The Fab fragment (Fragment of antigen binding), t, is the region in the antibody structure that can bind to the antigen. The Fab fragment consists of a complete light chain and the VH and CH1 domains (Fd segment) of the heavy chain, with a molecular weight of about 5×104. Both the light chain and the heavy chain have a constant region and a variable region, and there are disulfide bonds between the light and heavy chains.