Structure of Antibody

Antibody Structure:

IgG has been studied extensively and serves as a model of basic structural unit of all Igs.

An antibody molecule consists of the following parts.

(i) Heavy and Light Chains:

An antibody molecule is made up of 4 peptide chains, two small called light chains and two longer called heavy chains. Hence an antibody is represented as H2L2. The heavy chain has larger number of amino acids while light chain has smaller number of amino acids. Heavy and light chains may be either lambda or Kappa type.

(ii) Constant and Variable Regions:

There are two different regions the constant region and variable region in each chain of the antibody.

(iii) Disulfide Bonds and Hinge Region:

A disulfide bond joins a light chain with a heavy chain. Two disulfide bonds also link the two heavy chains. This part of the antibody displays considerable flexibility and is called the hinge region. Because the antibody “arms” can move somewhat as the hinge region bends, an antibody can assume a Y shaped molecule.

(iv) Fragment Antigen Binding (Fab) and Fragment Crystallisable (Fc):

Two iden­tical fragments of Y-shaped molecule possess the antigen-binding sites and are thus named fragment-antigen binding (Fab). The antigen-binding sites bind to the specific antigens in a lock and key pattern, forming an antigen-antibody complex. The third fragment which lacks the ability to bind to antigen and can be crystallized, is, therefore, known as fragment crystallizable (Fc).

The stem of the Y-shaped antibody monomer is called the Fc region, so named because when antibody structure was first being identified, it was a fragment (F) that crystallized (c) in cold storage.