POSTIRRADIATION TREATMENT OF MiCE AND RaTS 147 



making the suspension of spleen cells accounts for the increased 

 number necessary compared with the exudate which could be 

 obtained without damage to the cells. 



Whereas 10^ strain- A spleen cells are necessary to produce 

 a state of transplantation immunity, in the experiments above 

 15 X 10^ spleen cells were injected intravenously into the 

 CBA mice. It is possible that these cells were favourably 

 distributed in the normal or irradiated recipient and accounted 

 for the positive results recorded for tissues in the first days 

 after injection. However, in the unirradiated animals these 

 positive results were soon reversed to negative as was expected 

 on the basis of immunity following the implant of foreign and 

 incompatible tissue. In the irradiated animals the positive 

 results not only persisted in tissues that were manifestly and 

 measurably enlarged and hyperplastic, but even showed 

 evidence of increase for which the original paper should be 

 consulted. Moreover, it is noteworthy that a peritoneal 

 exudate induced at a late stage was also positive. 



One must conclude that the persistence of the A antigen 

 against all the laws of tissue grafting is a result of the massive 

 dose of radiation ; and that the apparent increase in A antigen 

 is the result of the growth of the grafted strain-A cells or the 

 incorporation of the A antigen by the CBA host. Recent work 

 from other laboratories leads to the same conclusions. Main 

 and Prehn (1955) demonstrated that mice injected with 

 homologous cells after lethal doses of X-rays survived and 

 would then take skin grafts which normally would be incom- 

 patible. Lindsley, Odell and Tausche (1955), using rats, could 

 identify red blood cells characteristic of the donor in rats 

 irradiated with near lethal doses and treated with homologous 

 bone marrow. Finally, No well and co-workers (1955) have 

 reported that mice injected with rats' bone marrow develop 

 myeloid cells and circulating leucocytes which give positive 

 histochemical reactions for alkaline phosphatase — a property 

 of the rat but not of the mouse. 



2. The rather improbable explanation involving incorpora- 

 tion of the donor's antigens into the host need no longer be 



