Each antibody consists of four polypeptides two heavy chains and two light chains joined to form a Y shaped molecule. The amino acid sequence in the tips of the Y varies greatly among different antibodies. This variable region, composed of 110-130 amino acids, give the antibody its specificity for binding antigen.The variable region includes the ends of the light and heavy chains.Treating the antibody with a protease can cleave this region, producing Fab or fragment antigen binding that include the variable ends of an antibody. Material used for the studies shown below originated from Fab.The constant region determines the mechanism used to destroy antigen.reating the antibody with a protease can cleave this region, producing Fab or fragment antigen binding that include the variable ends of an antibody. Material used for the studies shown below originated from Fab. The constant region determines the mechanism used to destroy antigen.