412 HUMAN BIOLOGY 



factors of injury, treatment with the toxins alone will 

 induce the formation within the animal of a neutralizing 

 constituent in the blood, the specific antitoxin, which will 

 not only protect the tissues of the immunized animal from 

 injury by any absorbed or injected toxin, but can be used, 

 by taking the serum from such an animal, to protect others, 

 i.e., antitoxin treatment. And to emphasize that these so- 

 called bacterial toxins are merely a special group of a larger 

 class of similar things in nature, it is well to state here that 

 what we have said about bacterial toxins applies as well to 

 snake poisons, spider poisons, the vegetable poisons (ricin, 

 crotin and abrin) and to certain enzymes. 



In cases in which the invasive power of the bacteria 

 is relatively more important than their toxin production the 

 immunity is antibacterial rather than antitoxic, and the blood 

 serum of the animal treated with the attenuated or dead 

 bacterial bodies acquires a substance which specifically unites 

 with the bacteria (sensitizat on) and alters them so that they 

 are more susceptible to a number of destructive effects. 

 Bacteria mixed with such an immune serum will rapidly clump 

 into masses, precipitating to the bottom of the tubes. By 

 suitable experiments (absorption tests) it can be shown that 

 in the course of this phenomenon the bacteria have absorbed 

 out of the serum the substance responsible for the reaction. 

 Also, it can beshown that the actual clumping of the bacteria 

 is due to the fact that union with this immune constituent of 

 the serum has rendered the cells susceptible to electrolytes in 

 the fluid, probably altering their suspension-equilibrium, so 

 that they are precipitated by the electrolytes just as are col- 

 loidal suspensions. At the same time the union with this anti- 

 body or "sensitizer," as it is called, has rendered the bacteria 

 susceptible to an enzyme-like normal constituent of the 

 serum, the "alexin" or "complement," which can often 

 kill sensitized bacteria of varieties which it cannot injure in 

 the unsensitized state. Again, the wandering and fixed 

 phagocytic cells of the body, leucocytes and various endothelial 

 cells, can take up and destroy virulent bacteria much more 

 effectively after their union with the serum antibody than 

 in their native condition. Thus the immunization has induced 

 the formation of a specific reaction body which becomes free 



