Hindrance in the simultaneous binding of two distinctly separate antibody populations to the hemoglobin antigen

Abstract

Two subpopulations of goat anti-human hemoglobin a 1 (HbA 1) Fab' monomers were isolated. One binds at the α subunit of hemoglobin and not at the β subunit. The other binds only at the β subunit of hemoglobin. The total number of antigenic sites on hemoglobin bound by the two Fab' subpopulations exceeds the number of antigenic sites bound by an equimolar mixture of the two populations. Also, the amounts of each Fab' subpopulation needed to reach equivalence point when titrating HbA 1 already bound to the other Fab' were 25% less than the amounts of Fab' needed to reach equivalence point when titrating HbA 1 alone. These results indicate that part of the Fab' present in each of the two subpopulations interfere with one another in binding antigen. These particular Fab' monomers bind at the α and β subunits of HbA 1 either at adjacent but independent determinant sites or at nonadjacent sites, the conformations of which are affected by the binding of Fab' at the other antigenic site.