What class switch recombination changes
Class switch recombination (CSR) lets an activated B cell swap the constant region of its antibody heavy chain while keeping the same antigen-binding variable region. The cell starts producing IgM, then can switch to IgG, IgA, or IgE. The specificity for the antigen stays identical; only the effector tail changes, which determines whether the antibody fixes complement, crosses the placenta, coats mucosal surfaces, or arms mast cells.
The reaction deletes the intervening DNA between switch regions upstream of each constant-region gene, so a switch is generally irreversible for that cell and its descendants. This is distinct from somatic hypermutation, which tunes binding affinity rather than antibody class.
Why AID is the enzyme that drives it
Activation-induced cytidine deaminase (AID) initiates the process by deaminating cytidine to uracil in single-stranded DNA at repetitive switch regions. Downstream repair machinery converts those lesions into the double-strand breaks that allow recombination. AID also underlies somatic hypermutation, which is why loss-of-function mutations in the AICDA gene cause a hyper-IgM syndrome: B cells make IgM but cannot switch or mature affinity.
Cytokines from helper T cells steer which class is chosen. As a broad pattern, interferon-gamma favors certain IgG subclasses, IL-4 favors IgE and some IgG, and TGF-beta plus other mucosal signals favor IgA. This is general educational information about immunology, not medical advice.