 In an antibody, the Fab (fragment, antigen-binding) region is formed from the amino-terminal end of both the light and heavy chains of the immunoglobulin polypeptide.This region, called the variable (V) domain, is composed of amino acid sequences that define each type of antibody and their binding affinity to an antigen.The combined sequence of variable light chain (VL) and variable heavy chain (VH) creates three hypervariable regions (HV1, HV2, and HV3).