The Protection of Health 601 



the free toxin, and curative in breaking down the chemical 

 union of cells and toxin. 



294. Metchnikoff's biological theory. The biological en- 

 ergies that work to the preservation of health or confer immunity 

 have been explained most clearly by Metchnikoff and his school, 

 in what is known as the phagocytic theory. Certain white cells 

 of the blood plasma, and perhaps certain other cells of the body, 

 are phagocytes, devouring or absorbing not only the toxins 

 produced by bacteria and animal parasites, but the bacteria 

 and animal parasites as well. They may be considered the 

 scavengers of the organism. Reference has been made to this 

 function of the phagocytic cells in connection with the malarial 

 parasite. The number and absorptive capacity of these cells 

 will vary with the general health of the individual. The 

 phagocytes may be sufficient to confer on the individual natural 

 immunity, by neutralizing the toxins, or by attacking and 

 destroying the bacteria or animal parasites. Immunity ac- 

 quired through infection is explained by the fact that the pro- 

 tective cells have increased sufficiently in capacity and in 

 number to destroy the invading germs and to neutralize their 

 poisonous products. 



Both biological and chemical factors seem to be necessary in 

 understanding the conditions of immunity and of recovering 

 from disease with the aid of antitoxin. The phagocytic cells, 

 multiplying under stress of infection, attack and digest some 

 species of invading microorganisms. The chemical substances, 

 formed by the cells as side chains to receive food materials, 

 split off in abnormal conditions and act as antitoxins. These 

 combine freely with the respective toxins to form new chemical 

 compounds that are inert and not injurious to the body cells 

 or tissue. Thus the antitoxins are protective energies in the 

 service of the cells. Apparently immunity is dependent upon 

 the biological phagocytes and the chemical substances which 

 form antitoxins. 



Practical value of the chemical theory. The knowledge which 



