The rapidly changing nature of acute immune complex disease.

Antibody was measured in acute serum sickness with a modified ABC Farr test. Under the conditions of antigen excess which were employed in this test, antibody could be detected before antigen was eliminated from the circulation. This approach made it possible to directly measure the ratio of antigen to antibody in the serum. The antigen/antibody ratio changed daily. The immunologic conditions could be divided into three phases. In phase I the serum antigen/antibody ratio showed great antigen excess, with values of 2:1 or greater. Cold-insoluble complexes were not seen in this phase, and antigen was cleared slowly from the circulation. The serum antigen/antibody ratio in phase II varied from 1:1 to 1:20 corresponding to conditions of slight antigen excess. Partial cold insolubility of circulating complexes was observed. The rate of antigen clearance increased during this phase. Phase III began just before antigen was completely eliminated. The serum antigen/antibody ratio was 1:40 or less. At this time the complexes in the circulation exhibited marked cold insolubility, and the rate of antigen clearance was extremely rapid.