IMMUNITY AND SUSCEPTIBILITY 713 



case recovery would be indicated. When vaccines are injected in the 

 treatment of infections the opsonic index has been shown to vary from 

 time to time. Within a few hours after the injection the opsonic 

 index falls below what it was at the time of the injection. This lower- 

 ing of the index is known as the "negative phase." Following the 

 fall in the index there is a continuous rise to a point equal to what it 

 was in the beginning and above this point. This rise in the opsonic 

 index is known as the "positive phase." The individual receiving the 

 vaccine usually shows an increase in the symptoms during the "nega- 

 tive phase." Obviously, it is necessary not to give a subsequent in- 

 jection of vaccine until the patient is at the height of the "positive 

 phase." This can be best determined by determining the opsonic 

 index. 



Occasionally counts are made of the number of leucocytes which are 

 actually taking up bacteria, disregarding the number of bacteria within 

 the cells. The determination is always made on the basis of 100 and 

 the per cent of leucocytes which are phagocytic is taken as the so-called 

 percentage index. The percentage index also gives an idea of the re- 

 sistance of the individual. It has been shown that in the practical 

 work of treating infections with vaccines it is not absolutely necessary 

 to determine the opsonic index or percentage index. The positive 

 and negative phase may be determined fairly well by general clinical 

 observations on the infected individual. Virulent bacteria are not 

 readily phagocytized. For example, virulent streptococci and pneu- 

 mococci are not phagocytized as easily as non- virulent forms. It. 

 seems in this instance that there is some toxic or poisonous substance 

 produced by the bacteria that is antagonistic to the opsonins or perhaps 

 an antiopsonin is formed. 



The presence of opsonins in the body fluids of an animal is not 

 absolute proof that such animal is highly resistant to infections. The 

 resistance really depends on the activity of the phagocytes and in 

 certain cases where the opsonins are high in concentration the phago- 

 cytes are not active. In other cases the reverse is true and in these 

 cases opsonins and phagocytosis are of the utmost importance in the 

 immunity of individuals. For example, in anthrax the immunity of 

 the dog is due to opsonins and phagocytosis, while in the rat, although 

 opsonins are present, there is no phagocytosis and immunity is due to 

 antibacterial substances in the blood serum. In certain infections, 



