V region

variable region the N-terminal portion, composing one homology region, of an immunoglobulin heavy chain (V_H) or light chain (V_L); it varies in amino acid sequence among chains of a single type. The antigen binding sites of immunoglobulin molecules are formed by parts of the V_H and V_L regions; thus the V_H and V_L amino acid sequences determine the antigenic specificity of the antibody molecule. Although variable regions vary among antibodies of different specificity (idiotype variation), all of the immunoglobulins produced by a single clone of plasma cells (a clonotype) have the same variable regions but may have different constant regions. See illustration at immunoglobulin.