58 



INFECTION, PHAGOCYTOSIS, OPSONINS 



FIG. 24 



the latter have entirely disappeared. It can also be shown that the 

 anthrax bacilli which the frog's leukocytes take up are first alive and 

 virulent and remain so until the process of intracellular digestion 

 and destruction has progressed to a certain point. 



A similar experiment can be made by injecting a bouillon culture 

 of anthrax bacilli into a chicken. Here, again, the leukocytes of the 

 chicken act as phagocytes and destroy the anthrax bacilli. However, 

 if the temperature of the chicken is reduced by immersing its feet in 

 cold water, or by administering to it large doses of chloral or anti- 

 pyrin, the leukocytes will be so damaged that they lose their phago- 



cytic power. They do not now 

 take up and destroy the bacilli; 

 these multiply and the chicken 

 dies From anthrax. It can be 

 shown that the lack of suscep- 

 ^*fe A tibility of pigeons to human 



I* ^ 9tr k tuberculosis is due to the power 



/ ^jfly^ t /^rj^ M jof that bird's leukocytes to de- 



** " -' i stroy tubercle bacilli of human 



derivation. 



Guinea-pigs are ordinarily not 

 susceptible to an infection with 



x ^ &ite*~7 ^ e Streptococcus pyogenes. If 



^1^ fa 1 cultures are injected into the 



^vjr peritonea] cavity of the guinea- 



pig the cocci are taken up and 

 Numerous dipiococci (gonococci) inside of destroyed by phagocytes. How- 



the protoplasm of two polynuolear leukocytes i i 



(phagocytes). X 1000. (Author's preparation.) ever, Very Virulent Cultures OI 



streptococci frequently kill 



guinea-pigs, and it can be shown that such very virulent cocci are 

 not destroyed by phagocytosis. Rabbits are very susceptible to the 

 tetanus toxin, but if washed tetanus spores free from toxin are 

 injected into the peritoneal cavity, phagocytes take up these spores 

 and make them innocuous. 



Immunity. All of these examples show that animals without 

 receiving any preliminary treatment can protect themselves against 

 invasion and multiplication of pathogenic bacteria by phagocytosis. 

 Animals may be very susceptible, however, to certain pathogenic 

 bacteria because phagocytosis does not naturally occur, and, in con- 

 sequence, a natural immunity does not exist. Fortunately in these 

 cases an artificial immunity can generally be brought about. 



It is well, at this point, to understand that we mean by immunity 

 the power of an animal to resist the invasion of pathogenic bacteria 

 (see below). This power may be brought- about artificially by in- 

 jecting into a susceptible animal less than a fatal dose of a pathogenic 

 bacterium or a weakened, that is, an attenuated, bacterium or a weak- 

 ened virus or vaccine. 



