Zeta potentials, van der Waals forces and hemagglutination

It has recently become possible to determine the van der Waals (Hamaker) coefficient of erythrocytes, whilst their zeta-potential has been known for some time. With these two data the net potential energy of interaction versus distance diagrams could be elaborated for unsensitized human erythrocytes suspended in saline water, as well as for erythrocytes monogamously sensitized with anti-D (Rh0) antibodies of the IgG class. Unsensitized erythrocytes can approach each other, to within approximately equal to 79 A of their sialoglycoprotein surfaces, leaving a distance between their actual cell membranes of approximately equal to 180 A, which is considerably more than the maximum distance between the two valencies of an IgG molecule (approximately equal to 120 A). This explains why unaided anti-D (Rh0) antibodies of the IgG class cannot cross-link two D (Rh0)-positive erythrocytes, although cross-linking can easily be achieved with IgM class antibodies. D (Rh0)-positive erythrocytes, monogamously sensitized with antibodies of the IgG class, can approach each other to within approximately equal to 60 A (between the Fc ends of the protruding antibodies), which makes cross-linking by means of anti-IgG antibodies of the IgG class feasible.