Fd fragments of antibody alone show very low binding activity toward the specific hapten.

As is the case for the combination of heavy and light chains, the combination of Fd fragments and light chains also requires that both components come from antibody from the same rabbit in order to give binding sites. When they are derived from different rabbits producing antibody against the same antigen, they still give Fab fragments as shown by immunoelectrophoresis but do not have competent binding sites.

An important observation is that the subunits of the papain digest fractions, Fab_I and Fab_II, have the capacity to cross-combine to form active Fab fragments with competent binding sites. Fd_I from Fab_I combines with L_II chains from Fab_II to give the composite (Fd_I-L_II) with good binding activity. Likewise, the composite (Fd_II-L_I) has good binding activity. The composites from the two types of antibody molecules yielding different Fab fragments have antibody activity although heretofore these molecules have appeared to be different on the bases of chromatography and amino acid analysis.

There is also a preferential combination of the Fd fragments to combine with the correct L fragments to give binding sites since this combination takes preference over the combination of Fd fragments of antibody with light chains of normal globulin (or of light chains of antibody with Fd fragments of normal globulin).