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].