RAG1/2 recombinases are essential for which process?

• A. V(D)J recombination
• B. Somatic hypermutation
• C. Class switch recombination
• D. Antigen presentation by MHC II

Answer: (A)

RAG1/2 recombinases are the enzymes that initiate the rearrangement of antigen receptor genes in developing B and T cells. They recognize the recombination signal sequences flanking V, D, and J gene segments and introduce DNA double-strand breaks. This cleavage allows the segments to be rejoined in new combinations by the cell’s nonhomologous end-joining machinery, generating diverse V(D)J coding sequences that form the variable regions of B-cell receptors and T-cell receptors. Without RAG1/2 activity, lymphocytes cannot assemble functional antigen receptors and cannot mature properly.

Somatic hypermutation and class switch recombination rely on activation-induced cytidine deaminase (AID), not RAG, and occur later in mature B cells during immune responses. Antigen presentation by MHC II is a separate process involving processing of extracellular proteins and loading of peptides onto MHC II molecules in antigen-presenting cells, not related to RAG function.