PRINCIPLES OF THE OPSONIC INDEX 199 



demonstrated that phagocytosis occurs in most cases only in the presence 

 of serum. If the phagocytes are thoroughly washed, so that they are 

 entirely serum-free, phagocytosis will not take place, or will do so imper- 

 fectly. The belief of some authors that " spontaneous phagocytosis" 

 without serum was altogether impossible was disproved, especially by 

 Lohlein. The manner in which the serum acts, whether it stimulates the 

 digestive activity of the leucocytes or whether it so changes the bacteria 

 that they can more readily be taken up by the phagocytes, has been settled 

 in favor of the latter view through researches, especially of Wright and his 

 followers as well as of Neufeld. The substances within the serum which 

 thus modify the bacteria have been designated by Wright as "opsonins." 

 C'opsono" = I prepare food for.) 



Opsonins are demonstrated by mixing bacteria, serum and washed 

 leucocytes, allowing this mixture to remain in the incubator for a short 

 time, and then staining smear preparations. Wright then counts a 

 certain number of leucocytes and the number of bacteria found within 

 these leucocytes. The relation between the number of ingested bacteria 

 and the counted number of phagocytes is designated as the 

 The Opsonic phagocytic count. Wright compared the phagocytic counts 



Index. of infected individuals with those of normal persons and found 

 that those of the former were much lower. The relation existant 

 between the two he expressed in the form of a fraction and that is known as 

 the opsonic index. Thus a smear made from a mixture of equal parts of 

 an emulsion of staphylococci, leucocytes and the patient's serum showed 

 for example 75 cocci to 100 leucocytes; while one made from a mixture 

 of equal parts of the same bacterial emulsion and leucocytes, but a 

 normal individual's serum, demonstrated 150 bacteria to 100 leucocytes. 

 The opsonic index of the patient's serum was therefore one-half (0.5). 



According to Wright, the opsonic index expresses the animal's resist- 

 ance to infection. He believes that a low opsonic index for a given 

 bacterium indicates a susceptibility on the part of the individual for that 

 particular infective agent. Furthermore, the opsonic index he claims 

 can be used as an aid in the diagnosis of infectious diseases, inasmuch as 

 opsonins are specific. Thus the opsonic index in a tuberculous individual 

 is low only for the tubercle bacillus and not for other bacteria. 



When an animal is immunized, its opsonic index toward the respective 

 bacterium is considerably increased. The question has been asked 

 whether the immune opsonins formed during this process are identical 

 with the normal opsonins. Wright and a number of the more recent 

 authorities believe that they are different. Neufeld, who discovered these 

 immune opsonins independently of Wright, named them Bacteriotropins, 

 and pointed out that while the normal opsonins are destroyed when heated 

 to 56, the bacteriotropins remain unharmed. As yet the exact nature 



