Question

Explain the structure of an immunoglobulin molecule/antibody.

Answer 1

An antibody or immunoglobulin (Ig) is a globular protein produced by plasma cells on stimulation by B cells, in response to a specific antigen.

Structure:

(i) Each antibody is made up of four polypeptide chains — 2 identical short light (L) chains and 2 identical long heavy (H) chains; represented as H₂L₂. 

(ii) Each light chain is bonded to one heavy chain, and the four chains are held together by disulphide (-S-S-) bonds, forming a Y-shaped molecule.

(iii) The top two tips of the Y-shaped molecule are the antigen-binding sites — they recognise and bind to a specific antigen in a lock-and-key fashion, forming an antigen-antibody complex. Both arms of the Y have the same antigen-binding sites, so they recognise the same antigen.

(iv) Each of the four chains has a constant (C) region and a variable (V) region.

(v) Due to the variable regions there is a huge variety in the shape of the antigen-binding sites, so different antibodies can react with different antigens.

(vi) The stem of the antibody is formed by the constant regions of the heavy chains. In humans there are five classes of heavy chains, giving five types of antibodies — IgA, IgD, IgE, IgG and IgM.

The Y-shaped structure with two identical antigen-binding sites gives the antibody its specificity and its ability to bind and neutralise a specific antigen.