566 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



The Opsonic Index. The concentration of opsonins may be recorded 

 in an individual. Suppose the leucocytes of the infected individual take 

 up a certain number of bacteria, say an average of 5, after counting 50 

 to 100 polymorphonuclear leucocytes. In this case the phagocytic index 

 is said to be 5. Again, suppose the leucocytes of the normal individual 

 take up 15 of the bacteria in question, the average after counting 50 to 

 100 leucocytes being always taken. The phagocytic index in this case 

 would be 15. In order to determine the opsonic index of an infected 

 individual the phagocytic index of the normal individual is taken as a 

 denominator of a fraction and the phagocytic index of the infected 

 individual as the numerator of the fraction. In our above illustration 

 this would be 5/15, 1/3 or reduced to decimals 0.33 + . The opsonic 

 index, it can be seen, is a fair indication of the resistance of the par- 

 ticular individual to the infecting microorganism in question so far as 

 phagocytic activity is concerned. By the use of vaccines the opsonic in- 

 dex may be raised to at least i.o or even more, showing that the leucocytes 

 are more actively phagocytic due to increased concentration of its opsonic 

 in the blood serum. In such a case recovery will be indicated. 



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 per- 

 centage index. The percentage index and the opsonic index give a very 

 fair idea as to the resistance of the particular individual to an infecting 

 microorganism. 



The virulent bacteria are not readily phagocytized. For example, 

 virulent streptococci and pneumococci 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 possession 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, as well as the 

 serum, and in certain cases where the opsonins are high in concentration 

 the phagocytes 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 



