578 IMMUNITY 



lysis. This, however, is not justifiable, since normal thermo- 

 labile opsonin can, as we have seen, act by itself, as can also 

 the specific immune-opsonin after normal opsonin has been 

 destroyed by heating, and we know of no corresponding action in 

 the case of an immune-body. The subject is one of considerable 

 complexity, but it may be said that the most important varia- 

 tions in the opsonic content observed in infections depend on 

 the specific immune-opsonins, though the presence of immune- 

 body may play a part in raising the index, by leading to the 

 union of more normal-complement-opsonin. 



Increased phagocytic action had long been known by the work 

 of Metchnikoff to be associated with the development of active 

 immunity, and the theory of stimulation of leucocytes was 

 supported by many. The work on opsonins has caused a swing 

 of the pendulum in the other direction, and points to the 

 development of anti-substances in the serum as the all-important 

 factor. Some recent researches, however, go to show that in 

 some cases the leucocytes of the immunised animal are more 

 actively phagocytic than those of the normal animal. It remains 

 to be determined to what extent the opsonic and directly 

 bactericidal properties, taken together, will explain the phenomena 

 of natural and acquired immunity. 



(c) Agglutination. Charrin and Roger in 1889 observed 

 that when the bacillus pyocyaneus was grown in the serum of 

 an animal immunised against this organism, the growth formed 

 a deposit at the foot of the vessel ; whereas a growth in normal 

 serum produced a uniform turbidity. Griiber and Durham, in 

 investigating Pf eiffer's reaction, found that when a small quantity 

 of an anti-serum is added to an emulsion of the corresponding 

 bacterium, the organisms become agglutinated into clumps, this 

 phenomenon depending upon the presence of definite bodies in 

 the serum called agglutinins. 



It had already been found that the serum of convalescents 

 from typhoid fever could protect animals to a certain extent 

 against typhoid fever, and, in view of the facts experimentally 

 established, it appeared a natural proceeding to inquire whether 

 such serum possessed an agglutinative action and at what stage 

 of the disease it appeared. The result, obtained independ- 

 ently by Griinbaum and Widal, but first published by the latter, 

 was to show that the serum possessed this specific action shortly 

 after infection had taken place; in other words, the develop- 

 ment of this variety of anti-substance can be demonstrated at 

 an early stage of the disease. Agglutination may be said to be 

 observed generally in bacterial infections, though the degree of 



