class switching, class switch recombination A somatic gene recombination process in activated B cells that replaces one heavy-chain constant region with one of a different isotype, switching the isotype of antibodies from IgM to the production of IgG, IgA, or IgE. This affects the antibody effector functions but not their antigen specificity. Also known as isotype switching. Cf. somatic hypermutation.
classes The class of an antibody is defined by the type of heavy chain it contains. There are five main antibody classes: IgA, IgD, IgM, IgG, and IgE, containing heavy chains α, δ, μ, γ, and ε, respectively. The IgG class has several subclasses. See also isotypes.
classical C3 convertase The complex of activated complement components C4b2a, which cleaves C3 to C3b on pathogen surfaces in the classical pathway of complement activation.
classical MHC class I genes MHC class I genes whose proteins function by presenting peptide antigens for recognition by T cells. Cf. nonclassical MHC class Ib.
classical monocyte The major form of monocyte in circulation capable of recruitment to sites of inﬂammation and differentiation into macrophages.
classical pathway The complement-activation pathway that is initiated by C1 binding either directly to bacterial surfaces or to antibody bound to the bacteria, thus ﬂagging the bacteria as foreign. See also alternative pathway, lectin pathway.
classically activated macrophage  See M1 macrophages.
cleavage stimulation factor A multi-subunit protein complex involved in the modification of the 3ʹ end of pre-messenger RNA for the addition of the polyadenine (polyA) tail.
clonal deletion The elimination of immature lymphocytes when they bind to self antigens, which produces tolerance to self as required by the clonal selection theory of adaptive immunity. Clonal deletion is the main mechanism of central tolerance and can also occur in peripheral tolerance.
clonal expansion The proliferation of antigen-specific lymphocytes in response to antigenic stimulation that precedes their differentiation into effector cells. It is an essential step in adaptive immunity, allowing rare antigen-specific cells to increase in number so that they can effectively combat the pathogen that elicited the response.
clonal selection theory The central paradigm of adaptive immunity. It states that adaptive immune responses derive from individual antigen-specific lymphocytes that are self-tolerant. These specific lymphocytes proliferate in response to antigen and differentiate into antigen-specific effector cells that eliminate the eliciting pathogen, and into memory cells to sustain immunity. The theory was formulated by Macfarlane Burnet and in earlier forms by Niels Jerne and David Talmage.
clone A population of cells all derived from the same progenitor cell.
clonotypic   Describes a feature unique to members of a clone. For example, the distribution of antigen receptors in the lymphocyte population is said to be clonotypic, as the cells of a given clone all have identical antigen receptors.
Clostridium difficile Gram-positive anaerobic toxogenic spore-forming bacterium frequently associated with severe colitis following treatment with certain broad-spectrum antibiotics.
c-Maf A transcription factor acting in the development of TFH cells.
coagulation system A collection of proteases and other proteins in the blood that trigger blood clotting when blood vessels are damaged.
coding joint   DNA join formed by the imprecise joining of a V gene segment to a (D)J gene segment during recombination of the immunoglobulin or T-cell receptor genes. It is the joint retained in the rearranged gene. Cf. signal joint.
codominant Describes the situation in which the two alleles of a gene are expressed in roughly equal amounts in the heterozygote. Most genes show this property, including the highly polymorphic MHC genes.
collectins A family of calcium-dependent sugar-binding proteins (lectins) containing collagen-like sequences. An example is mannose-binding lectin (MBL).