ZOOLOGY, ENTOMOLOGY, AND MEDICINE 241 



immune against a second. The same is true of animals having 

 had hoof and mouth diseases. This is called natural acquired 

 immunity. 



We can produce artificial immunity either active or passive. 

 Vaccination or the injection of bacterial toxins, produces active 

 immunity, while the injection of an immunizing serum such as 

 diphtheria antitoxin confers passive immunity. In other words 

 in the first instance the patient supplies his own antibodies — 

 active immunity (in the second instance the anti-bodies are 

 supplied to the patient), passive immunity. 



In active immunity, following recovery from either an idio- 

 pathic infection or an artificially produced infection, there are 

 developed in the blood the anti-bodies which are inimical to 

 the toxin or the activity of the bacteria of themselves, or which 

 accomplish the destruction of the causative agent by the ac- 

 tion of the phagocytes. Normal blood serum has a powerful 

 destructive effect upon many varieties of bacteria, and this 

 power is found to be greatly increased in a patient who has 

 been infected with these bacteria, either naturally or arti- 

 ficially. There can be no doubt that in all cases of acquired 

 immunity, either active or passive, the leucocytes have per- 

 formed the large and important work of destroying and ab- 

 sorbing the process of phagocvtosis. 



Pathogenic bacteria secret very powerful soluble toxins 

 which enter the general circulation; whenever such toxins 

 circulate in the blood, there is a tendency to the formation of 

 bodies which neutralize them, and bring about a cure of the 

 conditions, provided that the toxins are not over abundant and 

 have not already done irreparable damage. These bodies 

 which neutralize the soluble toxins are called anti-toxins. The 

 action of the anti-toxin upon a toxin, is best understood by 

 comparison with the well known chemical reaction between 

 acids and alkalies. Just as hydrochloric acid can be neutral- 

 ized in a test tube with ammonia, so can a soluble toxin be 

 neutralized with its anti'toxins. The principle is the same, al- 

 though the process is much more complicated in the neutraliza- 

 tion of toxin by anti-toxin than of acids by alkalis. The anti- 

 toxin mixture can be injected into a susceptible animal with- 

 out producing any ill effects. Thus the formation of anti- 

 toxin is another means by which the body protects itself against 

 pathogenic bacteria, and their most important products, toxins. 



