Fab and Fc fragments

Papain,  a  protease, cuts  the  immunoglobulin G molecule   to  release  the  two  arms  of  the Y-shaped molecule, every of that has one antigen-binding  site and is known as an Fab fragment  Fragment  antigen binding. Hence Fab fragments have only   one   antigen-binding    site   for instance is univalent,   they   cannot   cross-link antigens. The defined stem of the Y-shaped molecule which is consisting of the identical C-terminal  components  of  the  two  H  chains  is  named  the  Fc  fragment because  it readily  crystallizes.  The fragment Fc carries the effector sites which switch the destruction of the antigen, for instance triggering of the complement system and inducing phagocytosis of pathogens through other white blood cells. In contrast to pepsin, papain, (another protease) cuts the immunoglobulin G molecule to release the two arms of immunoglobulin G since linked together and therefore this fragment has two antigen-binding sites example is bivalent and can still cross-link antigens. This is known as the F (ab) 2 fragments.

Figure: Papain digestion of an antibody molecule yields two univalent Fab fragments and an Fc fragment whereas pepsin digestion yields a bivalent F(ab )2  fragment.