  Despite the fact that red blood cells carry negative charges, under certain
conditions they form cylindrical stacks, or ``rouleaux''. It is shown here that
a form of the Casimir effect, generalizing the more well-known van der Waals
forces, can provide the necessary attractive force to balance the electrostatic
repulsion. Erythrocytes in plasma are modelled as negatively charged dielectric
disks in an ionic solution, allowing predictions to be made about the
conditions under which rouleaux will form. The results show qualitative and
quantitative agreement with observations, and suggest new experiments and
further applications to other biological systems, colloid chemistry and
nanotechnology.
