B cells initially express IgM and can switch to other isotypes (IgG, IgA, IgE) through class switch recombination (CSR), a DNA recombination process replacing the IgM constant region with another. AID initiates switching by generating U:G mismatches in switch regions; mismatch repair generates double-strand breaks that are repaired by NHEJ, deleting intervening DNA segments. Different Th cell subsets (Th1, Th2, Th17) drive switching to specific isotypes through cytokine signaling (IFN-γ→IgG, IL-4→IgE, TGF-β→IgA).
Diagram switch regions upstream of each constant domain and how CSR deletes intervening sequences. Map Th1/Th2/Th17 cytokines to their induced isotypes.
Every B cell begins life expressing IgM on its surface, but the immune system needs antibodies with different functional properties for different situations — IgG to opsonize bacteria in the blood, IgA to protect mucosal surfaces, IgE to combat parasites. Class switch recombination (CSR) is the DNA-level mechanism that changes the antibody's constant region (and therefore its isotype and effector function) while preserving the same antigen-binding variable region. The B cell keeps recognizing the same target but equips itself with a different weapon.
To understand the mechanism, picture the immunoglobulin heavy chain locus. After the rearranged VDJ region (which encodes antigen specificity), the constant region genes are arrayed in a fixed order: Cμ (IgM), Cδ (IgD), Cγ3, Cγ1, Cα1, Cγ2, Cγ4, Cε, Cα2. Upstream of each constant region gene (except Cδ) lies a switch (S) region — a stretch of repetitive DNA sequences 1-10 kilobases long. CSR works by physically deleting the DNA between two switch regions. The enzyme activation-induced cytidine deaminase (AID), which you encountered in the context of somatic hypermutation, initiates the process by deaminating cytosines to uracils in the donor switch region (Sμ) and a downstream target switch region. The resulting U:G mismatches are processed by base excision repair (UNG) and mismatch repair machinery, generating double-strand breaks in both switch regions. The cell then joins the broken ends by non-homologous end joining (NHEJ), looping out and deleting the intervening DNA. The VDJ segment is now directly upstream of the new constant region gene, producing a new antibody isotype.
The choice of which isotype to switch to is not random — it is directed by cytokine signals from helper T cells. This is one of the most elegant examples of immune regulation: the T helper cell subset activated during an immune response determines the antibody class that B cells produce. IFN-γ (produced by Th1 cells during intracellular infections) drives switching to IgG1 and IgG3, which are excellent at opsonization and complement activation. IL-4 (produced by Th2 cells during parasitic infections and allergic responses) drives switching to IgE, which arms mast cells and eosinophils. TGF-β (prominent at mucosal surfaces) drives switching to IgA, the dominant antibody in secretions. These cytokines work by inducing germline transcription through the target switch region, opening the chromatin and making it accessible to AID — the switch region that is transcribed is the one that gets recombined.
Two important features distinguish CSR from other recombination events. First, it is irreversible — the deleted DNA is lost as a circular episome that is eventually degraded, so a B cell that has switched to IgG cannot switch back to IgM. However, sequential switching is possible: a cell that switched to IgG can subsequently switch to IgE or IgA if the downstream switch regions remain intact. Second, CSR happens in the germinal center during T cell-dependent B cell responses, coordinated with somatic hypermutation and affinity maturation. This means the antibodies produced after class switching are not only of a different isotype but also of higher affinity — the immune system simultaneously upgrades both the targeting precision and the effector capability of its antibody response.