534 MALARIAL FEVER 



the apyrexial stages the total number of leucocytes may be 

 diminished but that there is always an increase of the mono- 

 nuclear cells, these frequently numbering 20 per cent 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 

 mononuclear 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 specially interesting in 

 the light of the leucocytic 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 has 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 immunity of the adult being 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 explanation. 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 parts played by the 

 development of artificial immunity on the one hand, and the 

 existence of a natural immunity on the other, in apparent un- 

 susceptibility to malaria. 



Our knowledge on the relationship of blackwater fever to 

 malaria is also in an unsatisfactory condition. Blackwater fever 

 is a condition often occurring, especially in Europeans, in tropical 



