The migrant cells in allotransplants of heart, kidney and skin. II. Monocyte-macrophage activity.

Within a few hours of transplantation the cells of the mononuclear phagocyte system migrate into the interstitial tissues of the organ. The cells arrive on the vascular endothelium of the allotransplanted organ either as macrophages or as monocytes but on entering the interstitial tissues they begin to enlarge, differentiate and undergo active pinocytosis. Along with the cell migration, which would appear to be their stimulus to migrate, is an extravasation of the protein-rich plasma. Erythrocytes also pass into the interstitium and, if effete, are contracted by monocyte-macrophages and phagocytosed in the classical manner. All through the natural history of an allotransplant new monocyte-macrophage recruits arrive the interstitial tissues of the allotransplant. Some of these cells escape via the lymphatics but the vast majority accumulate in the allotransplant, interdigitate and fuse with one another but make no attempt to damage the tissues into which they have migrated. The reaction to an allotransplanted organ fits into the category of an aseptic inflammatory reaction and the only obvious role played by the mononuclear phagocyte system is the classical role of scavenging interstitial debris.