Paper I
Question
Enumerate the four classes of immunoglobulins.
Answer
The five classes (isotypes) of immunoglobulins (the question asks for four, but all five are listed for completeness — IgG, IgA, IgM, and IgD/IgE are the commonly cited “four main” depending on the textbook’s emphasis):
- IgG — the predominant serum immunoglobulin (~70–75%), only class crossing the placenta, mediates secondary immune response, opsonization, and complement activation.
- IgA — predominant immunoglobulin in external secretions (saliva, tears, breast milk, mucosal surfaces); exists as monomeric (serum) and dimeric secretory (mucosal) forms.
- IgM — largest immunoglobulin (pentameric), first antibody produced in a primary immune response, most efficient complement activator, does not cross the placenta (so IgM in a neonate indicates congenital infection).
- IgE — lowest serum concentration, mediates Type I (immediate) hypersensitivity reactions by binding FcεRI receptors on mast cells/basophils, and provides defence against helminthic parasites.
- IgD — present in low concentration in serum, functions mainly as a membrane-bound antigen receptor (BCR) on naive B lymphocytes; its precise systemic function remains less well defined.