C-reactive protein An acute-phase protein that binds to phosphocholine, a constituent of the surface C-polysaccharide of the bacterium Streptococcus pneumoniae and of many other bacteria, thus opsonizing them for uptake by phagocytes.
CRIg (complement receptor of the immunoglobulin family)
A complement receptor that binds to inactivated forms of C3b.
Crohn’s disease   Chronic inﬂammatory bowel disease thought to result from an abnormal overresponsiveness to the commensal gut microbiota.
cross-matching A test used in blood typing and histocompatibility typing to determine whether donor and recipient have antibodies against each other’s cells that might interfere with successful transfusion or grafting.
cross-presentation The process by which extracellular proteins taken up by dendritic cells can give rise to peptides presented by MHC class I molecules. It enables antigens from extracellular sources to be presented by MHC class I molecules and activate CD8 T cells.
cross-priming Activation of CD8 T cells by dendritic cells in which the antigenic peptide presented by MHC class I molecules is derived from an exogenous protein (i.e., by cross-presentation), rather than produced within the dendritic cells directly. Cf. direct presentation.
cryptdins α-Defensins (antimicrobial peptides) made by the Paneth cells of the small intestine.
cryptic epitopes Any epitope that cannot be recognized by a lymphocyte receptor until the antigen has been broken down and processed.
cryptopatches Aggregates of lymphoid tissue in the gut wall that are thought to give rise to isolated lymphoid follicles.
CstF-64 Subunit of cleavage stimulation factor that favors polyadenylation at pAS leading to the secreted form of IgM.
C-terminal Src kinase (Csk) A kinase that phosphorylates the C-terminal tyrosine of Src-family kinases in lymphocytes, thus inactivating them.
CTLA-4 A high-affinity inhibitory receptor on T cells for B7 molecules; its binding inhibits T-cell activation.
C-type lectins Large class of carbohydrate-binding proteins that require Ca2+ for binding, including many that function in innate immunity.
cutaneous lymphocyte antigen (CLA) A cell-surface molecule that is involved in lymphocyte homing to the skin in humans.
CVIDs See common variable immunodeficiencies.
CX3CR1   Chemokine receptor expressed by monocytes, macrophages, NK cells, and activated T cells that binds CXCL1 (Fractalkine).
CXC chemokines One of the two main classes of chemokines, distinguished by a Cys-X-Cys (CXC) motif near the amino terminus. They have names CXCL1, CXCL2, etc. See Appendix IV for a list of individual chemokines.
CXCL12 (SDF-1) Chemokine produced by stromal cells in the dark zone of the germinal center that binds CXCR4 expressed by centroblasts.
CXCL13 Chemokine produced in the follicle and the light zone of the germinal center that binds CXCR5 expressed on circulating B cells and centrocytes.
CXCR5 A chemokine receptor expressed by circulating B cells and activated T cells that binds the chemokine CXCL13 and directs cell migration into the follicle.
cyclic dinucleotides (CDNs) Cyclic dimers of guanylate and/or adenylate monophosphate that are produced by various bacteria as second messengers and detected by STING.
cyclic guanosine monophosphate-adenosine monophosphate (cyclic GMP-AMP or cGAMP) See cyclic dinucleotides (CDNs).