combinatorial diversity The diversity among antigen receptors generated by combining separate units of genetic information, comprising two types. First, receptor gene segments are joined in many different combinations to generate diverse receptor chains; second, two different receptor chains (heavy and light in immunoglobulins; α and β, or γ and δ, in T-cell receptors) are combined to make the antigen-recognition site.
commensal microbiota, commensal microorganisms Microorganisms (predominantly bacteria) that normally live harmlessly in symbiosis with their host (for example the gut bacteria in humans and other animals). Many commensals confer a positive benefit on their host in some way.
common β chain A transmembrane polypeptide (CD131) that is a common subunit for receptor of the cytokines IL-3, IL-5, and GM-CSF.
common γ chain (γc) A transmembrane polypeptide chain (CD132) that is common to a subgroup of cytokine receptors.
common lymphoid progenitor (CLP) Stem cell that can give rise to all the types of lymphocytes with the exception of innate lymphoid cells (ILCs).
common mucosal immune system The mucosal immune system as a whole, the name reﬂecting the fact that lymphocytes that have been primed in one part of the mucosal system can recirculate as effector cells to other parts of the mucosal system.
common myeloid progenitor (CMP) Stem cells that can give rise to the myeloid cells of the immune system—macrophages, granulocytes, mast cells, and dendritic cells of the innate immune system. This stem cell also gives rise to megakaryocytes and red blood cells.
common variable immunodeficiencies (CVIDs) A relatively common deficiency in antibody production in which only one or a few isotypes are affected. It can be due to a variety of genetic defects.
complement A set of plasma proteins that act together as a defense against pathogens in extracellular spaces. The pathogen becomes coated with complement proteins that facilitate its removal by phagocytes and that can also kill certain pathogens directly. Activation of the complement system can be initiated in several different ways. See classical pathway, alternative pathway, lectin pathway.
complement activation The activation of the normally inactive proteins of the complement system that occurs on infection. See classical pathway, alternative pathway, lectin pathway.
complement proteins See C1, C2, C3, etc..
complement receptors (CRs) Cell-surface proteins of various types that recognize and bind complement proteins that have become bound to an antigen such as a pathogen. Complement receptors on phagocytes enable them to identify and bind pathogens coated with complement proteins, and to ingest and destroy them. See CR1, CR2, CR3, CR4, CRIg, and the C1 complex.
complement regulatory proteins Proteins that control complement activity and prevent complement from being activated on the surfaces of host cells.
complement system A set of plasma proteins that act together as a defense against pathogens in extracellular spaces. The pathogen becomes coated with complement proteins that facilitate its removal by phagocytes and that can also kill certain pathogens directly. Activation of the complement system can be initiated in several different ways. See classical pathway, alternative pathway, lectin pathway.
complementarity-determining regions (CDRs)   Parts of the V domains of immunoglobulins and T-cell receptors that determine their antigen specificity and make contact with the specific ligand. The CDRs are the most variable part of antigen receptor, and contribute to the diversity of these proteins. There are three such regions (CDR1, CDR2, and CDR3) in each V domain.
conformational epitopes, discontinuous epitopes Antigenic structure (epitope) on a protein antigen that is formed from several separate regions in the sequence of the protein brought together by protein folding. Antibodies that bind conformational epitopes bind only native folded proteins.