class switching
Summary
Class switching (isotype switching) is the process by which activated B cells change the constant region of their immunoglobulin heavy chain (e.g., from IgM to IgG, IgA, or IgE) without altering antigen specificity. It requires CD40-CD40L interaction with T helper cells and cytokine signals, and occurs in germinal centers via the enzyme activation-induced cytidine deaminase (AID).
Detail
Class switching allows B cells to change the effector function of the antibodies they produce while retaining the same antigen-binding variable region (VDJ segment). This occurs through a DNA recombination event called class switch recombination (CSR), which occurs in germinal centers of secondary lymphoid organs and is mediated by AID, which deaminates cytosine to uracil in switch regions of the immunoglobulin heavy chain locus, leading to double-strand breaks and recombination that deletes intervening constant region genes.
Key requirements: (1) CD40 on B cell binding CD40L (CD154) on activated Th cells - deficiency causes Hyper-IgM syndrome (X-linked, elevated IgM, low IgG/IgA/IgE, recurrent pyogenic infections); (2) Cytokine signals that determine which isotype is produced - IL-4 promotes switching to IgE (and IgG1 in mice), IL-5 promotes IgA, IFN-gamma promotes IgG2a/IgG3, TGF-beta promotes IgA switching.
Clinical relevance: AID deficiency causes autosomal recessive Hyper-IgM syndrome with similar phenotype to CD40L deficiency. Selective IgA deficiency, the most common primary immunodeficiency, may involve defective class switching to IgA. Understanding class switching is essential for understanding vaccine responses (memory B cells producing high-affinity IgG after repeated exposure), allergic reactions (IgE production), and mucosal immunity (IgA).
Distinguish from somatic hypermutation, which alters the variable region to increase antibody affinity (affinity maturation) and also occurs in germinal centers via AID, but changes antigen-binding specificity/affinity rather than isotype.
Sources
- First Aid for the USMLE Step 1
- Janeway's Immunobiology
- Kuby Immunology
- Robbins and Cotran Pathologic Basis of Disease
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