Immunology Terms Definition

immunology lec.2 b.sc. prof dr. abdalnabi almaamori

immunology terms definition
immunity is defined as resistance to infectious disease and the collection of cells and tissues that protects the body from infection is known as the immune system. the coordinated reaction of the cells of the immune system to a pathogen is known as the immune response.
• antigen is a general term that applies to molecules that bind to antibodies or t cell receptors with high affinity. antigens come in many forms: for example, small molecules in the environment and a huge array of bacterial and viral surface proteins might all act as antigens. many times, you will see the terms antigen and microbe used interchangeably, since most antigens are derived from larger pieces of a microbe.
• lymphocytes are cells found in the blood, lymphoid tissues and most organs of the body that express receptors for specific antigens and mediate immune responses. the lymphocytes that we will talk the most about are b cells and t cells. (b cell = b lymphocyte t cell = t lymphocyte)
• when b and t cells become activated, they divide and mature into effector cells that actually do the job of fighting the microbe. mature b cells are called plasma cells plasma cells secrete antibodies, which are glycoprotein molecules that bind antigens with high affinity and help to eliminate those antigens. mature t cells are called effector t cells. effector t cells either assist (“help”) leukocytes to kill ingested microbes or directly kill infected cells.
• humoral immunity is the type of adaptive immunity that is mediated by antibodies produced by plasma cells. humoral immunity is the main mechanism for defending against extracellular microbes and their toxins.
• cell-mediated immunity is the type of adaptive immunity mediated by t lymphocytes cell-mediated immunity is the main defense mechanism against microbes that survive within phagocytes (i.e. the bacteria that causes tuberculosis) or that infect the cytosol of non-phagocytic cells (i.e. many viruses).
• cytokines are secreted proteins that work as mediators of immune and inflammatory reactions. cytokines provide a mechanism for cells of the immune system to “talk” to one another to coordinate a response. interleukin is another term for a cytokine that acts on other leukocytes.

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 deletingrious 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.
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-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.
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.
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.
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.
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.
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..
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.
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.
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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.
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enchancing antibodies: antibodies which enhance the survival of a graft or of a tumour.
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.
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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.
hla complex: see major histocompatibility complex .
haplotype: a particular combination of closely linked genes on a chromosome inherited from one patient.
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.
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 deletingrious 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.
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.
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).
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.
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).
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.
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.