62 INFECTION, PHAGOCYTOSIS, OPSONINS 



Hektoen has shown that anthrax bacilli are taken up by the leuko- 

 cytes of the dog in the presence of dog's blood serum. If the leuko- 

 cytes are washed several times with physiologic salt solution, so that 

 they are entirely free from serum, and then mixed with a bacterial 

 emulsion, no bacteria are taken up by phagocytosis. Hektoen also 

 demonstrated that the opsonins in the dog's blood serum which 

 bring about the phagocytosis of anthrax bacilli are destroyed if the 

 serum is subjected to a temperature of 56 to 60 C. for thirty minutes. 

 The leukocytes themselves are made unfit for phagocytosis if heated for 

 thirty minutes at 45 C., but these cells of the dog contain a thermo- 

 stabile anthracidal substance, not destroyed by heating to 56 C., 

 which can be extracted with distilled water after self-digestion. In 

 working with anthrax bacilli to determine phagocytosis it is sometimes 

 difficult, on account of the length of the pseudofilaments, to determine 

 whether a given leukocyte is or is not engaged in phagocytosis. This 

 difficulty can be easily overcome by systematic comparisons of pre- 

 parations from experiments with or without dog's blood serum. 



Opsonic Index. When the blood serum of several healthy persons or 

 animals is examined with reference to the same pathogenic micro- 

 organism, it is found that the amount of opsonin present gives a 

 fairly constant average. The amount of the opsonin, of course, cannot 

 be estimated directly. Indirectly it is calculated by the amount of 

 phagocytosis which occurs in properly arranged, simultaneous, and 

 equivalent tests. It can also be shown that, as a rule, in many chronic 

 bacterial infections the amount of opsonin in the blood serum falls below 

 the normal. For instance, it will be found that one hundred cattle 

 leukocytes with the mixed blood sera from five healthy cattle will 

 take up by phagocytosis two hundred tubercle bacilli; while in the 

 same experiment with sera from five cases of mild chronic tuber- 

 culosis, each serum being used separately, the number of bacilli taken 

 up will be from 120 to 150. It is customary to compare the number 

 of bacilli taken up in the test with the mixed sera from the healthy 

 animals or persons with the number taken up in the test with a 

 serum from an infected animal, and the figure obtained by dividing 

 the latter by the former is called the opsonic index of the blood. In 

 the example above the opsonic index of the sick animals would be 

 -J-~-, or -|~f$, or -g-g-J; that is, 0.6 to 0.75. As a rule, the opsonic index 

 in chronic bacterial infections is always low; that is below 1 (one 

 being the normal standard). 



Vaccines. It has been shown experimentally that the opsonic 

 index for a pathogenic bacterium can be raised by injecting into 

 the infected person or animal a small dose of a vaccine or bacterine 

 prepared from the particular bacterium which causes the infection. 



Vaccines are generally prepared by obtaining first the bacterium 

 which causes the chronic infection in pure culture, and then heating 

 the bacterial emulsion thus obtained to a temperature which will just 

 kill the microorganisms without damaging them too severely. Such 



