8.2. Basic Structure and Isotypes

Unit 8- Antibody (Immunoglobulin)
8.2. Basic Structure and Isotypes
Antibody molecule contain 4 polypeptide chains, among them two are identical heavy chains and two identical light chains. These chains are held together by disulphide bonds, some are inter-chain and some are intra-chain. Each part of the molecule has different functions:
  • The fab fragment: Fab is for "fragment antigen binding" this Fab part of the antibody that binds to antigens. Variable amino acid sequences can be seen in this fragment.
  • The Fc fragment: Fc is for "fragment crystallizable". The Fc fragment is where complement binds and also the anti-antibodies (anti-IgG) will bind. The amino acid sequences present here are constant.
There are five different isotypes of antibody depending on their difference in heavy chain. These includes:
IgG, IgM, IgA, IgD and IgE.
Heavy chains of IgG, IgM, IgA, IgD, and IgE, are known as gamma, mu, alpha, delta, and epsilon, respectively.
This is the principle antibody found in blood and body fluids. Nearly 75% of the antibody circulating in the blood is IgG. IgG is a monomeric immunoglobulin, built of two heavy chains and two light chains. Each molecule has two antigen binding sites. This is the only isotype that can pass through the placenta, thereby providing protection to the fetus in its first weeks of life before its own immune system has developed.
IgA represents about 15% to 20% of immunoglobulins in the blood. IgA is involved in mucosal immunity and prevent the colonization of bacteria in the digestive and respiratory tracts. It does not activate complement.
The IgM isotype is expressed on the surface of B cells and it is also secreted by plasma cells. IgM present in the form of polymers where multiple immunoglobulins are covalently linked together with disulfide bonds, normally as a pentamer or occasionally as a hexamer. Each monomer has two antigen binding sites, so an pentameric IgM has 10 antigen binding sites, however it cannot bind 10 antigens at the same time because they hinder each other. It is also a "natural antibody" where it is found in the serum without any prior contact with antigen. Due to its polymeric nature, a single IgM-antigen complex can trigger the complement cascade, whereas multiple IgG-antigen complexes are required.
The function of IgD is not well known but it makes up about 1% of proteins in the plasma membranes of immature B-lymphocytes. It is also in serum in very small level. It is monomeric in nature. IgD's function is currently unknown but it may function as a regulatory antigen receptor.
IgE is a monomeric immunoglobulin which is heat labile and plays an important role in defending against parasitic worms. IgE is mainly responsible for allergies and this is through their ability to trigger the release of chemicals from the granulocytes when the antibody reacts with specific antigen. The IgE antibodies do not activate complement.

Last modified: Thursday, 21 June 2012, 7:01 AM