Structure of Antibody
Different parts and regions of the antibody. Antibody is made of two types of chains Heavy (H) and Light (L) chains. These chains are held together by disulfide bonds forming Y shaped molecule. The Tip part forms antigen binding site. There are two regions within each type of chain. The Tip regions are called as variable regions and base regions are called as constant regions.
Antibodies are the proteins that are involve in the defense mechanism of living beings i.e. antibodies are those which involve in the immune system. They provide immunity to the living bodies. Antibodies play different roles in defending with external stimuli like disease causing agents and protect the living body from these disease causing agents. The system of bodies involved in immunity and antibody production is called as immune system which is continuously monitoring the body. Antibodies are based on a Y-shaped molecule i.e. almost all antibodies are made of a Y-shaped structure consisting of four polypeptide chains held together with the help of disulfide bonds. Among the two pairs, one pair is called as Heavy (H) and next pair is called as Light (L).
The base of the Y is called as a constant region because the amino acid composition on this region is fixed in an individual and only varies among the animal species. While the tips of the arms are called as variable region, because amino acid composition in the tips varies among the different antibodies within a single animal species. The ends of both heavy and light chains are variable and form antigen binding site. The variable regions are further divided into two areas, the framework regions and hypervariable regions. There are four framework regions and three hypervariable regions per binding sites.
Antibodies can be broken down by enzymes producing different subunits. Papain is an enzyme that can digest antibodies giving three fragments; two antigen binding fragments (fab) and one constant fragment. Fab fragments retain the antigen binding capacity while constant fragment has no practical use.
There are five classes of antibodies and these are also called as immunoglobulins (Ig). These are immunoglobulin A, D, E, G and M. Immunoglobulin G is the most common type of antibody and all share the common structure Y-shaped structure. IgM is produced by newly activated immature B lymphocytes exposed to an antigen for the first time. It is found on the surface of B cells in association with IgD. IgM is formed from five IgG linked to mu chain through constant regions. It is also called as natural antibody as its production rises significantly after first exposure to the antibody and it constitutes the primary immune system. This type of antibody is only found in serum.
IgA is a dimeric form constituting two IgG linked to each other end to end (tail to tail linkage). It is predominantly found in secretion from mucosa and it is resistant to enzyme degradation. It is found on the surface of B cells in the mucosa and secreted through mucosal secretion.