Lymphocyte-like cells in these fish express highly variable lymphocyte receptor genes, which undergo somatic rearrangements reminiscent of the manner in which mammalian immunoglobulin genes are rearranged during development. In summary, allorecognition, the ability to distinguish self from non-self, is basic to all life, unicellular as well as multicellular. The earliest recognition systems were innate, and were based on the recognition of self molecules. The evolution of multicellular forms brought about selective pressures for ever-increasing sophistication to innate immune systems. Adaptive immune systems, based on the recognition of non-self, have arisen independently in two lines of chordates, and exploit molecules and cellular systems which had a previous role in innate immune responses. Allorecognition as it currently exists in mammals can be traced back as the result of sequential modification to immunity mechanisms dating back to some of the earliest multicellular organisms.