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.