 Fragment antigen-binding

A Fab fragment contains one antibody binding site, allowing comparison with the two-site intact IgG in the antibody surface plasmon resonance experiment.

A Fab fragment combines one complete immunoglobulin light chain with one immunoglobulin heavy chain $V_H,C_{H1}$ portion, including antibody constant regions.^[ref]

The single paratope makes a Fab fragment useful for separating antibody affinity from two-arm avidity. If an intact IgG attaches strongly to a surface but its matched Fab fragments detach more readily, loss of the second arm is a candidate explanation even if the individual paratope is unchanged. Comparing the formats asks a structural question; it does not by itself establish which binding mechanism dominates.