Antibodies are globulins (globular proteins) produced by plasma cells and are usually called immunoglobulins (Ig) to emphasize their role in immunity. Because the immune system generates antibody to thousands of antigens (foreign bodies) with which the host may or may never come into contact, general antibody of unknown specificity is referred to as immunoglobulin until it can be defined by its specific antigen. A simple way to view this point is that antibody is an immunoglobulin, but immunoglobulin is not necessarily an antibody.2
There are five different classes of immunoglobulins:
- Immunoglobulin M (IgM)
- Immunoglobulin G (IgG)
- Immunoglobulin A (IgA)
- Immunoglobulin D (IgD)
- Immunoglobulin E (IgE)
Although immunoglobulins differ somewhat from one another in their chemical composition, molecular weight and size, they all have the same basic structure: two matched pairs of polypeptide (protein) chains joined by chemical bonds. One pair is called heavy chain Ig. The second pair is only half as long as the heavy chain and is called light chain Ig.
The arrangement of Ig chains somewhat resembles the appearance of a fork. The ends of the Ig chains that combine with antigen can be compared with its prongs. The "prong" end of an immunoglobulin molecule, which is different in each antibody, is called the variable part of the molecule. It is this part that imparts specificity to the molecule. Because of this structure, the antibody can react only with a specific antigen that induced its formation.
Mechanisms of action employed by monoclonal antibodies.
Credit: PubMed Central
The constant part of the chain, which can be compared with the handle of the fork, is the same for each major class of antibody. The "handle" end does not combine with antigen but determine other properties of the antibody, such as the ability to activate complement or fix to the surface of cell membranes.
All Ig molecules have the same four-chain unit structure, but some immunoglobulins characteristically aggregate to form clusters of two, three, or five individual units. For example, IgM is usually a cluster of five units, and IgA is usually a pair of units.
IgM is the first Ig formed in response to foreign antigen (primary immune response). It is present in bloodstream but not in tissues. Its pentamer (cluster of five) configuration is very effective for combining with foreign antigens.
IgG is the most prevalent Ig produced in large amounts (secondary immune response) to replace IgM. It is found both in blood and tissues. IgG crosses placenta to protect fetus until infant immune system can produce antibodies.
IgA is present in bloodstream, in secretions from mucous membranes (respiratory and GI tract), and breast milk to provide maternal antibody protection to infant.
Small amount of IgD is present in blood and on the surface of lymphocytes which have not been exposed to antigens. Its precise function is still under investigation.1
IgE is present in bloodstream. It is responsible for anaphylaxis and other allergic reactions, as well as such diseases as asthma, allergic rhinitis, atopic dermatitis, and vernal conjunctivitis. High blood level is also found in parasitic diseases. Anaphylactic antibodies are also found in rabbits and dogs. Unlike other immunoglobulins, IgE antibodies remain firmly fixed when injected into the human skin, where they are bound to mast cells. Contact with allergen leads to their activation and release of various chemicals responsible for inflammatory and allergic response.3
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- Pathobiology of Ocular Disease: A Dynamic Approach, Volume 1 By Alec Garner, Gordon K. Klintworth