University of Central Florida (UCF) PCB3233 Immunology Exam 2 Practice Test

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What is class switch recombination and what enzyme mediates it?

CSR changes the antibody isotype to a different heavy-chain class via RAG enzymes.

CSR changes the antibody's antigen-binding specificity.

CSR increases the number of light chains.

CSR changes the antibody isotype (e.g., IgM to IgG) and is mediated by AID.

Class switch recombination changes the heavy-chain constant region to produce a different antibody isotype (for example IgM to IgG, IgA, or IgE) while keeping the same antigen-binding specificity. The variable region that recognizes the antigen stays the same, so the antibody still binds the same target, but its effector functions change because of the new constant region.

This switch is driven by activation-induced cytidine deaminase (AID). AID targets the switch regions upstream of the constant region gene segments, deaminating cytosine bases and initiating DNA breaks. Through DNA repair and recombination, the VDJ variable region is now joined to a different constant region gene, yielding a different isotype. Cytokines from T helper cells influence which isotypes are favored during CSR.

CSR does not alter antigen specificity—that’s determined by the variable region and is affected by processes like somatic hypermutation, not CSR. It also doesn’t change the number of light chains, and it isn’t initiated by RAG (which handles V(D)J rearrangement earlier in B cell development).