RAG expression in B cells in secondary lymphoid tissues.

The clonal selection theory accounts for many of thecharacteristics that define the adaptive immune system,such as specificity, memory, and tolerance (Burnet 1959;Talmage 1959). This model rests on the assumption thateach lymphocyte expresses antigen receptors of a singlespecificity. It was, therefore, a surprise when in 1996 twogroups provided evidence that recombination activatinggene (RAG) 1 and RAG2 were expressed in mouse germinal center (GC) B cells (Han et al. 1996; Hikida et al.1996). Before these reports, RAG expression was thoughtto be terminated before B cells left the bone marrow(Grawunder et al. 1995). Therefore, the presence of RAGin GCs was proposed to be due to reinduction...