Complement and Ig uptake by erythrocytes in low ionic strength solutions

We studied the uptake of immunoglobulin (Ig) and complement from plasma by red blood cells (RBC) under low ionic strength (LIS) conditions using Rh-negative RBC and anti-Rh antibodies. The ionic strength of plasma/RBC mixtures was lowered by dialysis against 5% mannitol, pH 6.0. The studies revealed that bromelin-modified RBC failed to bind either Ig or complement in low ionic strength conditions, but that unmodified RBC bound the majority of anti-D. The bound anti-D (consisting of IgG1, IgG3, IgA, and IgM molecules) could be eluted into 0.9% NaCl yielding a higher recovery of antibody than that obtained by specific absorption-elution techniques. Although uptake of Ig by RBC was independent of complement activation, the subsequent elution of bound Ig into normal ionic strength solutions was not. Small amounts of Ig remained on EC43 and EC4, but not on RBC that did not become complement sensitized during low ionic strength exposure. These findings suggest that Ig remained attached to RBC via a complement-RBC bond.