THE UNIQUENESS OF THE INDIVIDUAL 



the antibodies which are the chemical effectors of the immunity 

 reaction must be able to pass from the mother''s circulation into 

 the circulation of the unborn child. In effect, this means that 

 the membranes which separate mother from foetus must be of 

 such a kind as to let the antibodies through. This first condi- 

 tion is satisfied by rabbits, rats and mice, and also by human 

 beings, but not by cattle, sheep and horses. Second, the foetus 

 must reach before birth a stage of development at which the 

 immunizing substances present in its blood corpuscles have 

 reached maturity, for if they are still undeveloped at birth the 

 mother can have no normal opportunity to become immunized 

 against them; and even if the mother were to be artificially 

 immunized, the antibodies so formed could not attack the 

 foetal blood. This second condition is satisfied by cattle, sheep 

 and horses, and also by human beings, but not by mice and 

 probably not by rats. 



On the face of it, mice should be specially liable to be 

 immunized by their own young, for a female mouse may give 

 birth to a quarter of her own body-weight of young in a single 

 pregnancy and to ten times her body-weight in a lifetime; 

 furthermore, antibodies formed in the mother have ready 

 access to the embryos within it. But it seems to be impossible to 

 give mice haemolytic disease, even when their mothers are 

 deliberately immunized against the red blood corpuscles of 

 their young. Mitchison attributes this to their extreme im- 

 maturity at birth; if the degree of maturity of red blood cells is 

 taken as a yardstick, birth in mice takes place at a stage equi- 

 valent to a human embryo that is still six months from term. 

 Conversely, cattle, horses and sheep can only get haemolytic 

 disease after birth — a stage at which it is clinically manageable 

 — for it is only at birth that antibodies pass from the mother to 

 her young, in the colostrum, the first watery milk. 



Human beings qualify on both counts: maturity at birth and 

 the ability of antibodies to gain access to the unborn young. 



124 



