GERM-KILLING POISONS IN THE BLOOD 353 



formed. This is a far more efficacious method than the 

 supposed mithridatic " habituation " or " toleration " of a 

 poison, with small doses of which you have to be gradually 

 prepared. The healthy blood converts any one of a large 

 series of microbe poisons into antitoxins. It is true that 

 apparent " opposites " are often closely allied in Nature. 

 Evil smells and tastes are closely allied to sweet perfumes 

 and flavours, and what is healthy and agreeable to some 

 men acts as virulent poison to others (e.g. shell-fish, egg, 

 quinine, opium). The smallest change in the substance 

 administered or the smallest difference in the living sub- 

 stance of an individual (what is called " idiosyncrasy ") 

 makes all the difference between " poison " and " meat." 



If the phagocytes and similar cells in the blood of a man 

 or animal exposed to the poison produced by localised 

 microbes (such as those of tetanus, diphtheria and septic 

 growths) cannot'produce enough antitoxin so as to quickly 

 destroy the poison, we can, and do, nowadays, save his life, 

 by injecting into his blood the required antitoxin, obtained 

 from another animal which we have caused (by injection 

 of the toxin) to produce the antitoxin in excess. That 

 is one sort of " immunity " or " resistance " which we can 

 confer, and is largely in use at the present day the " anti- 

 toxin " treatment. 



The second poison-repelling chemical activity of the 

 blood, produced by the living cells in and about it, 

 consists in the blood becoming directly poisonous to 

 injurious microbes. It becomes " bactericidal," produces 

 a bactericidal poison (called an alexin) which is usually 

 present in normal blood, but is greatly increased when 

 large numbers of certain poisonous microbes (e.g. those 

 of typhoid fever) get into the blood. Again, by other 

 chemical substances produced in it, the blood may, 

 without actually killing the invading bacteria, only para- 

 lyse them, and cause them to " agglutinate " (that is, to 



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