Like physicists striving to develop a unified field theory, immunologists are attempting to bring order to the microcosmos of defense reactions. Indications are that one of the most important constants in this microcosmos is the major histocompatibility complex (MHC) of the species. A test of any interpretation of the MHC's role in immunity is how well it explains this system's polymorphism. One of the most crucial questions an MHC hypothesis must answer is: Why are there so many alleles at this complex?