Basic structure of proteins of the immunoglobulin supergene family

The immunoglobulin molecules of the T-cell receptors
(TCR) and the class I and class II MHC
molecules (1) are basically similar. They consist
of variable Ig-like domains (V), constant Ig-like
domains (C), or primordial Ig-like domains (H).
Although their genes are located on different
chromosomes, the gene products form
functional complexes with each other. Others,
such as the V, D, and J gene segments of all antigen
receptors and their genes for the C domain
lie close together in gene clusters. In addition,
genes of the MHC loci and for the two CD8
chains lie together. The basic structures of accessory
molecules (2) such as CD2, CD3, CD4,
CD8, and thymosine 1 (Th-1) are relatively
simple. Other members of the Ig superfamily
(3) are the Fc receptor II (FcRII); polyimmunoglobulin
receptor (pIgR), which transports antibodies
through the membranes of epithelial
cells; NCAM (neural cell adhesion molecules);
and PDGFR (platelet-derived growth factor receptor)
(3).