At Tl'RE OF THE PROTECTIVE DEFENCES OF THE BODY 171 



and so allowed the complement to penetrate the cell substance or 

 that, as the French put it, changes its nature so it can combine with 

 its complement. Muir has shown that when cells are saturated with 

 both immune body and complement that the addition of fresh cells 

 causes a splitting oft' of immune body but not of complement. This 

 throws further doubt upon the direct union of immune body and com- 

 plement. Most of the experiments which have been made with the 

 purpose of clearing up these difficult problems have been made upon 

 red blood cells. Here the absorption of the immune bodies at low 

 temperatures and the lack of noticeable injury until the complement 

 is added, at a temperature of 20 to 30 C., is very striking. The 

 immune bodies are very numerous and fairly specific in their action. 

 The complement substance is much less specific and, although probably 

 multiple, each variety acts upon widely different bacteria after they 

 have united with the immune body. There is little reason to think that 

 the complement of one animal is any more capable to attack bacteria 

 prepared by immune bodies developed in its blood than by immune 

 bodies developed in some other species. 



The building of immune bodies in the infected animals is believed 

 by most to take place the more rapidly the more virulent the infecting 

 organisms are. In our experiments this has not been evident. Increase 

 of virulence for one species of animal does not mean increase for all 

 animals ; so that the animal upon which the virulence is tested must be 

 the same variety as the one being immunized to draw conclusions. 



Origin of Bactericidal Substances. Ehrlich and his followers consider 

 the immune bodies to be built up in the same way as the antitoxins. 

 They are cell atom-groups, which are similar to those which were 

 attacked by the bacterial substance and which were overproduced as 

 the cell attempted to replace what had been destroyed or bound up. 



Their source must apparently be attributed to the cells, but probably 

 only certain cells produce them. The red blood cells, for instance, seem 

 rather to destroy than to increase them. The nuclein derived from the 

 cells, although it has a general bactericidal action, and may enter into 

 the complements (alexins), has different properties, and so cannot 

 itself be one of these bodies. The cells which have abundant nuclear 

 substance, such as the leukocytes and lymph cells, seem especially to 

 be a source, and Metchnikoff asserts their pre-eminent role as the pro- 

 ducers of both complements and immune bodies. Buchner and others 

 have found that through the irritation of bacterial filtrates the leukocytes 

 were attracted in great numbers to the region of injection, and that the 

 fluid here, which was rich in leukocytes, was more bactericidal than 

 that of the blood serum elsewhere. Some claim to have demonstrated 

 that along with increased leukocytosis there is a general increase in the 

 complement in the blood; still, it has not yet been positively established 

 that the complement is derived solely from the leukocytes, nor from 

 all leukocytes, and a mere increase in them does not always mean an 

 increase in the complement. Immune bodies appear to be more abun- 



