Antibody molecule. Crystal structure of a monoclonal immunoglobulin (IgG2a). IgG antibodies are composed of 2 long heavy chains and 2 shorter light chains. This antibody adopts a T-shape: the arms of the T make up the Fab or antigen binding region, while the base part or Fc region plays a role in interacting with the immune system. Atoms are represented as spheres and are colour-coded: hydrogen (white), carbon (grey), oxygen (red), sulfur (yellow) and nitrogen (blue).