IMMUNOLOGY. IMMUNITY AND IMMUNIZING AGENTS 239 



sulphate and other salts. Remarkably enough, reactions have been ob- 

 served which would indicate that antitoxin is not a proteid substance; for 

 example, it is not destroyed (digested) by trypsin. 



It has furthermore been found that variably small amounts of antitoxin 

 exist in normal blood; that is, in the blood of animals that have not been 

 naturally or artificially immunized, and also in still lesser amounts in the 

 milk of normal animals. As to the origin of the antitoxins the physio'ogic 

 evidence points to their formation in the body cells rather than in the 

 blood serum. 



Another important discovery was that normal blood could actively de- 

 stroy (lake) bacteria, and in common with antitoxins, this bactericidal 

 property was found to be specific. That is, serum found to be quite 

 destructive to the typhoid bacillus is not destructive to the cholera bacil- 

 lus. These germ destroying or bactericidal substances are designated 

 lysins. Ehrlich has discovered that there are in fact three distinct blood 

 lysins; namely, cytolysin, a substance which is capable of destroying 

 (laking) body cells; hemolysin, which is capable of destroying red blood- 

 corpuscles; and bacteriolysin as already explained. By injecting tissue 

 cells, as those of kidney or of some other organ, into an animal, there are 

 developed in the blood of the inoculated animal lysins which will dissolve 

 kidney cells or other organ cells used. If the blood of a bird or other 

 animal is injected into an animal of a different species, hemolysins will 

 appear in the blood of the animal thus injected. This hemolysin is specific, 

 as it will only dissolve or destroy the red blood cells in the blood of the kind 

 of animal of which the blood was used for injecting. An animal inocu- 

 lated with the typhoid bacillus will produce a bacteriolysin which destroys 

 the typhoid bacillus. Lytic sera become inactive when heated to 55 C. 

 for one-half hour and such sera are said to be inactivated. However, if 

 normal serum is added to the inactivated serum the bactericidal power is 

 fully restored. The bactericidal power of the serum can be greatly in- 

 creased by the use of highly virulent bacterial cultures, thus producing a 

 serum of high potency. In actual practice, as in the manufacture of 

 bactericidal sera for the prevention and cure of disease, the animal (as 

 horse) is first inoculated with attenuated cultures, then with normally 

 virulent cultures and finally with hyper-virulent cultures of the specific 

 pathogenic microbe. Such sera act by destroying the disease-producing 

 bacteria, but they have no effect upon the toxins produced by the bacteria, 

 thus showing that they are entirely distinct from the antitoxins. 



The eminent bacteriologist Metchnikoff made the very interesting 

 discovery that the white blood-corpuscles (leucocytes) had the power of 

 feeding upon and digesting bacteria with which they came in contact. 

 That is the white blood corpuscles, called phagocytes, act as the defenders 



