338 FOUNDATIONS OF BIOLOGY 



without any apparent discomfort. But if the intermediate 

 hosts, which are biting Flies, transfer for example Trypano- 

 soma brucei to imported Horses or Cattle, a serious disease 

 results which is usually fatal. Indeed, the opening up of cer- 

 tain regions of Africa has been greatly retarded by the 

 ravages of this Trypanosome in new hosts to which it is not 

 adapted. Generally speaking, pathogenic species may be 

 regarded as aberrant forms which are not yet adapted to 

 their hosts or are not in their normal hosts. And these are 

 the parasites which are forced upon our attention, though 

 there are few organisms without their specially adapted para- 

 sites the parasites themselves not excepted. 



4. Immunity 



At best, however, the part played by the host cannot be 

 regarded as ideal, and devious types of adaptations against 

 parasites exist which, insofar as they are effective, bring 

 about IMMUNITY. Usually among the higher animals, includ- 

 ing Man, immunity to pathogenic Bacteria seems to have its 

 foundations in specific chemical substances in the blood, 

 termed ANTIBODIES. These either modify the activities of 

 certain cells of the body, chiefly the white blood corpuscles, 

 or act directly upon the invaders themselves and the poisons 

 (TOXINS) which they produce. The white blood corpuscles 

 have been called the 'policemen of the body' because, under 

 the influence of invading organisms and of certain antibodies 

 called OPSONINS, some of them make their way through the 

 walls of the capillaries in the region of the infection and, in 

 amoeboid fashion, engulf and digest the intruders. When 

 acting in this capacity the corpuscles are referred to as 



PHAGOCYTES. 



Among the various classes of antibodies are also the ANTI- 

 TOXINS which neutralize the poisonous products of Bacteria, 



