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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.
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