100 The Lymphocyte and Lymphocytic Tissue 



nitrations at sites of tumor grafts and concluded that these were associated 

 with the resistance of the recipient's tissues to the graft. 2 Bash ford el al. 

 had reported that mice could be rendered relatively immune to a cancer 

 graft by the injection of homologous living tissue at least ten days before." 

 Later, Murphy found that a sharp increase in the absolute lymphocyte 

 count of the circulating blood followed the implantation of malignant tis- 

 sue and showed correlation between the degree of lymphocytic reaction and 

 the retardation of growth of implanted tumor or tissue. In these and others 

 of a long series of studies on this subject, Murphy postulated an association 

 of the lymphocytic reaction with an immune process. In support of such 

 an association, Murphy was able to present substantial evidence from his 

 data. 4 This lymphocytic infiltration was found in the case of heterologous 

 as well as homologous grafts, and where this cellular reaction did not occur, 

 in the irradiated animal or in the brain or in the embryo, the heterologous 

 graft grew with no evidence of host resistance. In such instances, however, 

 resistance to the graft could be reestablished by the introduction of adult 

 lymphoid tissue, even, in the case of the chick embryo, when the adult-splenic 

 tissue was introduced at some distance from the heterologous graft. Murphy 

 was not, however, able to determine how the lymphocytic infiltration or the 

 regional lymphatic hyperplasia exerted their effects on the implanted tis- 

 sue. Loeb also described lymphocytic infiltrations about homografts and like- 

 wise considered that their presence was associated with the defense of the 

 host tissue against the graft."' 



In the past fifteen years there has been a new sequence of studies of lym- 

 phocytic reaction at sites of tissue transplantation. In the period between 

 that of the earlier studies described above and these more recent ones, atten- 

 tion had been drawn to the immunologic functions of the lymphatic system 

 by the direct experimental evidence which began with the work of McMaster 

 and his colleagues in 1935.° Perhaps this development contributed to the 

 more definitive association, by recent workers, of the infiltrating lympho- 

 cytes with formal immunologic mechanisms of defense of the host tissue. 



In the classic study of skin homografting by Medawar in 1944, he found 

 that by the eighth day after placing of a homograft, shortly before the begin- 

 ning of the overt processes of graft rejection, the graft was densely invaded by 

 cells, largely lymphocytes with some monocytes, frequently in the form of 

 halos around blood vessels. The local lymphatic vessels were also hyper- 

 trophic^!. Subsequently these lymphocytes became pyknotic and then broke 

 down altogether, and additional lymphocytes, released by rupture of the 

 lymphatic vessels, showed similar changes. By this time the breakdown of the 

 graft was complete. These changes were observed in the case of homografts 

 but not of autografts. 7 In this study by Medawar, the importance of classic 



