class I MHC protein One of two classes of major histocompatibility complex (MHC) protein. Found on the surface of almost all vertebrate cell types, where it can present foreign peptides derived from a pathogen such as a virus to cytotoxic  T cells. (Figures 24–35 and 24–36A) class II MHC protein One of two classes of major histocompatibility complex (MHC) protein. Found on the surface of various antigen-presenting cells, where it presents peptides to helper and regulatory T cells. (Figures 24–35 and 24–36B) class switching   Change from making one class of immunoglobulin (for example, IgM) to making another class (for example, IgG) that many B cells undergo during the course of an adaptive immune response. Involves DNA rearrangements called class-switch recombination. (Figure 24–30) class-switch recombination   An irreversible change at the DNA level when a B cell switches from making IgM and IgD to making one of the secondary classes of immunoglobulin.
classical cadherins   Family of cadherin proteins, including E-cadherin, N-cadherin, and P-cadherin, that are closely related in sequence throughout their extracellular and intracellular domains.
clathrin   Protein that assembles into a polyhedral cage on the cytosolic side of a membrane so as to form a clathrin-coated pit, which buds off by endocytosis to form an intracellular clathrin-coated vesicle. (Figure 13–6) clathrin-coated pits   Specialized regions typically occupying about 2% of the total plasma membrane area at which the endocytic pathway often begins.
clathrin-coated vesicles Coated vesicles that transport material from the plasma membrane and between endosomal and Golgi compartments.
cleavage (1) Physical splitting of a cell into two.
(2) Specialized type of cell division seen in many early embryos whereby a large cell becomes subdivided into many smaller cells without growth.
clonal selection   From a population of T and B lymphocytes with a vast repertoire of randomly generated antigen-specific receptors, a given foreign antigen activates (selects) only those lymphocyte clones that display a receptor that fits the antigen. Explains how the adaptive immune system can respond to millions of different antigens in a highly specific way.  (Figure 24–15) co-receptor   In immunology: an accessory receptor on  B cells or T cells that does not bind antigen but binds to a co-stimulatory signal and helps activate the lymphocyte, by helping to activate an intracellular signaling pathway.
co-stimulatory signal   In immunology: a secreted or membrane-bound signal protein that helps activate an antigen-responding B cell or T cell.
co-translational   Occurring as translation proceeds. Examples include the import of a protein into the endoplasmic reticulum before the polypeptide chain is completely synthesized (co-translational translocation, Figure 12–32), and the folding of a nascent protein into its secondary and tertiary structure as it emerges from a ribosome. (Figure 6–79) coat-recruitment GTPases Members of a family of monomeric GTPases that have important roles in vesicle transport, being responsible for coat assembly at the membrane.
coated vesicle Small membrane-enclosed organelle with a cage of proteins (the coat) on its cytosolic surface. Formed by the pinching off of a coated region of membrane (coated pit). Some coats are made of clathrin, others are made from other proteins.
codon   Sequence of three nucleotides in a DNA or mRNA molecule that represents the instruction for incorporation of a specific amino acid into a growing polypeptide chain.