Antibody activity and complement-mediated solubilization of precipitates during the immune response

The avidity of antibodies against human serum albumin (HSA) and the complement-mediated solubilization of HSA anti-HSA precipitates has been investigated during the immune response in rats, mice, rabbits, and guinea-pigs. The avidity of the anti-HSA antibodies was quantitatively determined and expressed as the amount of HSA required to solubilize 50% of 10 ng HSA anti-HSA precipitate. In rats, from 25 to 100 days after primary immunization the avidity of the anti-HSA antibodies increased approximately 100-fold, whereas the affinity did not change significantly. The increase in avidity was accompanied by a thirty-six-fold decrease in the amount of immune precipitate that could be solubilized by complement. Booster injections of the rats did not prevent the increase in avidity (thirteen-fold) that occurred from 25 to 50 days after the first immunization; however, the subsequent eight-fold increase in avidity from 50 to 100 days could partially or completely be abolished by secondary immunizations. A close, inverse relationship between the avidity of the anti-HSA antibodies and the rate of complement-mediated solubilization was observed. In addition, an increase in avidity without a change in affinity of anti-HSA antibodies and a decrease in the rate of complement-mediated solubilization was found during the immune response against HSA not only in rats, but also in mice, rabbits, and guinea-pigs.