Fragment antigen-binding
![An antibody digested by papain yields three fragments: two Fab fragments and one Fc fragment](/Images/godic/202501/14/2fab_fc.svg5856.png")
![An antibody digested by pepsin yields two fragments: a F(ab')2 fragment and a pFc' fragment](/Images/godic/202501/14/F_ab2_pFc5856.png")
![Heavy and light chains, variable and constant regions of an antibody.](/Images/godic/202501/14/Antibody_je25856.png")
The fragment antigen-binding (Fab) fragment is a region on an antibody that binds to antigens. It is composed of one constant and one variable domain of each of the heavy and the light chain. The variable domain contains the paratope (the antigen-binding site), comprising a set of complementarity determining regions, at the amino terminal end of the monomer. Each arm of the Y thus binds an epitope on the antigen.