Lymphocytes detect the antigens in the environment

by means of antibodies on the surface of B cells and T-cell receptors (TCR) on the T cells. With the diverse and expanding array of antigens, the generation of antibody/TCR diversity using limited genetic resources remained a question that baffled scientists for decades. An almost limitless number of antigens exist in the environment and recent research suggests that among the millions of lymphocytes, each one expresses a structurally different antigen receptor to combat this plethora of antigens. How is the genetic information for all of these antigen receptors encoded in the DNA? Do cells carry enough DNA to encode all the antibody specificities? Or is it that random mutations generate this enormously diverse repertoire of antibodies? Two theories arose initially to answer these questions. Somatic STA-9090 mouse mutation/variation theory BAY 80-6946 order suggested that a few inherited genes, with time, encountered mutations or recombinations to encode each antibody.[1] In contrast, germline theory proposed that the genome contains a large repertoire of antigen receptor genes and each of them encodes for separate, specific antibody.[2] Arguments supporting

and opposing these theories were put forward and remained unresolved for several years. In this review, we summarize the basic principles that presently govern the generation of diversity of antibody and TCR with special emphasis on V(D)J recombination. We also discuss the role of recombination activating genes (RAGs) in the generation of antibody diversity and chromosomal translocations. In the early 1990s, it was shown that isothipendyl two tightly linked genes, RAG1 and RAG2, which were unique to vertebrates, were responsible for the generation of antigen receptor diversity.[3, 4] An elegant series of experiments involving genomic DNA transfections into mouse

3T3 fibroblasts lacking V(D)J recombination activity, showed that the transfer of a single genomic locus could make these cells proficient for V(D)J recombination.[5] Following this, using the technique of ‘genome walking’, the RAG1 gene was discovered. Comparative sequence analysis of RAG1 genes from various species indicated that they were evolutionarily conserved.[3] Further studies demonstrated that the locus contained two closely linked genes, RAG1 and RAG2 on chromosome 11p in humans and chromosome 2p in mice.[4, 6] The coding and 3′ untranslated sequences of RAG1 and RAG2 were contained in a single exon.[6] The proteins encoded by the RAG genes play a crucial role in the generation of antigen receptor diversity as discussed below. There are two major antigen receptors for the lymphoid system, antibodies and TCR in B and T cells, respectively. Antibodies or immunoglobulins are glycoproteins that are either secreted out from B cells or remain bound to their membrane.