English: Schematic diagram of an antibody (immunoglobulin).

It consists of two heavy chains (blue, yellow) and two light chains (green, pink). One of the two antigen-binding regions is circled: they are formed by the variable regions at the tip of the antibody. The heavy chains have (starting from the N-terminus at the tip) a variable domain (V_H), followed by a constant domain (C_H1), a hinge region, and two more constant domain (C_H2, C_H3). The light chains have a variable domain (V_L) and a constant domain (C_L).

The C_H2 and C_H3 regions form the crystallizable fragment (Fc). The variable and C_H1, C_L regions form a pair of antigen-binding fragments (Fab).

Disulfide bonds between the chains are drawn as S-S. Their exact number and location varies for different isotypes.