Accelerated serum sickness in the rabbit. IV. Characteristic endarteritis in the pulmonary artery.

A unique angiitis developing in acute serum sickness in the rabbit was studied by immunofluorescence and light microscopy and by transmission and scanning electron microscopy. The angiitis occurred predominantly in the pulmonary artery and was characterized by a prominent accumulation of monocytes in the subendothelial space. During the early stage, scanning electron microscopy revealed the occurrence of various endothelial alterations, especially the formation of many endothelial holes leading to a subendothelial emigration of monocytes. There was neither emigration of polymorphs nor significant thrombotic lesion, and it is possible that this was due to a paucity in the amount of immune complexes deposited. The monocytes ingested platelets and fibrin, suggesting their preventive role in the development of thrombotic changes. The intimal change resolved within several days, leaving only slight fibrosis. A distinct resemblance in the disease process between this type of angiitis and acute serum sickness nephritis is emphasized.