Growing up on the streets: why B-cell development differs from T-cell development

B-cell development differs significantly from T-cell development in that ne gative selection of autoreactive B cells can occur in the same microenviron ment in which productive immune responses begin. Here, Sarah Townsend and c olleagues discuss how this 'growing up on the streets' might provide a mech anism that fills holes in the B-cell repertoire, much as major histocompati bility complex polymorphism fills holes in the T-cell repertoire.