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The vertebrate immune system is remarkable because it can recognize virtually any foreign material or antigen and yet not react against itself except in those rare cases when autoimmunity results. Over the last 20 years the mechanisms by which antigens are recognized by the B cell antigen receptor (Ig) and the T cell antigen receptor (TcR) have been well characterized. Both of these groups of proteins are built up from Ig-like domains. The variable or V domain provides the specificity to react with the range of foreign antigens and constant or C domains provide the effector functions. One fascinating question is how such a complex system evolved, and this is the topic addressed by a paper in this issue of PNAS by Strong et al. (1). The domain structure of Igs and TcRs is illustrated in Fig. 1. Both V and C domains have similar structures called the Ig fold, which is formed by antiparallel β-strands arranged into two sheets linked by a disulfide bond (Fig. 2). V domains can be distinguished from C domains because they show the variability associated with antigen recognition and also the domain is longer with two extra strands being accommodated in the middle of the domain (C′ and C" in Fig. 2). The binding site is built up from two chains in both the TcR (e.g., αβ) and Igs (heavy and light chains). Additional chains are associated with both Ig and the TcR that mediate signaling to the cytoplasm. CD79a and CD79b associate with membrane Ig and contain an immunoreceptor tyrosine activation motif (ITAM) motif involved in signal transduction. The TcR heterodimer is associated with the CD3 chains including ζη, which contain ITAM motifs. The variability that allows Ig and TcR to bind such a diverse range of proteins is determined largely …

Ig-like domains: Evolution from simple interaction molecules to sophisticated antigen recognition