ClassSwitching

Class switching (also called isotype switching) occurs after B cells are activated by antigen and interact with CD4+ helper T cells via CD40-CD40 ligand (CD40L) binding, along with co-stimulatory cytokine signals. This process allows a B cell to change the constant region of its heavy chain (switching from IgM/IgD to IgG, IgA, or IgE) while keeping the same variable region, thus preserving antigen specificity. The mechanism involves somatic recombination at switch regions in the immunoglobulin heavy chain locus, catalyzed by activation-induced cytidine deaminase (AID), which deaminates cytosine to uracil, triggering DNA repair pathways that excise intervening DNA. Cytokines direct which isotype is produced: IFN-gamma promotes switching to IgG (important for opsonization and complement activation), IL-4 promotes switching to IgE (important for parasitic defense and allergic responses), IL-5 promotes IgA production (important for mucosal immunity), and TGF-beta also contributes to IgA switching. Clinically, defects in this pathway have major significance: Hyper-IgM syndrome results from CD40L deficiency (X-linked, seen in T cells) or CD40 deficiency, leading to elevated IgM but low IgG, IgA, and IgE, causing recurrent pyogenic infections and opportunistic infections (e.g., Pneumocystis jirovecii). AID deficiency causes a similar Hyper-IgM-like phenotype with normal CD40L. Understanding this pathway is essential for explaining immunodeficiencies, vaccine responses (memory B cell development), and allergic/parasitic immune responses on USMLE Step 1 and Step 2 exams. This process is distinct from somatic hypermutation (affinity maturation), though both occur in germinal centers and both require AID.