The regions (domains) of the antibody (molecule’s) “heavy chain” that vary in their amino acid sequence. The “chains” (of atoms) comprising the antibody (immunoglobulin) molecule consist of a region of variable (V) amino acid sequence and a region in which the amino acid sequence remains constant (C). An antibody molecule possesses two antigen binding sites, and it is the variable domains of the light (VL) and heavy (VH) chains which contribute to this (antigen binding ability).