A VERTEBRATE is an attractive culture medium for viruses, bacteria, fungi, and metazoan parasites. It is therefore understandable that vertebrates have had to develop a body defense mechanism directed toward these pathogens for their emergence and survival. This defense mechanism is the See also pp 1834 and 1837. immune system. The crucial event in mounting an immune response is the recognition of chemical markers that are present on infectious organisms and foreign bodies. The molecules entrusted to this task are the B-cell-synthesized antibodies and the antigen-specific T-cell receptors (TCRs). Antigens are numerous and structurally diverse. The fact that the body can produce antibodies against many artificially made chemicals and can discriminate among them attests to the vastness of the antigenic world. The antigen-binding specificity of an antibody or a TCR is determined by the amino acid sequence of the polypeptide chains that compose these proteins. The enormous diversity of antigens,