Structure of an Antibody (Gk. anri-against, body-body) Antibody is a glycoprotein, called immunoglobulin, which have a specific amino acid sequences by which they can interact with specific antigens. Antibodies form 20% of the plasma proteins. Each antibody has a combination of at least 2 light (L) and 2 heavy (H) polypeptide chains. The heavy chains has larger number of amino acids while lighter chain has smaller number of them. Usually, the polypeptides form a Y-shaped configuration. The stem of Y is exclusively formed by heavy chains. In the arms of Y, both light and heavy chains occur parallel to each other except for antigen binding sites. Attachments and bendings occur by means of disulphide bonds (—S—S—). In certain immunoglobulins the number of chain pairs can be JO. An antibody has a variable portion in the arms. It is called V-region or antigen-binding fragment, Fab. The remainder of the antibody is called constant portion or crystalline fragment, Fc.
Most of the antibodies function as monomers. A few such as IgA, IgM can occur both as monomers and polymers.
Fig. A Typical antibody molecule
