Fragment antigen-binding region (Fab)

The fragment antigen-binding region (Fab) is a part of an antibody composed of one constant and one variable domain from each of the antibody's heavy and light chains[1][4][8]. The Fab region is responsible for the highly specific binding to antigens through its variable domains, which form the paratope recognizing unique epitopes[1][8]. Fab fragments can be generated in the laboratory by enzymatic cleavage (e.g., papain digestion), discarding the Fc region and leaving only the antigen-binding arms[1][4][5]. Fab fragments are not enzymes themselves, but forms derived from immunoglobulins—most often used in research, diagnostics, or as the basis for engineered therapeutic antibody fragments, such as abciximab (used in cardiovascular disease), idarucizumab (antidote for dabigatran), and ranibizumab (for macular degeneration)[7]. "Fab enzyme" is not a correct target name; confusion may arise from references to enzymes that generate Fab fragments (e.g., papain, IdeS), or from the use of Fab fragments in the inhibition of enzymatic targets via their antigen-binding capacity[5][7].

Other names

Fab fragmentFab regionFragment antigen binding

Antigen binding; neutralization or inhibition of target antigens (e.g., receptor blockade or ligand sequestration, dependent on the Fab’s antigen specificity)