Which molecules are required for isotype switching?

Which molecules are required for isotype switching?

• A. CD40 on B cells and CD40L on T cells
• B. CD28 on T cells and B7 on APCs
• C. Fc receptor engagement on NK cells
• D. TGF-β signaling in B cells

Answer: (A)

Isotype switching requires help from helper T cells delivered through CD40 on B cells engaging CD40L on T cells. This interaction provides the essential second signal that allows B cells to undergo class switch recombination, with activation-induced cytidine deaminase mediating the recombination at switch regions to replace the constant region of the heavy chain. The cytokine environment then influences which downstream isotype is produced (for example, IL-4 promotes IgG1/IgE, TGF-β promotes IgA), but without CD40-CD40L contact, B cells cannot switch from IgM. The other options don’t provide this critical T cell–B cell cooperation: the CD28–B7 axis is more about initial T cell activation; Fc receptor engagement on NK cells relates to antibody-dependent cellular cytotoxicity; TGF-β signaling alone can influence isotype outcomes but cannot drive CSR without CD40 signaling.

Isotype switching requires help from helper T cells delivered through CD40 on B cells engaging CD40L on T cells. This interaction provides the essential second signal that allows B cells to undergo class switch recombination, with activation-induced cytidine deaminase mediating the recombination at switch regions to replace the constant region of the heavy chain. The cytokine environment then influences which downstream isotype is produced (for example, IL-4 promotes IgG1/IgE, TGF-β promotes IgA), but without CD40-CD40L contact, B cells cannot switch from IgM. The other options don’t provide this critical T cell–B cell cooperation: the CD28–B7 axis is more about initial T cell activation; Fc receptor engagement on NK cells relates to antibody-dependent cellular cytotoxicity; TGF-β signaling alone can influence isotype outcomes but cannot drive CSR without CD40 signaling.