C3 convertase Enzyme complex that cleaves C3 to C3b and C3a on the surface of a pathogen. The C3 convertase of the classical and lectin pathways is formed from membrane-bound C4b complexed with the protease C2a. The alternative pathway C3 convertase is formed from membrane-bound C3b complexed with the protease Bb.
C3(H2O)Bb See fluid-phase C3 convertase.
C3a See anaphylatoxins.
C3b See C3.
C3b2Bb The C5 convertase of the alternative pathway of complement activation.
C3bBb The C3 convertase of the alternative pathway of complement activation.
C3dg Breakdown product of iC3b that remains attached to the microbial surface, where it can bind complement receptor CR2.
C3f A small fragment of C3b that is removed by factor I and MCP to leave iC3b on the microbial surface.
C4   Complement protein of the classical and lectin pathways. C4 is cleaved by C1s to C4b, which forms part of the classical C3 convertase.
C4b-binding protein (C4BP) A complement-regulatory protein that inactivates the classical pathway C3 convertase formed on host cells by displacing C2a from the C4bC2a complex. C4BP binds C4b attached to host cells, but cannot bind C4b attached to pathogens.
C4b2a C3 convertase of the classical and lectin pathways of complement activation.
C4b2a3b C5 convertase of the classical and lectin pathways of complement activation.
C5 convertase Enzyme complex that cleaves C5 to C5a and C5b.
C5a See anaphylatoxins.
C5a receptor The cell-surface receptor for the pro-inﬂammatory C5a fragment of complement, present on macrophages and neutrophils.
C5b Fragment of C5 that initiates the formation of the membrane-attack complex (MAC).
C5L2 (GPR77) Non-signaling decoy receptor for C5a expressed by phagocytes.
C6, C7, C8, C9 Complement proteins that act with C5b to form the membrane-attack complex, producing a pore that leads to lysis of the target cell.
calcineurin A cytosolic serine/threonine phosphatase with a crucial role in signaling via the T-cell receptor. The immunosuppressive drugs cyclosporin A and tacrolimus inactivate calcineurin, suppressing T-cell responses.
calmodulin Calcium-binding protein that is activated by binding Ca2+; it is then able to bind to and regulate the activity of a wide variety of enzymes.
calnexin A chaperone protein in the endoplasmic reticulum (ER) that binds to partly folded members of the immunoglobulin superfamily of proteins and retains them in the ER until folding is complete.
calprotectin A complex of heterodimers of the antimicrobial peptides S100A8 and S100A9, which sequester zinc and manganese from microbes. Produced in abundance by neutrophils, and in lesser amounts by macrophages and epithelial cells.
calreticulin A chaperone protein in the endoplasmic reticulum that, together with ERp57 and tapasin, forms the peptide-loading complex that loads peptides onto newly synthesized MHC class I molecules.
cancer immunoediting A process that occurs during the development of a cancer when it is acquiring mutations that favor its survival and escape from immune responses, such that cancer cells with these mutations are selected for survival and growth.
cancer-testis antigens Proteins expressed by cancer cells that are normally expressed only in male germ cells in the testis.
capping A process occurring in the nucleus in which the modified purine 7-methylguanosine is added to the 5ʹ phosphate of the first nucleotide of the RNA transcript.
capsular polysaccharides See capsulated bacteria.