l62 DISCUSSION 



by no means "neutral" when it is injected with a larger number of cells. 

 In this critical first postnatal week tolerance induction is therefore 

 entirely dose-responsive. The work of Silvers and Billingham, for 

 example, has shown that it depends also on the strength of the antigen, 

 which in itself may depend on the genetic relationship between the 

 donor and the recipient ; thus for the sex-linked antigen the period in 

 which tolerance can be induced after birth is very much longer than for 

 the H-2 or the H-3 antigens. 



