638 MALARIAL FEVER 



in these cells absorbs their pigment and thus destroys their 

 function ; this is further evidenced by the activity displayed by 

 the red marrow in its attempts to make good the loss sustained 

 by the blood. One of the most interesting events in malaria, 

 and one that links it with bacterial infections, is the reaction of 

 the colourless cells'of the blood. It has been shown that during 

 the apyrexial stages the total number of leucocytes varies greatly, 

 but that there is always an increase of the mononuclear cells, 

 these frequently numbering 20 per cent, or more of the. whole, 

 and sometimes even outnumbering the polymorphs. This is 

 such an important feature that in cases where the parasites 

 themselves cannot be demonstrated in the blood, the mono- 

 nuclear reaction along with the presence of pigment in the 

 mononuclear cells (due to phagocytosis of pigmented parasites) 

 has been taken as evidence that the case is really one of malaria. 

 The mononuclear reaction is specially interesting from the fact 

 that in other protozoal diseases an activity of the same elements 

 has been observed. 



The question of the possibility of immunity to malaria being 

 developed naturally arises, and this is especially interesting in 

 the light of the leucocyte reaction which we have seen must be 

 looked on as an element in immunity against bacterial infection. 

 With regard to Europeans developing immunity, it is difficult to 

 speak. In such a malaria-stricken region as the West Coast of 

 Africa, the death-rate in residents of more than four years' 

 standing is less than in the previous years, but this may be due 

 to the survival of the more resistant immigrants. But there can 

 be little doubt that malaria in the negro is a much less serious 

 condition than in the European. Koch from his observations in 

 New Guinea attributes this to the infection of the native children 

 leading to the development of immunity in the adult community. 

 He found, what had been independently noted by Stephens and 

 Christophers in West Africa, that the greater number of the 

 children harboured malarial parasites in their blood. The wide- 

 spread presence of parasites in children might appear to preclude 

 the possibility that the immunity of the adult is due to survival 

 of the most resistant, but the infant mortality in these regions 

 may be very high, and such a survival may be the real explana- 

 tion-. On the other hand, Koch states that while an immunity 

 appears to exist in native adults in malarial districts, this is 

 only true of those born in the locality, — natives coming from 

 neighbouring non-malarial districts into the malarial region being 

 liable to contract the disease. At present it must be held that 

 the facts available do not enable us to determine the relative 



