Antibody variable region ($V_H,V_L$)

The paired antibody variable regions of an immunoglobulin heavy chain and an immunoglobulin light chain contribute to an antibody binding site. Variation refers to differences between antibody sequences, not to a tip continually changing identity while binding.

A complementarity-determining region is a variable-region segment contributing to an antibody binding site. Transfer of these regions can transfer specificity, but framework residues also influence conformation and affinity.