Immunoglobulins
The basic immunoglobulin molecule (Ig) comprises four protein molecules,
two heavy chains and two light chains, with a variable Fab end (a ntigen b inding end) and Fc ( the constant end) that
determines the properties of the molecule. There are five classes of
immunoglobulin, IgG, IgM, IgA, and IgD, each having a specialized
function determined by its heavy chain. Following gene rearrangement,
antigen activated B cells differentiate into plasma cells, which are
designed for high protein production and secretion. Initially, the
splicing of variable region genes onto the new heavy chain gene leads to
the production of IgM. This immunoglobulin is expressed on the surface
of the developing B cells and acts as a trigger to further cell
proliferation. Subsequently ‘gene switching’ leads to the production of
IgG and the other immunoglobulins. IgG is the most abundant.
immunoglobulin in the blood. IgM is a pentamer and is the first to be
produced in an immune response. IgA is a dimer and is secreted by
mucosal plasma cells. It passes through the epithelium and resides in
the gut ready to deal with ingested antigens. On passage through the
epithelium, a polypeptide secretory component is attached to the
immunoglobulin which helps prevent digestion by the gut enzymes. IgE
binds to specific regions of mast cells which recognize the Fc region of
the immunoglobulin molecule. This immunoglobulin is important in
combating helminth infections and also plays a role in hypersensitivity
reactions [19, 20].