. 



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. By the 

 use of vaccines the opsonic index may be raised to at least i.o or even more, 

 showing that the leucocytes are actively phagocytic and the opsonins in- 

 creased in concentration 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 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 fuch 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 phagocytes are not 

 3-ctive. In other cases the reverse is true and in these cases opsonins 

 and phagocytosis are of the utmost importance in the immunity of in- 

 dividuals. For example, in anthrax, the immunity of the dog is due to 

 opsonins and phagocytosis, while in the rat, although opsonins are present, 



