Abstract

Adaptive immunity of vertebrates is centered on leukocytes, lymphocytes and other cells of the hematopoietic system like the antigen presenting cells.

By scrutinizing the data bases of various invertebrates, from Cnidarians to Tunicates, genes related to the receptors that build the different leukocyte lineages of vertebrates have been found (only lectins and immunoglobulin superfamily members have been investigated), sometimes with high homology and often on conserved linkage groups. One of these linkage groups is that of the leukocyte receptor complex and neighbouring region of the human chromosome 19q and 3 proposed paralogs.

Following the hypothesis of 2 genome duplications occurring during the evolution of chordates one could imagine that preduplication ancestors should have only one such region. Indeed it seems to be the case in the tunicate Ciona. So, perhaps, the basic leukocyte population context of the future vertebrates was common to the ancestor of both jawless and jawed vertebrates, that ultimately diverged by the choice of different protein families for generating somatically their specific antigen receptors.