variable region

n.
The portion of the amino terminal of an immunoglobulin's heavy and light chains having a variable amino acid sequence.

The N-terminal portion of heavy and light chains of immunoglobulin molecules in which the amino acid sequence varies as a consequence of somatic mutation and recombination during ontogeny of B lymphocytes and also occurring after antigen exposure. The variable amino acid sequence which provides more than 107 different antibody molecules is responsible for the antigen-binding specificity.