The end can justify the means.

The generation of a pre-immune repertoire of B lymphocyte specificities is a prerequisite for subsequent antigen-dependent antibody responses. In avian species the bursa of Fabricius plays a central role in the development of B lineage cells and in the generation of antibody diversity. Apart from its segregation into the bursa, the pathway of avian B cell development differs from that seen in mammalian species by a number of other criteria. In particular the chicken has evolved a remarkable mechanism for the diversification of immunoglobulin V region genes. Here we review some of the key features by which avian and mammalian B cell development differ.