It has recently become possible to determine the van der Waals (Hamaker) coefficient of erythrocytes, whilst their ζ-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 suspensed in saline water, as well as for erythrocytes monogamously sensitized with anti-D (Rh(0)) antibodies of the IgG class. Unsensitized erythrocytes can approach each other, to within ≈79Å of their sialoglycoprotein surfaces, leaving a distance between their actual cell membranes of ≈180Å, which is considerably more than the maximum distance between the two valencies of an IgG molecule ( ≈120Å). This explains why unaided anti-D (Rh(0)) antibodies of the IgG class cannot cross-link two D (Rh(0))-positive erythrocytes, although cross-linking can easily be achieved with IgM class antibodies. D (Rh(0))-positive erythrocytes, monogamously sensitized with antibodies of the IgG class, can approach each other to within ≈60Å (between the Fc ends of the protruding antibodies), which makes cross-linking by means of anti-IgG antibodies of the IgG class feasible.

By means of potential energy versus distance diagrams, derived from electrophoretic and surface tension data, the minimum distances of approach of normal (NOR) and of neuraminidase-treated (NEU) and papain-treated (PAP) human erythrocytes could be determined. The minimum distances between the actual cell membranes of two opposing red cells are: 184 Å (NOR), 111 Å (NEU), and 113 Å (PAP), which agrees well with the fact that anti-D (Rh(0)) antibodies of the IgG-class (which have a maximum distance of ≈120 Å between the two antibody-active sites) can hemagglutinate NEU and PAP cells, but are incapable of hemagglutinating normal D (Rh(0))-positive erythrocytes.