8 

 EXPERIMENTS IN HOMOGRAFT SURVIVAL 



M. Kent 

 The Department of Surgery, University of Melbourne, Melbourne 



This paper is presented to introduce the subject of homotransplantation 

 and, in particular, to discuss the influence of total body irradiation on the 

 mechanisms of the homooraft reaction. It contains an interim report on 

 work Ijcing carried out within the Department of Surgery of the University 

 of Melbourne. 



THE NATURE OF THE HOMOGRAFT REACTION 



With the exception of the cornea, cartilage and blood vessels, all homografts 

 undergo complete disintegration at varying time intervals. This process 

 can be observed in its simplest form in the case of whole-thickness skin 

 grafts in rabbits. An adecjuate capillary blood supply develops in four days. 

 The epithelium, in the case of autotransplants, shows a gradual thickening 

 over the first two weeks, reducing slowly to normal in four weeks. By 

 contrast, homografts show a less marked thickening of the epithelium, with 

 rejection of the graft at about ten days. The process of graft disintegration 

 is accompanied by neighbouring infiltration of small round cells, mainly 

 polymorphs of the inflammatory reaction. 



Many hypotheses have been put forward to explain the mechanism of 

 homograft rejection^. The theory of actively acquired immunity, implying 

 an antigen-antibody reaction, has considerable experimental backing. 

 Accelerated disintegi'ation follows re-exposure of the host to homografts 

 from the same donor; antil)odies to certain homotransplants (particularly 

 tumours) have been demonstrated ; the homograft reaction is not seen in 

 grafts in the anterior chamber until contact with the host's circulation 

 occurs; and finally, methods which suppress the activity of the reticulo- 

 endothelial systein (the presumed site of antibody production) have the 

 effect of prolonging homotransplant survival. The grafts, however, are 

 rejected shortly after recovery of the reticulo-endothelial system. 



Some rejected theories worth mentioning are: — 

 (7) The theory of natural immunity on the analogy of blood group incom- 

 patibility. There is no sound evidence that natural antibodies ])lay a part 

 in homograft rejection; the time lag between contact and rejection of the 

 homotransplant is not in keeping with this theory. 



{2) The theory of lack of vascular supply is not borne out by the histological 

 evidence, by the time of rejection of the graft in relation to its vascularization, 

 nor ])y the fact that destruction of anterior chamber implants follow on this 

 vascularization. 

 {3) The theory of local cellular reaction. That tissue cells have no toxic 



68 



