250 ACQUIRED IMMUNITY 



scale is thus provided up which she reacts. Her foetus receiving 

 enzymes and attenuated toxins from her tends to acquire immunity 

 at the same time. 



417. But young individuals are usually less tolerant of disease, 

 less capable of acquiring immunity, more liable to be poisoned 

 even by attenuated toxins than older individuals. 1 Consequently 

 in the earlier stages of the mother's illness the foetus tends to 

 perish. In the later stages, protected by the mother's enzymes, 

 and little injured by the much attenuated toxins, the child may be 

 born with every appearance of health ; but after birth, when both 

 the enzymes and the attenuated toxins from the mother have 

 disappeared, the microbes may reassert themselves, and the child 

 may perish, or waste and sicken for a time till its own immunity is 

 thoroughly established. If the placental vessels are uninjured, so 

 that only the mother or the child is infected, the non-infected 

 individual tends to acquire immunity more easily and quickly than 

 when microbes with their concentrated toxins are present. It 

 undergoes, in fact, a serum treatment strictly analogous to that 

 which has proved so successful in diphtheria or rather strictly 

 analogous to that by which we secure immunity to a person in 

 imminent danger of infection by diphtheria. Consequently a non- 

 infected child, born to an infected mother, tends to acquire im- 

 munity and survive, though it is possible that its immunity, being 

 ' passive,' may not be permanent Doubtless if a mother, who has 

 recovered, becomes pregnant of healthy children, she confers no 

 immunity on them, for no toxins or antitoxins are present to cause 

 reaction. So also a mother who has recovered from measles or 

 smallpox confers no immunity on her children subsequently con- 

 ceived. But doubtless, also, if she becomes pregnant of an infected 

 child, she confers immunity ; for toxins elaborated in the child pass 

 to her, and returning as antitoxins induce a reaction similar to that 

 which arises in the children she bears in the later stages of her 

 own disease. At any rate, as far as I am aware, no instance is 



1 Doubtless this is because they represent a stage in the life-history at which 

 powers of making resistance had not been evolved, or so much evolved. Young 

 embryos represent a stage when the very power of making use-acquirements did not 

 exist. Probably, therefore, the survival even of foetuses is determined less by their 

 own resisting powers than by those of their mothers. It must be noted, however, 

 that since variations occur in all stages of development, and since children as 

 well as adults are selected by most diseases, they are usually more resistant than 

 their remote ancestors to diseases of which the race has had experience as is 

 proved, for example, by the fact that English children are more resistant to 

 tuberculosis than American Indians, who therein resemble the ancestors of the 

 English (see 442). 



