ARTIFICIAL IMMUNITY 547 



Those who seek the cause for this condition in the cells 

 find that phagocytosis, so ably described and demonstrated by 

 Metchnikofif, is the source of the individual defence. Those 

 who find the cause in the humors of the body rely upon the 

 germicidal action of the serum itself or of the substances 

 set free from the cells that are present in the liquids. 

 Metchnikoff seems to believe that the microcytase elaborated 

 from the leucocytes acts as a solvent directly upon the 

 bacteria. 



The lateral side-chain theory of Ehrlich assumes that the 

 cells of certain animals do not possess the necessary combining 

 molecules (receptors) with which the haptophore group of 

 atoms of the toxin molecule could unite, thus the destructive 

 action of the toxin upon the cell is prevented. This theory 

 refers immunity to the resistance of the animal tissues to the 

 toxin. A study of the subject, however, shows that while 

 many interesting facts have been brought to light a satis- 

 factory exposition of the cause of natural immunit}^ has not 

 appeared. 



§ 443. Artificial immunity. As the term implies, this 

 is immunity brought about in the individual after birth. The 

 most common form of artificial immunity is found in individ- 

 uals that have survived an attack of an infectious disease. 

 The most striking examples of this are cases of recovery from 

 small pox and yellow fever in man and Texas fever in cattle. 

 All of the exanthematous diseases leave the individual with 

 more or less immunity. In some other infectious diseases like 

 tuberculosis there seems to be very little if any increased 

 power of resistance imparted to a patient who has recovered 

 from the first attack. In such acute diseases as diphtheria, 

 the duration of the immunity resulting from a natural attack 

 of the disease is somewhat variable. In artificial immunity 

 there is great variation in the period of duration. 



The fact that individuals that had pa.ssed through certain 

 diseases were rendered immune to a second attack led Pasteur, 

 Salmon, Smith and others to inquire into methods for artifi- 

 cially immunizing animals against the infectious diseases most 



