496 INFECTION AND IMMUNITY. 



Natural im- It is probable that no disease is more perplexing 

 ?i* from the standpoint of immunity than anthrax. 

 ty. rpk e variations in susceptibility and immunity 

 among different animals are extreme: Guinea- 

 pigs, rabbits and mice are probably more suscepti- 

 ble than sheep and cattle ; compared with these the 

 dog and rat are relatively immune, whereas fowls 

 and cold-blooded animals can be infected with dif- 

 ficulty. Although the microbe is readily killed by 

 suitable serums (rabbit, e. g.), such an effect is not 

 an index of immunity. The serum of the highly 

 susceptible rabbit is strongly bactericidal in test- 

 glass experiments, whereas that of the more resist- 

 ant dog, or rat, has little or no bactericidal power. 

 Because of this inconsistent relationship of the 

 serum to immunity, and since the leucocytes have 

 a high phagocytic power for the anthrax bacillus, 

 Ptruschky, Frank and others agree with Metchni- 

 koff in assigning variations in the natural immun- 

 ity of different animals to variations in phago- 

 cytic power. Bail and Pettersson, in extensive ex- 

 perimental work, discovered conditions which, they 

 believe, explain the lack of correspondence between 

 serum properties and natural immunity. In the 

 serum of the relatively immune dog and chicken 

 they found bactericidal amboceptors but no com- 

 plement; hence, the serum could show no bacteri- 

 cidal action in the test-glass. If, however, leuco- 

 cytes from the same animals were added to the 

 serum, the latter became bactericidal. It may be 

 assumed that in the course of infection the ambo- 

 ceptors are activated by complement which is dis- 

 charged from the leucocytes. The failure of the 

 bactericidal substances of the rabbit's serum to 

 protect the animal was ascribed to the ability of 



