Constant (C) Region 
 
 The heavy and light chains of immunoglobulins contain (a) variable (V) regions of approximately equal size (ie, about 110 amino acid residues), and (b) constant (C) regions that contain a multiple of 110 residues, one for the L chain and three or four for the H chains, depending upon the isotype. Whereas the V regions are involved in antigen recognition, C regions are devoted to effector functions, such as complement fixation, transplacental passage, or attachment to various cell types. C regions are organized on a basic domain structure, corresponding to the 110-residue subunit of Ig with an autonomous three-dimensional protein structure, as shown by X-ray crystallography. This structure, known as the immunoglobulin constant domain fold, provides the basis for the general organization of a large number of proteins, known as the Ig superfamily.
 The existence of different C regions was first shown by immunochemistry, because specific antisera could distinguish two types of light chains, known as l) light chains, and five discrete isotypes of H chains, which dictate the organization into the five classes of immunoglobulins (IgA, IgD, IgE, IgG, IgM). Definitive understanding of the basis of isotype structure was obtained by protein sequencing and finally by DNA sequencing. Within each Ig class, one may distinguish subclasses, as in the case of the g chain or the a chains in humans.
Most of the effector functions of immunoglobulins are supported by the last two or three COOH-terminal domains of the heavy chains, depending upon the class. The most interesting feature of constant regions is their ability to bind to a receptor family, termed Fc receptors (FcRs), because binding involves the Fc region of Ig. FcRs represent adaptors that can confer on a variety of cell types any antibody specificity, depending on which Ig will bind to the receptor. There is an FcR specific of each soluble isotype, IgG, IgA, and IgE. One distinguishes FcRs of high affinity (FcR type I) that fix Ig monomers and FcRs of low affinity (FcR type II) that bind Ig only when aggregated or as multivalent complexes. Depending on which cell type FcR is anchored, they express an cytoplasmic tail of variable size, which may result from alternative splicing and that contains various signaling motifs such as an immunoreceptor tyrosine-based activation motif and an immunoreceptor tyrosine-based inhibition motif. Fc RI, present on mast cells, basophils, and eosinophils, plays a major role in anaphylaxis, because it will fix monomeric IgE. Upon cross-linking by antigen, this FcR is triggered, leading to the cascade of events that are responsible for degranulation of the corresponding cells and thus to the liberation of pharmacologically active molecules that account for immediate hypersensitivity. Fc RII (CD23) binds aggregated IgE, or IgE already cross-linked by antigen. It is present at the surface of many cell types, including B cells. It may also be cleaved and release several soluble products that have a cytokine-like activity. FcgRI is found on monocytes, eosinophils, and neutrophils and binds to monomeric IgG, whereas FcRaI is present on macrophages, neutrophils, and T and B cells.
This demonstrates the extraordinary plasticity and amplification of molecules of the immune system and illustrates the huge diversity of functions that are linked to the constant regions of immunoglobulins.