Figure 2.

Diversification mechanism of antigen receptors (TCRs and BCRs) in jaw vertebrates. Somatic recombination is the mechanism of V(D)J segments recombination that occurs in maturating T- and B- lymphocytes at an early stage of development in the thymus and bone marrow, respectively. This process mediates by a protein complex led by the RAG protein that initiates random rearrangement of V, J, and D segments, which leads to the emergence of novel antigen-binding regions of TCRs and BCRs. An important role in repertoire diversification belongs to TdT, a template-independent DNA polymerase that randomly adds non-template nucleotides to the coding-end of an open hairpin, thereby forming an N-insert immediately after the P-insert formation. Further reorganization of the variable region is possible only in B cells during the process of somatic hypermutations that mediate by AID in the course of affinity maturation. This process relies on the gene conversion-like mechanism during the reparation of double-strand DNA breaks. RAG, recombination-activating gene; TdT, terminal deoxynucleotidyl transferase; AID, activation-induced deaminase. The illustration is not drawn to scale.