Immunoglobulins
immunoglobulins
Immunoglobulins (IG) or antibodies are antigen receptors of the B cells of the adaptive immune response, acquired by jawed vertebrates (or gnathostomata) more than 450 million years ago and found in all extant jawed vertebrate species from fishes to humans. IG are anchored in the membrane of a B cell as part of a signaling B cell receptor (BcR = membrane IG+CD79) or are secreted by plasmocytes. IG recognize antigens in their native (unprocessed) form. IG are made of two identical heavy (H) chains and two identical light (kappa or lambda) chains, encoded by genes located in three major loci: the IG heavy (IGH) locus at 14q32.33, IG kappa (IGK) locus at 2p11.2 and IG lambda (IGL) locus at 22q11.2. Genes outside of these major loci are orphons. There are four IG gene types, variable (V), diversity (D) (only for IGH), joining (J) and constant (C) genes which contribute to the IG chain synthesis. The variable domain at the N-terminal end of each IG chain results from a V-(D)-J rearrangement whereas the remaining of the chain, or constant region, is encoded by a C gene. The combinatorial and junctional diversity together with IG somatic hypermutations create a huge diversity of 1012 specific IG per individual, the limiting factor being only the number of B cells that an organism is genetically programmed to prod