Immunity acquired by natural means 453 



municate her disease to other persons by means which are without 

 effect on her own infant. This example comes under the law of 

 Profetta. Here again the mechanism of the acquired immunity is 

 absolutely unknown. 



It must be admitted that, generally, we are still very imperfectly 

 informed concerning immunity as acquired by natural paths. In 

 cases where this immunity is developed as the result of an attack of 

 an infective disease the phenomena observed closely resemble those 

 that have been observed after vaccination by living, fully active, or 

 attenuated viruses, by micro-organisms which have been killed, or by 

 the products of these micro-organisms. These vaccinations which 

 bring about isopathic (von Behring) or active (Ehrlich) immunity give 

 rise to transient and mild diseases and are confined almost completely 

 to the diseases contracted by natural means which terminate in [475] 

 recovery and give rise to a refractory condition. The immunisation 

 of the foetus comes into the same series. 



On the other hand, the immunity which was believed to be 

 hereditary and which results merely from the direct passage of the 

 antibodies of the blood or of the milk of the mother to the foetus 

 and to the child come into a group of cases characterised by what 

 Ehrlich has termed a condition of passive immunity. We have 

 already discussed (Chapter x) the thesis that this term " passive " is 

 applicable only in rare cases. Most frequently it is necessary that 

 the living cells of the organism which receives the antibodies 

 antitoxin, fixatives or others should contribute their quota in order 

 to ensure the refractory condition. This rule is undoubtedly applic- 

 able to the examples of immunity acquired by the new-born progeny 

 of unaffected mothers. 



