The five classes and where each works
Antibodies come in five isotypes defined by their heavy-chain constant region. IgG is the most abundant in blood, provides long-term protection, and crosses the placenta to protect newborns. IgM is the large, first-responder antibody made early in an immune reaction. IgA dominates mucosal surfaces and secretions including saliva, tears, and gut fluid. IgE is scarce but central to allergy and parasite defense. IgD sits mostly on naive B cells with a less defined role.
Their structures differ to match these jobs: IgM circulates as a pentamer for strong early binding, while secretory IgA travels as a dimer suited to the mucosal environment.
Class switching tunes the response
A B cell can change which isotype it makes without altering what it recognizes, a process called class switching driven by signals from helper T cells. This lets the immune system deploy the right tool, such as IgG for systemic memory or IgA for the gut surface, against the same target.
This is general educational information, not medical advice; questions about immune function or antibody testing should be directed to a qualified clinician.