352 IMMUNOLOGICAL STUDIES 



can now react to a tolerated skin graft. Evidently the host's own 

 antibody-forming cells do not respond to the antigen received 

 in fetal life, but antibody-forming cells received from another 

 animal (same strain) can react to antigens in the graft or else- 

 where in the body. 



This work has now been extended to a number of other sys- 

 tems. In rats it has been reported (Woodruff and Simpson, 1955) 

 that the tolerance to skin grafts can be induced by injection with 

 cells of the prospective donor at the day of birth. In experiments 

 with tumor transplants, mouse Crocker sarcoma S 180 has been 

 found to grow to large size, but not indefinitely, when implanted 

 into rats that had been injected during fetal life with mouse tissue 

 (BoUag, 1955). An ascites tumor of mice {6CSHED lymphosar- 

 coma) has been gotten to grow in strains of mice in which it 

 would ordinarily regress by injection of the 16- to 17-day fetuses 

 with donor strain blood or tumor cells ( Koprowski, 1955 ) . In the 

 latter experiments the tumor cells undergo a change in antigenic 

 specificity upon growth on the tolerant, foreign strain, mouse. A 

 great many earlier experiments particularly by Snell, Kaliss, and 

 their co-workers (see Snell, 1952; Kaliss, 1955) have demon- 

 strated a "conditioning the host" to tumor homografts by pre- 

 injection with various tissue (normal or tumor) homogenates or 

 extracts. This appears to be related to the acquired tolerance phe- 

 nomenon, although the effect is produced by injection of adult 

 animals immediately prior to tumor grafting and can be produced 

 also by injection of antisera (prepared in mice or rabbits) to 

 donor strain mouse tissues. 



Acquired tolerance is also indicated in experiments in which 

 chick embryos have been injected with killed Salmonella piillorem 

 and the hatched chickens later found to show marked decrease 

 in ability to form antibodies against this antigen ( Buxton, 1954 ) . 

 Similarly the antibody response of cattle to Trichomotms foetus 

 ( freeze-dried or acetone-dried) has been inhibited (Kerr and 

 Robertson, 1954) by injection of this antigen into newborn calves. 

 Specific inhibition of antibody-forming capacity has also been 

 demonstrated by use of simpler protein antigens, such as bovine 

 serum albumin and human serum albumin injected into rabbits 



