148 IMMUNITY IN PROTOZOAL INFECTIONS 



withstand the disease. It is known that young animals recover more 

 easily than older ones, and that the disease is less severe at a certain 

 season. It has been shown by a number of observers that by inoculating 

 young animals with Babesia bigefnina at this particular season it is possible 

 to obtain a higher percentage of recoveries, and hence of permanently 

 immune animals, than if they had been exposed to natural infection. 

 In the case of East Coast fever also young animals are less seriously 

 affected than older ones, and it would be expected that a similar method 

 of protection could be applied. As will be shown below, it is not as. 

 a rule possible to transmit this disease by the inoculation of the blood 

 of an infected animal, but Meyer (1909) found that this could be effected 

 by inoculating the macerated spleen and lymphatic glands in which the 

 reproducing forms occur. By the inoculation of young animals with 

 emulsions of these organs Theiler (1911a, 19126) noted that though a 

 number acquired a severe and fatal disease, a much larger number survived 

 and recovered completely. As many as 50 per cent, of those which 

 survived proved resistant when exposed to infection by ticks under 

 natural conditions. Somewhat similar results were obtained by Wolfel 

 (1912) and Spreull (1914). In the production of immunity by these 

 methods it is important, as demonstrated by Theiler (1908) and Lignieres 

 (1903), to employ the particular strain of virus to which subsequent 

 exposure will occur. A previous infection with Babesia bigemina of 

 European origin w^ill not produce immunity against the parasite of South 

 Africa. 



Mechanism of lynvnunity. — During the development of an immunity 

 the blood of the animal acquires certain properties which it did not pre- 

 viously have, but which are possessed by the blood of naturally immune 

 animals. It has already been pointed out that the serum of such an 

 animal will produce a degree of passive immunity when injected into a 

 healthy animal, which is thereby protected against inoculation with the 

 organism. Such passive immunity is usually of much shorter duration 

 than active immunity, which is due to the production of antibodies by the 

 host itself as a result of an actual infection, or the introduction of modified 

 or dead parasites, or the products of their dissolution, which stimulate the 

 host to produce the antibodies without actually giving rise to an infection. 

 Where active immunity is produced without infection, the substance 

 introduced is termed a vaccine. It is evident that the immunity produced 

 is dependent upon the presence of several distinct substances, each of 

 which has its special action. It was first shown by Laveran and Mesnil 

 (1901a) that during the course of an infection with Trypanosoma lewisi 

 the leucocytes of the rat's blood are constantly ingesting trypanosomes, 

 which are ultimately destroyed. It appears that the serum of an immune 



