[(Science: immunology) The portion of the immunoglobulin or antibody molecule which contains the binding Site for antigens. The exact sequence of amino acids in the area varies widely from molecule to molecule to accommodate a wide variety of antigens which the body may encounter. there are two such regions on each molecule (individually called fab fragments or f-ab fragments). The molecule is shaped like the letter Y and the F(ab)2 fragments are located on the upper halves of the two fork parts. (The rest of the molecule, the stem and the lower parts of the forks, are the fc fragment).