IMMUNITY 73 



opsonins. They are specific, just as are the antitoxins, 

 the bacteriolysins, and the agglutinins; that is to say, 

 when the body is injected with typhoid bacilli, only 

 the opsonic power for typhoid bacilli is affected; when 

 staphylococcus pyogenes is employed, only the opsonic 

 power for this germ is affected; when pneumococci 

 are injected, only the opsonic power for pneumococci 

 is affected. Wright devised a special techni-c for measur- 

 ing the amount of opsonin present in a serum, and ex- 

 pressed this in what he calls the opsonic index. This 

 is merely the opsonic power of the patient's serum, as 

 compared with the opsonic power of several known nor- 

 mal sera, using the same leukocytes and the same bac- 

 teria in the one test. Although Wright believes that 

 this opsonic index is essential in the bacterial vaccine 

 treatment of infections, most other observers have 

 failed to find the index of any real help. 



Precipitins and Other Antibodies. We have seen 

 above that the injection into the animal body of bac- 

 teria or other cells is followed by the production of 

 a number of different antibodies. If, instead of inject- 

 ing bacteria, we inject solutions of albuminous material, 

 for example, inject a rabbit with chicken egg-albumin 

 (white of egg), we shall find that the rabbit's serum 

 acquires the power to produce a precipitate when mixed 

 with chicken egg-albumin. This action is highly specific, 

 so that if the serum is tested against albumin from any 

 other animal e. g., from a duck no precipitate will be 

 produced. If a rabbit is injected with human blood 

 (which, of course, is really an albuminous solution) the 

 rabbit serum will produce a precipitate when mixed with 

 human blood, but not when mixed with any other blood. 



