156 THE BLOOD 



forming staphylococci a weaker effect, and on the streptococci, the diphtheria 

 and anthrax bacilli, no effect at all. On the other hand the serum of the 

 rabbit kills the bacteria of anthrax and of typhoid fever, but is harmless 

 for the pus-forming staphylococci, etc. 



It has long been known that many diseases confer on the individual who 

 survives them, as an after effect,, an immunity or unsusceptibility toward those 

 particular diseases. Now it has been shown (Behring and Kitasato, 1890) 

 that the blood or the serum of an individual who has become immune in this 

 way against an infectious disease, has the property, when it is injected in 

 sufficient quantity, of conveying the immunity to another individual previously 

 susceptible to the disease. A blood or serum of an individual who has success- 

 fully withstood the disease receives therefore as an after effect properties which 

 it did not possess before. 



These discoveries represent the point of departure of numerous investiga- 

 tions on the various changes which appear in the blood after injection of dif- 

 ferent substances. In general, one may say that if a foreign substance of a 

 certain kind (such as the toxins formed by the Bacteria, foreign blood, various 

 proteid substances, finely minced organs, etc.) is brought into the animal by 

 subcutaneous, intraperitoneal or intravenous injection, the blood of the animal 

 acquires the ability to change the foreign substance in some way, and thus to 

 neutralize its effect on the organism. Since in such cases there are found in 

 the blood specific antibodies, we infer that the changes appearing in the blood 

 are of different kinds according to the nature of the substance injected. 



If a bacterial toxin is injected into the blood, there arises in the latter an 

 antitoxin specific for just this toxin, which has the power to abolish the effect 

 of the former, probably by a kind of neutralization. Different toxins possess 

 an elective power for different cells of the body; and precisely those cells which 

 are attacked by a definite toxin appear to be most active in the formation of 

 the antitoxin. By a kind of internal secretion the antitoxin is given off to 

 the blood and in such abundance that it may be used (for example, diphtheria 

 serum) as a remedy in other animals. 



The power of the blood to destroy blood corpuscles, Bacteria and foreign 

 cells generally, like its antitoxic properties, can be increased by the addition of 

 appropriate substances. Here also we have to suppose that under the influence 

 of the foreign cells, the cells of the body (certain leucocytes especially) are the 

 seat of production of the antibodies. As with the antitoxin serum, the cytolytic 

 serum can also exercise its specific effects outside of the body from which it 

 follows that these are not bound up in the formed constituents of the blood, and 

 in general not in the living' substance. 



If the serum of an animal immunized against say typhoid fever is mixed 

 with a fluid culture of Bacillus t'yphosus, the latter become stuck together, agglu- 

 tinated. There has been formed in the blood of the animal therefore a substance 

 which produces this effect. The same influence may be observed also on the 

 blood corpuscles. When an animal has been treated with a foreign blood, its 

 own serum added to the blood in question brings about an entirely similar agglu- 

 tination of the corpuscles of the foreign blood. 



After intraperitoneal injection of cow's milk the serum of the animal em- 

 ployed acquires the ability to throw down a precipitate in the cow's milk i. e., 

 a precipitin has been formed in the blood. By injection of different kinds of 

 blood or proteid solutions, one may obtain different precipitins which on the 

 whole are specific since they produce precipitates especially well in solutions of 



