Sketch and label the structure of antibody.

Answer: [Diagram: a Y-shaped antibody molecule (immunoglobulin) drawn with two long heavy (H) chains forming the stem and both arms, and two short light (L) chains lying alongside the outer side of the arms. Labels: heavy chain, light chain, disulphide bonds (–S–S–) joining the two heavy chains at the hinge and each light chain to a heavy chain, hinge region, variable region (V_H, V_L) at the tips of the arms, constant region (C_H, C_L), antigen-binding sites at the two tips, Fab fragment (arms) and Fc fragment (stem).]

An antibody is a Y-shaped glycoprotein made of four polypeptide chains, two identical heavy chains and two identical light chains (H₂L₂), held together by disulphide bonds. The tips of the arms have variable regions which form two antigen-binding sites; the rest is the constant region, which decides the class of antibody (IgG, IgA, IgM, IgE, IgD).