PHAGOCYTOSIS 189 



meets with resistance, and when the numbers are not too great, all the 

 spores are taken into leukocytes and digested. When the spores are 

 too numerous, they develop and pour out a secretion which kills and 

 digests the leukocytes, then fatal infection follows. In the trans- 

 parent daphnia all stages of this combat can be followed under the 

 microscope. Metchnikoff applied this knowledge to a study of anthrax 

 infection. He found that when animals naturally immune to this 

 organism, such as dogs and frogs, are inoculated, the bacilli are 

 engulfed in leukocytes which speedily gather at the point of inocula- 

 tion. On the other hand, in highly susceptible animals, such as guinea- 

 pigs and mice, phagocytosis is wholly wanting or but slightly in evi- 

 dence. In moderately susceptible animals, such as rats and rabbits, 

 phagocytosis is more marked and may or may not protect, depending 

 on the virulence and number of the bacilli introduced. Further studies 

 in many laboratories have shown that the greater the susceptibility of 

 an animal to anthrax, the less markedly does phagocytosis occur on 

 inoculation, and the more highly immune the animal the more marked 

 is the phagocytosis. It is generally admitted that natural immunity to 

 anthrax is due to phagocytic effectiveness. 



In the study of certain anaerobic bacteria, as the bacilli of tetanus, 

 symptomatic anthrax and malignant edema, interesting facts concern- 

 ing phagocytosis have been ascertained. Paradoxical as it seems, all 

 animals have a natural immunity to tetanus, but this immunity is sup- 

 pressed when certain non-pathogenic bacteria are introduced into the 

 animal along with the tetanus bacillus. The injection of a large 

 amount of tetanus bacilli or their spores into animals are without 

 effect provided there is no ready-made tetanus toxin injected at the 

 same time. Under this condition, a large number of leukocytes collect 

 at the point of inoculation and soon devour the bacilli and spores. But 

 when the ready-made toxin is present, phagocytosis does not result 

 and a fatal infection follows. The explanation has been found to be 

 due to the fact that the toxin protects the tetanus bacilli by exerting 

 a negative chemotaxic effect on the leukocytes. It holds back or repels 

 the phagocytes until the spores and bacilli develop and produce more 

 toxin. A capillary glass tube filled with tetanus toxin and introduced 

 under the skin of an animal remains for a long time free from leuko- 

 cytes, while a similar tube filled with bacilli and spores, free from 



