The immune response is highly specific for each invader, and that’s because the cells of the adaptive immune response have unique receptors that are able to differentiate friendly bacteria from potentially deadly pathogens from their unique parts - called antigens.
The key cells of the adaptive immune response are the lymphocytes - the B and T cells.
B cells develop in the bone marrow where they undergo a process called VDJ rearrangement to generate a massively diverse set of B cell receptors.
The B cell receptor is essentially an antibody except that has a transmembrane part that goes through the membrane attaching the receptor to the surface of the B cell.
The B cell receptor, has two heavy chains and two light chains, and the region or fragment of the B cell receptor that binds the antigen is called the fragment-antigen binding or Fab region.
The Fab region is where the ends of the heavy and light chains meet, and there are two Fab fragments on each B cell receptor.
The remainder of the heavy chain makes up the constant region or constant fragment region, also called Fc.
The two heavy chains are linked to one another by disulfide bonds and each heavy chain is also linked to a light chain by a disulfide bond.
Each B cell receptor, has two identical heavy and light chains, resulting in two identical antigen binding sites.
As the B cell develops into a plasma cell, the B cell receptor gets secreted as an antibody with the exact same antigen specificity.
However, the heavy chain actually changes as the B cell develops.
There are 5 major types of heavy chains which encode the isotypes or classes of immunoglobulins: IgM, IgD, IgG, IgA, and IgE.
