Early simultaneous appearance of antigen binding cells in the foetal sheep

THE two classes of theory on the generation of diversity in the immune system differ as to when the genetic capacity for producing a full repertoire of antibody clonotypes is achieved. The germline theory 1 proposes that diversification occurs mainly on an evolutionary timescale at the species level, with the immune capacity of an individual predetermined at the instant of fertilisation; the various somatic theories 2–8 propose a much briefer period of diversification from a minimal complement of V genes, occurring during the ontogeny of the individual or even later in life. We have attempted to study the time course of generation of diversity by following the appearance of lymphoid cell receptors binding any of several antigens in foetal sheep.