<P> The 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 . </P> <P> In an experimental setting, Fc and Fab fragments can be generated in the laboratory . The enzyme papain can be used to cleave an immunoglobulin monomer into two Fab fragments and an Fc fragment . The enzyme pepsin cleaves below the hinge region, so a F (ab') fragment and a pFc' fragment is formed . Recently another enzyme for generation of F (ab') has been commercially available . The enzyme IdeS (Immunoglobulin degrading enzyme from Streptococcus pyogenes, trade name FabRICATOR) cleaves IgG in a sequence specific manner at neutral pH . The F (ab') fragment can be split into two Fab' fragments by mild reduction . </P> <P> The variable regions of the heavy and light chains can be fused together to form a single - chain variable fragment (scFv), which is only half the size of the Fab fragment, yet retains the original specificity of the parent immunoglobulin . </P>

How many antigen-binding sites are present on an igm molecule