Hi everyone, why do we call the "fragment antigen binding region" this way ? Isn't only the Variable domains (HV and LV) of an Immunoglobulin(Ig) responsible for binding to the antigen ?????? If so, why does the region "antigen binding" contain both the variable and one constant domain ??

 

Thanks in advance.

Hi everyone, why do we call the "fragment antigen binding region" this way ? Isn't only the Variable domains (HV and LV) of an Immunoglobulin(Ig) responsible for binding to the antigen ?????? If so, why does the region "antigen binding" contain both the variable and one constant domain ??

 

Thanks in advance.

 

This nomenclature dates back to the days before anyone knew the structure of the protein. If you digest antibodies with papain, you get Fab fragments. If you use pepsin instead, you get F(ab')2 fragments. With either digestion, you also get Fc fragments.

 

Papain cleaves the molecule above the hinge, resulting in two Fab fragments and one Fc for each antibody molecule. Pepsin cleaves below the hinge (where the two heavy chains are linked together), so you get the two Fab fragments still joined together. Since the pepsin-derived Fab are just a bit longer, they're called Fab' fragments (and you can get individual Fab' fragments by breaking the disulfide bond that links them).

 

So, they're called Fab (or Fab') fragments because those were the fragments that could still bind antigen, as opposed to the Fc fragment, which could not. (The Fc fragments are called that because they are homogeneous and can be crystalized.)