measure of the overall strength of antibody-antigen binding, determined by both the affinity of the interaction between epitopes and paratopes and the valencies of the antibody and antigen.

Adjuvant. A substance that non-specifically stimulates the Immune Response to an antigen.

Freund's adjuvant. A Water-in-oil emulsion used to enhance the immunogenicity of an antigen administered within it. Complete Freund's adjuvant, unlike incomplete Freund's adjuvant, additionally contains killed Mycobacterium tuberculosis Cells.

Reactive oxygen and nitrogen species. Bactericidal metabolites generated by phagocytic cells. These include the superoxide radical, hydrogen peroxide, hypohalites, and nitric oxide NO.

Allelic exclusion. The phenomenon of expression of only a single Gene allele—from either the paternal or maternal chromosome. Characteristic of immunoglobulin and T-Cell receptor genes. Allergen. An agent (such as pollen, dust, or animal dander) that induces IgE antibody-mediated hypersensitivity reactions.

Allergy. Originally, this term referred to an altered form of an Organism's reaction upon re-exposure to an antigen. Currently, it is used to denote type I hypersensitivity reactions.

Allotypes. Allelic variants of Proteins that differ in antigenic Specificity among individuals of the same species.

Alternative pathway of Complement activation. The process of complement system activation involving component C3 and factors B, D, P, H, and I, which interact in close proximity to an activating surface to form The alternative pathway C3-convertase. Anaphylaxis. An antigen-specific immune response mediated mainly by IgE Antibodies, manifested by vasodilation and smooth Muscle contraction. The resulting bronchoconstriction can cause the death of the animal.

Anaphylatoxins. Peptide fragments of complement components (C3a and C5a) that induce mast cell degranulation and smooth muscle contraction.

Antiviral proteins. Proteins whose synthesis is induced by interferons. They are activated upon cell infection by a virus and restrict its Replication.

Antigenic determinant. See Epitope.

Antigenic Peptides. Protein-derived peptide fragments bound by MHC molecules that activate T cells.

Antigen-presenting cells (APCs). Various cell types that display processed antigen on their surface.

Antigens. Substances that interact with specific receptors on T AND B cells.

Blood Group Antigens. A series of allotypically variable molecules expressed On the surface of erythrocytes (and sometimes tissue cells) that can serve as target antigens in transfusion reactions.

T-dependent/T-independent antigens. The immune response to T-dependent antigens requires their recognition by both T and B cells. T-independent antigens can induce antibody synthesis by acting directly on B cells.

Ly antigens. A group of cell surface markers found on mouse T cells that allow the differentiation of T-cell subpopulations. Currently, many of these are classified as CD markers.

Antibodies. Molecules produced by the body in response to exposure to an antigen and which specifically bind to that antigen.

Antibody-dependent cell-mediated cytotoxicity (ADCC). A cytotoxic reaction based on the recognition of target cells by killer cells bearing an Fc receptor, with the participation of specific antibodies.

Antibody-forming cell (AFC). A plasma cell that secretes antibodies.

Arthus reaction. Inflammation developing in the Skin several hours after antigen injection—a type III hypersensitivity reaction.

Atopy. Clinical symptoms of type I hypersensitivity reactions, including eczema, asthma, and rhinitis.

Autoimmunity. A state in which the immune system acquires The ability to recognize and react against the body's own self-antigens.

Autocrine. A term referring to the action of a cytokine on The Cell that produces it.

Affinity. The strength of the bond between an antigenic determinant (epitope) and the binding site of an antibody (paratope).

White pulp. The lymphoid component of the Spleen, consisting of periarteriolar sheaths formed by lymphocytes and antigen-presenting cells.

Acute-phase proteins. Serum proteins whose levels increase during infections or inflammatory processes.

Protein A and Protein G. Cell wall components of certain staphylococcal strains that bind to the Fc region of most IgG isotypes.

Plaque-forming cell. An antibody-producing cell detected in vitro by its ability to lyse antigen-sensitized erythrocytes.

Large granular lymphocytes (LGLs). Lymphocytes of a specific morphological type that possess most of the activities of K and NK cells. They express markers of both lymphocytes and monocytes/macrophages.

Bradykinin. A vasoactive nonapeptide and the most important mediator in the kinin system.

BCG (from Bacillus Calmette–Guérin). An attenuated strain of Mycobacterium tuberculosis used as a vaccine; also employed as an adjuvant or biological response modifier.

Vasoactive amines. Products secreted by basophils, mast cells, and platelets—such as histamine and 5-hydroxytryptamine—that act on the endothelium and locally on vascular smooth muscle.

High endothelial venules. A specialized region of venules through which lymphocytes migrate into Lymph Nodes.

Epstein–Barr virus (EBV). The CAUSATIVE AGENT OF Burkitt's lymphoma and infectious mononucleosis. It has the ability to transform human B cells in such a way that they form stable cell lines.

Inflammation. The complex of processes that trigger cell migration and the influx of immune system molecules to the site of infection or injury. It manifests as increased blood supply, elevated vascular permeability, and enhanced leukocyte migration across the endothelium.

Respiratory burst. A sharp increase in oxidative metabolic processes in phagocytes following the engulfment of opsonized particles. Secondary immune response. An immune reaction that occurs after repeated or multiple exposures to a given antigen.

Haplotype. A set of genetic determinants (alleles) on a single chromosome.

Hapten. A small molecule that can act as an epitope but is unable by itself to induce antibody production.

Genetic association. The Link Between a specific genotype and a particular process, such as a specific disease.

Genetic restriction. See MHC Class I/II antigen restriction.

Genome. The entire set of genetic material (genes) of a cell.

Genotype. The genetic material inherited by an individual from its parents; not always fully expressed.

Immune response genes (Ir genes). Genes that influence the level of the immune response. MHC class II genes play a crucial role in regulating immune responses to specific antigens. RAG-1 and RAG-2 genes (recombination activating genes). Genes that activate recombination and are necessary for the rearrangement of V, D, and J gene segments during The formation of functionally active antigen receptor genes.

C genes. Gene segments encoding the constant region of immunoglobulin heavy and light chains, as well as the α-, β-, γ-, and δ-chains of the T-cell receptor.

D genes. Gene segments located between the V and J segments of immunoglobulin heavy chain genes and T-cell receptor β- and δ-chain genes; they recombine with V and J genes during ontogeny.

J genes. Groups of gene segments that are part of the genes encoding immunoglobulin heavy and light chains and T-cell receptor chains. During DNA Rearrangement in lymphocyte differentiation, J genes join with other segments of genes encoding variable domains.

Heterologous. A term denoting interspecies antigenic differences.

Heterogeneous nuclear RNA (hnRNA). A fraction of nuclear RNA representing primary, unprocessed precursors of cytoplasmic mRNA.

Hybridoma. A hybrid cell line produced in vitro by the fusion of two cell types, typically lymphocytes and tumor cells.

Giant cells. Large multinucleated cells occasionally observed in granulomatous reactions, presumed to form via macrophage fusion.

5-Hydroxytryptamine. A vasoactive amine found in platelets; a major inflammatory mediator in rodents.

Hypervariable region. Three highly variable regions within the V-domains of IMMUNOGLOBULINS and T-cell receptors. These regions lie in close proximity at the distal end of the V-domain, forming the antigen-binding site.

Delayed-type hypersensitivity (DTH). A term denoting delayed cutaneous reactions characteristic of type IV hypersensitivity.

Histamine. A primary vasoactive amine released from the granules of mast cells and basophils.

Histocompatibility. The acceptance of transplanted Organs or Tissues, determined by the identity of tissue antigens between donor and recipient.

Major Histocompatibility Complex (MHC). A gene cluster found in all mammals, whose products are primarily responsible for allograft rejection and the interaction of lymphocytes with antigen-presenting cells.

Nude (athymic) mice. Mice with a Congenital absence of the Thymus, which also carry a closely linked gene causing hairlessness. Homologous restriction factors: Complement components that limit the action of the lytic complex on host cells.

Granzymes. Enzymes contained within the granules of cytotoxic T cells and large granular lymphocytes.

Granulomatous reaction. A chronic inflammatory process (often a manifestation of type IV hypersensitivity) caused by the prolonged persistence of an antigen in tissues.

Granulocytes. Neutrophils, eosinophils, and basophils.

Humoral. Pertaining to extracellular fluids, including blood serum and lymph.

Dendritic cells. Tissue-resident cells that capture antigens and migrate to LYMPH NODES AND the spleen, where they act as potent APCs.

Defensins. A group of low-molecular-weight proteins produced by neutrophils that exhibit antibacterial activity.

Domain. A region of a peptide possessing a coherent tertiary Structure. Domains are present in immunoglobulin molecules and class I and II MHC molecules.

C-Domains. Constant domains of antibodies and T-cell receptors. These domains do not form the antigen-binding site and are highly conserved among various TCRs, thus contributing minimally to their Variability.

SCR-Domains (complement control Protein domains) (also known as short consensus repeats). Domains found in many Components of the classical and alternative complement pathways, certain complement receptors, and regulatory proteins.

V-Domains. N-terminal domains of the heavy and light chains of Ig, and the α-, β-, γ-, and δ-chains of the T-cell receptor. The V genes encoding them recombine with corresponding D and J genes during lymphocyte differentiation.

Dominant idiotypes. Individual idiotypes that predominate among the antibody idiotypes elicited by a given antigen.

Germline. The Lineage of cells that give rise to Gametes, whose genetic material has not yet undergone modification by somatic recombination or maturation. Idiotype. The antigenic determinant of the V region of antibodies, defined by a set of idiotopes.

Idiotope. A single antigenic determinant located within the V region of an Ig molecule.

Isotypes. Genetic variants within a given family of proteins or peptides. Every isotype of the family is represented in The Genome of every individual of a species (e.g., immunoglobulin classes).

ICcosomes. Immune complexes found as small inclusions on follicular dendritic cells.

Immunogenicity. The capacity to induce immune responses mediated by B and/or T cells.

Immune enhancement. The prolongation of graft survival for transplanted organs or tissues upon administration of antibodies against graft alloantigens.

Immune complex. The product of an Antigen-Antibody Reaction, which may also incorporate components of The Complement System.

Immunoblotting (Western blotting). A method for identifying proteins and determining their properties using antibodies.

Immunoglobulins. Serum antibodies classified into IgG, IgM, IgA, IgE, and IgD classes.

Immunofluorescence. A microscopic technique for identifying antigens in tissues or on cells using conjugates of specific antibodies with a fluorescent dye.

Integrins. A large family of cell surface adhesion molecules. Some of them interact with Cell Adhesion molecules, others with complement fragments, and still others with Extracellular matrix components.

Interleukins (IL-1 - IL-18). A group of molecules involved in signaling between Cells of the immune system.

Interferons (IFs). A group of molecules through which immune system cells exchange information (signals) and provide the body with defense against viral infections.

Intron. A gene segment located between exons, typically representing a non-coding sequence.

Intradermal test (Skin test). The intradermal injection of a small amount of antigen to test for type I hypersensitivity.

Karyotype. The chromosomal complement of somatic cells, which may vary among individuals of a given species depending on the composition of Sex Chromosomes and translocations between regions of different chromosomes.

Framework segments. Fragments of antibody V regions located between hypervariable regions.

Kinin. A group of vasoactive mediators generated during tissue injury.

Classical complement pathway. Complement activation by antigen-antibody complexes involving components C1, C2, and C4, leading to the Formation of the classical pathway C3 convertase. Immunological memory cells. Long-lived lymphocytes primed by a specific antigen but not terminally differentiated into effector cells. Upon re-exposure to the same antigen, they respond more rapidly than unprimed lymphocytes.

Kupffer cells. Phagocytic cells lining the hepatic blood sinusoids.

Langerhans cells. Antigen-presenting cells of the skin that migrate to regional lymph nodes, where they differentiate into dendritic cells; highly active in presenting antigens to T cells.

Myeloid cells. Phagocytes of Bone Marrow origin, including neutrophils, eosinophils, and monocytes.

Passenger leukocytes. Leukocytes contained within a donor graft that are capable of sensitizing the recipient to graft alloantigens.

B cells. Lymphocytes that develop in the adult bone marrow and produce antibodies.

K cells (Killer cells). A population of lymphocytes capable of lysing target cells in antibody-dependent cell-mediated cytotoxicity. They express Fc receptors.

T cells. Lymphocytes that undergo differentiation primarily within the thymus. They play pivotal roles in the development and Regulation of the immune response.

γ/δ T cells. A minor subpopulation of T cells expressing the γ/δ T-cell receptor.

Cell-mediated (cellular) Immunity. Immune responses mediated by cells rather than antibodies or other humoral factors.

Cell Cycle. The process of Cell Division, consisting of 4 phases: G1, S, G2, and M. METABOLISM/36.html">DNA replication occurs during the S phase, and cell division takes place in the M phase (mitosis).

B-Cell Coreceptor Complex. A group of cell surface molecules consisting of complement receptor type 2 (CD21), CD81, and CD19 molecules. Acts as a costimulatory receptor for mature B cells.

B-Cell Receptor Complex. The B-cell surface immunoglobulin and its associated signaling molecules CD79a and CD79b.

T-Cell Receptor (TCR). The T-cell antigen receptor, which is a heterodimer composed of α and β chains (TCR-2) or γ and δ chains (TCR-1) in association with the CD3 molecular complex.

Clonal Selection. One of the fundamental postulates of immunology, which states that a given antigen induces the selective activation, division, and differentiation only of those cells that express receptors capable of binding to that antigen.

Collectins. A group of high-molecular-weight proteins, including conglutinin and mannan-binding lectin (MBL), capable of opsonizing microbial cells.

Colony-Stimulating Factors (CSFs). A group of cytokines that regulate the differentiation of hematopoietic cells.

Complement. A system of serum proteins involved in The regulation of inflammatory processes, activation of phagocytosis, and lytic action on cell membranes. Complement activation occurs As a result of the interaction between its components and immune complexes (the classical pathway of activation).

Congenic Animals. Specially bred animal strains that are genetically identical except for a single specific locus.

C3 Convertases. Enzyme complexes and that cleave the C3 component of complement.

Concanavalin A (ConA). A T-cell mitogen. Contact hypersensitivity. A delayed-type inflammatory skin reaction seen in type IV hypersensitivity.

Constant Region. The relatively invariant region of light and heavy chains of immunoglobulins, as well as the α, β, γ, and δ chains of the T-cell receptor.

Conjugate. A reagent consisting of two covalently linked molecules, such as fluorescein and an immunoglobulin.

Costimulation. Signals required for lymphocyte activation In addition to the antigen-specific signal mediated by the antigen receptor. An important costimulatory molecule is CD28 for T cells and CD40 for B cells.

Xenogeneic. A term referring to interspecies antigenic differences.

Capping. The process of aggregation of cell surface molecules (usually induced by antibodies).

Leukotrienes. Pharmacologically highly active metabolites of arachidonic acid.

Leukocyte Function-Associated Antigens (LFAs). A group of leukocyte surface molecules that mediate antigen-nonspecific intercellular adhesion of leukocytes and other cells. LFA-1 is the CD11a/CD18 antigen, LFA-2 is CD2, and LFA-3 is CD58.

Lectin Pathway. A pathway of complement activation initiated by mannan-binding lectin (MBL) and converging with the classical activation pathway.

Ligand. A linking (binding) molecule.

Membrane Attack Complex (MAC). A complex consisting of terminal complement components C5b–C9 (see Lytic Pathway of complement activation); it inserts into cell membranes.

Bystander Lysis. Complement-mediated lysis of cells located in the immediate vicinity of a complement activation site but not responsible for its activation.

Gut-Associated Lymphoid Tissue. Clusters of lymphoid tissue in the walls of the gastrointestinal tract.

Mucosa-Associated Lymphoid Tissue (MALT). A collective term for lymphoid formations in the mucous membranes of the gastrointestinal tract, Bronchi, and other organs. Lymphokine-Activated Killer Cells (LAK cells). Cytotoxic cells generated ex vivo upon stimulation with IL-2 and possibly other cytokines.

Lymphokines. A collective term for non-immunoglobulin molecules produced by lymphocytes that function to transmit signals between cells of the immune system (cf. Interleukins).

Cell Line. A cell population resulting from the continuous growth of a cell culture in vitro. Typically contains A number of individual clones.

Lipopolysaccharide (LPS). A component of the outer cell wall of Gram-negative Bacteria that can act as a B-cell mitogen.

Lytic pathway of complement activation. Mediated by components C5–C9, leading to the lysis of The Plasma Membrane of cells.

CD markers. Surface membrane molecules on leukocytes and platelets, identified using Monoclonal Antibodies. They can serve as markers for various cell populations.

Myeloma. A lymphoma originating from the B-cell lineage of lymphocytes.

β₂-Microglobulin. A polypeptide that forms part of certain cell Membrane Proteins, notably class I MHC molecules.

Pokeweed mitogen. A B- and T-cell mitogen.

Mitogens. Substances that induce cell division, particularly in lymphocytes.

Cell adhesion molecules. A group of proteins belonging to the immunoglobulin superfamily that participate in cell-cell adhesion; includes ICAM-1 (CD54), ICAM-2 (CD102), ICAM-3 (CD50), VCAM-1, MAdCAM-1, and PECAM. Cell surface molecules on various leukocytes and non-hematopoietic cells that interact with LFA-1.

DM molecules. Molecules related to class II MHC antigens. Required for the formation of the antigenic peptide–class II MHC molecule complex.

MHC molecules (antigens) of classes I, II, and III. Molecules of three classes encoded by MHC genes. Class I molecules consist of an MHC-encoded peptide associated with β₂-microglobulin; class II molecules are dimers of non-covalently linked peptide chains (both are products of MHC genes); class III molecules comprise a heterogeneous group of proteins, including complement components.

Antigen receptor activation motif (ARAM). Amino acid sequences (Structural motifs) within the intracellular domains of CD79 and CD3 molecules that are phosphorylated upon lymphocyte activation triggered by receptor-antigen interaction.

Biozzi mice. Inbred mouse strains characterized by high or low immune response (antibody production) to various antigens (initially selected and bred based on the magnitude of the immune response to sheep red Blood Cells).

NK cells. Natural killer cells. A population of lymphocytes with the intrinsic ability to recognize and destroy certain virus-infected and tumor cells.

Linkage disequilibrium. The co-occurrence in a population of two alleles at frequencies greater than expected, i.e., calculated by multiplying the individual frequencies of each allele.

Knockout animals. Animals in which the gene for a specific protein has been either deleted or mutated to become non-functional using gene knockout techniques.

Carrier. An immunogenic molecule or part of a molecule recognized by T cells during a HUMORAL IMMUNE RESPONSE.

N regions. Gene segments present in rearranged antigen receptor genes that are absent from germline DNA.

T-cell education. The processes of positive and negative selection of developing thymocytes. Positive selection is undergone by immature T cells capable of responding to antigenic peptides presented in the context of self-MHC molecules. Negative selection, leading to apoptosis, affects T cells that respond to self-MHC antigens.

Opsonization. A process that facilitates phagocytosis, driven by the binding of opsonins (antibodies and the complement component C3b) to bacterial surface antigens.

Very late activation antigens (VLA-1 to VLA-6). A set of integrins sharing a common β₁ chain (CD29).

Paracrine mechanism. A term referring to the action of a cytokine on cells located at a distance from the cells that produced it.

Passive cutaneous anaphylaxis. A method for detecting antigen-specific IgE by intravenous injection of antigen and a dye into experimental animals whose skin has been previously sensitized with antibodies.

Pathogen. A disease-causing organism.

Peyer's patches. Aggregates of lymphoid cells in the intestinal wall that constitute secondary lymphoid tissue.

Antagonist peptides. Analogs of antigenic peptides that bind to MHC molecules and block the activation of specific T-cell clones.

Primary lymphoid tissues. The tissues of Lymphoid organs where lymphocytes undergo the Initial Stages of maturation. These include the embryonic Liver, adult bone marrow, thymus, and the bursa of Fabricius in birds.

Primary immune response. An immune reaction (cellular or humoral) that develops following the initial contact with an antigen.

Immunoglobulin isotype (class) switching. The transition of individual B cells (resulting from the recombination of immunoglobulin heavy-chain constant genes with the V gene) to the synthesis of antibodies of a different class while retaining the same specificity. As the immune response progresses, this process becomes widespread.

Cross-reactivity. The ability of two different antigens to elicit the same reaction due to shared or similar determinants they possess.

Periarteriolar lymphoid sheaths. Aggregates of lymphoid tissue that constitute the white pulp of the spleen.

Perforin. A granule protein of cytotoxic cells, homologous to the C9 component of complement. It forms Pores in the membrane of target cells.

Amplification loop. A positive feedback amplification mechanism in the activation of the alternative complement pathway: the Cleavage of C3 in the vicinity of an activating surface.

Plasma cell. An antibody-producing B cell at the terminal stage of differentiation.

Antigen presentation. The display (expression) of an antigen on The surface of (antigen-presenting) cells in a form accessible for recognition by lymphocytes.

Priming. The primary sensitization of a cell induced by an antigen.

Prostaglandins. Pharmacologically active derivatives of arachidonic acid capable of modulating Cell Motility and their immune response.

Proteasomes. Cellular Organelles responsible for the degradation of intracellular proteins that have been tagged for destruction by ubiquitin.

Flow cytometry. A method for analyzing cell populations in suspension based on the detection of differences between cells using surface markers.

Antigen Processing. The conversion of an antigen into a form accessible for recognition by lymphocytes.

Pseudoalleles. Genes that behave as true alleles in allelism tests but segregate during Crossing Over; they do not occupy a homologous position on the chromosome (e.g., C4).

Pseudogenes. Genes structurally similar to conventional genes but incapable of being expressed to produce functionally active products (e.g., Jk3 in mice).

Radioimmunoassay (RIA). A set of highly sensitive Methods for determining the titer of an antigen or antibodies using radiolabeled Reagents.

Graft-versus-host disease (GVHD). A process developing as a result of an immune reaction by allogeneic donor lymphocytes against the tissues of a recipient who is incapable of mounting an immune response against the donor cell antigens.

Recombination. The redistribution of genetic material during Meiosis. Recombination also occurs during somatic DNA rearrangement to form genes encoding antibody molecules and T-cell antigen receptors.

MHC class I/II restriction. The phenomenon whereby effective cooperation among immunocompetent cells in many immune responses is possible only if these cells share a common MHC haplotype (class I or II loci).

Reticuloendothelial system. A diffuse system of phagocytic cells originating from bone marrow stem cells, closely associated with the Connective Tissue of the liver, spleen, lymph nodes, and other serous cavities. This is now an outdated and rarely used term; the preferred designation is the mononuclear phagocyte system (see)

Receptor. A cell membrane molecule that specifically binds particular extracellular molecules.

Complement receptors (CR1–CR4 and C1qR). The first four molecules are surface receptors for fragments of the complement component C3. CR1 and CR2 molecules possess numerous CCP domains, whereas CR3 and CR4 belong to the integrins. The C1qR receptor binds C1q.

Fc receptors. Surface molecules on various cells that bind to the Fc fragment of immunoglobulins. They are specific for a particular class of antibodies and selective for isotype. "Recurrent" idiotype. An idiotype detected during the immune response of different animals (or animals of different strains) to a given antigen.

Rosette formation. A method used to identify or isolate specific cell types; the cells under study are mixed with particles or cells that they bind (e.g., human T lymphocytes with sheep erythrocytes). A rosette is a cellular aggregate consisting of a central cell and surrounding bound cells.

Secretory component. A polypeptide produced by certain types of secretory epithelial cells. It facilitates IgA Transport Across the epithelium and protects this immunoglobulin from gastrointestinal digestive enzymes.

Spleen. The major secondary lymphoid organ located in the Abdominal cavity near The Stomach.

Selectins. Three adhesion molecules: P-selectin (CD62P), E-selectin (CD62E), and L-selectin (CD62L). They are involved in arresting leukocytes to mediate their migration through venule endothelium.

Syngeneic animals. Animals of an inbred strain obtained by closely related breeding (sibling mating). All individuals of this strain share a high level of genetic homozygosity (identity of each autosomal pair).

Acquired Immunodeficiency Syndrome (AIDS). A progressive immune deficiency caused by the HUMAN IMMUNODEFICIENCY VIRUS (HIV) as a result of its destruction of CD4⁺ T cells.

Synergy. Cooperative action or effect.

Mononuclear phagocyte system. A system of fixed and mobile long-lived phagocytic cells related to blood monocytes and tissue macrophages.

HLA system (Human Leukocyte Antigen system). The major histocompatibility complex in humans.

Systemic lupus erythematosus (SLE). A human autoimmune disease typically characterized by the formation of antinuclear antibodies.

Scratch test (skin prick/scratch test for allergens). Application of an antigen to a limited area of the skin via scratching; a test for type IV hypersensitivity.

Mixed lymphocyte culture (MLC) or mixed lymphocyte reaction (MLR). A test system that allows the recognition of alloantigens by T lymphocytes to be determined based on cellular proliferation in the presence of stimulating cells.

Affinity maturation. The increase in average antibody affinity frequently observed during a secondary immune response.

Somatic (hyper)mutation (somatic mutagenesis). A process occurring during B cell maturation that affects the region of Ig genes responsible for increasing antibody specificity.

Superantigens. Antigens that stimulate T cell clones with diverse antigenic specificities but sharing the same TCR V genes. Immunoglobulin superfamily. Molecules possessing domains homologous to immunoglobulin domains. Members of the Ig superfamily include MHC class I and II molecules, the T-cell receptor, CD2, CD3, CD4, CD8, ICAM, VCAM molecules, and certain Fc receptors.

Suppressor T cells (Ts). Functionally distinct populations of T cells that suppress the immune response of other T or B cells or direct the response along an alternative pathway (switching the type of response).

Linkage. The close proximity of two genes on the same chromosome and their joint inheritance.

Network theory. First formulated by Niels Jerne and later further developed, this theory posits that T and B cells mutually regulate each other's activity by recognizing the idiotypes of antigen receptors.

Thymus. A primary lymphoid organ located in the thoracic cavity above The Heart.

Tissue typing. The determination of allotypic variants of MHC molecules in a given individual.

Tolerance. A state of specific immunological unresponsiveness.

Transgenic Animals. Animals whose genome incorporates one or more foreign genes (transgenes). Transgenes are often linked to specific promoters so that they are expressed only by specific tissues for a limited period.

Transformation. Morphological changes in lymphocyte cells associated with the onset of cell division. This term also refers to the changes characteristic of the autonomous division state of a tumor cell.

Transforming growth factor (TGF). This name groups together a cytokine family defined by their ability to stimulate fibroblast growth. They typically also exert an immunosuppressive effect.

Mast cells. Cells present in most tissues near Blood Vessels. They are packed with granules containing inflammatory mediators.

Complementarity-determining regions (CDRs - complementarity determining regions). PARTS OF THE V region of antibody molecules and T-cell receptors responsible for binding an antigen or an antigen-MHC complex.

Bursa of Fabricius. A lymphoepithelial organ in birds where B lymphocytes mature; located at the junction of the hindgut and the cloaca.

Phagocytosis. The process by which a cell engulfs various particles and encloses them in a cytoplasmic vacuole (phagosome).

Platelet-activating factor. A factor secreted by basophils that induces platelet aggregation.

Stem cell factor. A cytokine, also known as Steel factor, essential for the earliest stages of leukocyte development in the bone marrow.

Migration inhibition factor. A group of peptides produced by lymphocytes that can inhibit macrophage migration.

Tumor necrosis factors (TNF). A group of proinflammatory cytokines encoded by MHC genes.

Factors B, P, D, and I. Components of the alternative complement activation pathway.

Phenotype. The observable traits expressed by an individual (cf. Genotype).

Phytohemagglutinin (PHA). A T-cell mitogen.

Fluorescence-activated cell sorter. An instrument used for cell analysis via flow cytometry, enabling cells to be separated based on specific characteristics and various cell populations to be isolated.

Follicular dendritic cells. Antigen-presenting cells located in the B-cell-dependent areas of lymphoid tissue. They retain antigens for long periods.

Phosphorylcholine (PC). A widely used hapten that is also present on the surface of certain microorganisms.

Fab fragment. The portion of an antibody molecule containing the antigen-binding site. It consists of one light chain and a segment of a heavy chain, and can be generated by Enzymatic cleavage of the antibody molecule.

Fc fragment. The portion of an antibody molecule responsible for binding to antibody receptors on cells and to the C1q component of complement.

Helper T cells (Th cells). T cells that provide "help" for The Development of cytotoxic T Lymphocytes and antibody production by B cells. They recognize antigens presented in association with class II MHC molecules.

Chemokinesis. An increase in cell migratory activity characterized by a lack of directional movement.

Chemotaxis. Enhanced directed migration of cells, specifically along a concentration gradient of certain chemotactic factors.

Chimerism. The coexistence of cells from genetically distinct individuals within a single organism.

Germinal centers. Regions within secondary lymphoid tissue where B-Cell Differentiation and isotype switching take place.

J chain. A monomorphic polypeptide incorporated into polymeric IgA and IgM molecules; an essential structural component.

κ chains. One of the two immunoglobulin light chain isotypes.

λ chains. One of the two immunoglobulin light chain isotypes.

Cyclosporine A. An antibiotic used as a T-cell immunosuppressant, particularly valuable for preventing transplant rejection.

Cyclophosphamide. A cytotoxic agent frequently used for immunosuppression.

Cytokines. A collective term for molecules secreted by leukocytes and occasionally other cells that mediate intercellular interactions during the immune response.

Cytotoxic T cells. T lymphocytes capable of lysing virus-infected target cells expressing antigenic peptides in association with MHC class I molecules.

Hinge region. The segment of an immunoglobulin heavy-chain molecule located between the Fc and Fab fragments. It provides molecular flexibility and allows the two antigen-binding sites to operate independently. Typically encoded by a separate exon.

Eicosanoids. Arachidonic acid metabolites, including prostaglandins, leukotrienes, and thromboxanes.

Exon. A protein-coding segment of a gene.

Endothelium. The cell layer lining the inner surface of Blood and Lymphatic vessels.

Endotoxin. A lipopolysaccharide component of the outer membrane of Gram-negative bacteria; activates B cells and macrophages.

Endocytosis. The cellular uptake of extracellular material via phagocytosis or pinocytosis.

Epitope. The specific portion of an antigen molecule that interacts with the antigen-binding site of an antibody or a T-cell receptor.

Effector cells. Lymphocytes and phagocytes that have completed differentiation and carry out their ultimate functions.

Anchor residues. Specific amino acid residues within antigenic peptides that are required for binding to the peptide-binding cleft of an MHC molecule.

B7-1 (CD81) and B7-2 (CD86). Surface molecules on antigen-presenting cells that deliver a potent co-stimulatory signal upon binding to CD28 on T cells.

Bcl-2. An anti-apoptotic molecule transiently expressed on the surface of activated B cells that escape apoptosis.

C1-C9. Components of the classical and lytic pathways of complement activation responsible for inflammatory responses, particle opsonization, and cell membrane lysis.

c-Kit (CD117). The stem cell factor receptor, essential for the Cytology/cytology/16.html">Early stages of leukocyte development.

CTLA-4 (CD152). A T-cell surface molecule that exerts a down-regulatory effect by competing with CD28 for binding to B7 molecules on antigen-presenting cells.

Fas (CD95). A broadly expressed cell-surface molecule that interacts with its ligand, FasL, expressed on the surface of cytotoxic lymphocytes. H-2: The major histocompatibility complex in the mouse. T15: An idiotype associated with antibodies against phosphorylcholine, named after the prototypical TEPC15 myeloma sequence.