22 BIOLOGICAL LECTURES. 



but that phagocytosis is a factor of secondary importance in 

 resisting parasitic invasion ; also that general infection, at 

 least in some infectious diseases, is resisted, and in non-fatal 

 cases overcome, by an increase in the number of leucocytes 

 and in the alkalinity of the blood-serum, which favors solu- 

 tion of the germicidal proteids contained in the polynuclear 

 leucocytes." 



Recent researches indicate that the principal factor in the 

 production of acquired immunity is the presence in the blood 

 of the immune animal of some substance capable of neutral- 

 izing the toxic products of the particular pathogenic micro- 

 organism against which immunity exists, or of destroying the 

 "germ" itself. 



These substances are called antitoxins. As pointed out by 

 Buchner in a recent paper, the antitoxins differ essentially 

 from the so-called alexins, to which natural immunity is 

 ascribed. The alexins are characterized by their germicidal 

 and globulicidal action (they destroy both the red corpuscles 

 and the leucocytes of animals belonging to a different species 

 from that from which they have been obtained), and by their 

 coagulability and instability destroyed by sunlight and by a 

 temperature of 50 to 55 C. On the other hand, the anti- 

 toxins best known (diphtheria and tetanus) have no germicidal 

 or globulicidal action ; they resist the action of sunlight and 

 require a temperature of 70 to 80 C. for their destruction. 



Our knowledge of the antitoxins dates from the experiments 

 made in the Hygienic Institute of Tokio, by Ogata and 

 Jasuhara, in 1890. These bacteriologists discovered the im- 

 portant fact that the blood of an animal immune against anthrax 

 contains some substance which neutralizes the toxic products 

 of the anthrax bacillus. When cultures were made in the 

 blood of dogs, frogs, or of white rats, which animals have a 

 natural immunity against anthrax, they were found not to kill 

 mice inoculated with them. Further experiments showed that 

 mice inoculated with virulent anthrax cultures did not succumb 

 to anthrax septicaemia if they received at the same time a sub- 

 cutaneous injection of the blood of an immune animal. Further, 

 it was found that mice which had survived anthrax infection as 



