Folia Biologica
Journal of Cellular and Molecular Biology, Charles University 

As every high-school graduate knows, animals are classified into two large groups - those that have vertebrae and those that do not. What every high-school graduate should also know (but probably doesnít) is that the vertebrates are further divided into jawless (Agnatha) and jawed (Gnathostomata). The jawed vertebrates are distinguished not only by the possession of jaws, but also by a suite of other characters, including an immune system based on the use of the "holy trinity" or three sets of molecules - the major histocompatibility complex (MHC) molecules, the T-cell receptors (TCRs), and the B-cell receptors or immunoglobulins (BCRs or Igs; see Klein et al., 2000). The system is referred to as adaptive or acquired, but both designations are misleading and inaccurate; a term like anticipatory immune system (Klein, 1989) would have, perhaps, been more appropriate, but it did not take. Fortunately, the acronyms for all three names are the same - AIS. Immunologists originally believed that the AIS is present in both vertebrates and nonvertebrates (Cooper, 1976), but I have argued for some time (Klein, 1989, 1997) that the system is restricted to vertebrates and possibly to jawed vertebrates only. The first part of this proposition is now generally accepted; the second part is still debated, but this debate, too, is close to resolution. If we accept the holy trinity as the defining feature of the AIS, mounting evidence indicates that the agnathans lack the system, for the trinity appears to be present in all gnathostomes, but absent in jawless vertebrates.

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