Membranous and/or proliferative pneumonitis, similar in certain features to human interstitial pneumonitis, developed in rabbits making hyperactive antibody response to daily injections of bovine serum albumin (BSA) administered in multiple large doses sufficient to maintain the state of relative antigen-antibody equivalence. The pulmonary lesions were associated with deposition in alveolar capillary walls and interstitium of antigen, host globulin and complement, presumably in immune complexes. In some rabbits chronic interstitial pneumonitis, characterized by thickening of alveolar capillary walls, interstitial fibrosis and deposition of fibrinogen, was observed.

The production of immune complex pneumonitis seems to depend on the degree of the antibody response because rabbits developing chronic serum sickness with low doses of BSA, rabbits with acute serum sickness as well as nonresponders showed no pulmonary alterations. This observation is comparable to that described by Dixon in his studies on experimental immune complex glomerulonephritis. It is conceivable that the pulmonary pathology shown here is produced by formation of larger amounts of complexes which may persist longer at critical levels in the circulation than in rabbits immunized with a single daily injection of BSA.

In conclusion this study suggests: first, that experimental chronic serum sickness can be used as a model, not only for glomerulonephritis, but also for experimental systemic disease, comparable to human systemic diseases produced by circulating antigen-antibody complexes; and second, that the pathogenesis proposed here offers an alternative to using antilung basement membrane pneumonitis for the experimental approach to the study of human lung immunopathology.

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