INFECTION AND IMMUNITY 9 



Mechanism of Immunity. Infection, whether 

 natural or artificial, is usually followed by a remark- 

 able series of alterations in the tissues of the infected 

 host. Representing, as it does, all the tissues of 

 the body, it is natural that these changes are most 

 strikingly exhibited in the blood. The alterations 

 vary, however, both with the kind of bacterium, 

 and with the animal species involved. Against the 

 true toxins, the body produces antitoxins; against 

 the bacterial bodies it directs the action of the 

 leucocytes. 



Before leaving the consideration of the reaction 

 of the body to infection, attention should be called 

 to the comprehensive investigations of Opie. This 

 observer showed that the cells which accumulate in 

 response to an irritant contain enzymes, the enzyme 

 of the polynuclear leucocytes resembling trypsin 

 and the enzyme of the macrophages resembling 

 pepsin in its action. The blood serum, on the 

 other hand, contains an antienzyme. The varying 

 relation existing between these enzymes and the 

 antienzymes serves to explain how the same irritant 

 in the same quantity may cause two different types 

 of inflammation. This is well illustrated by the 

 following experiment made by Opie: 1 If a small 

 quantity of turpentine is injected into the sub- 

 cutaneous tissue of dog, a large fluctuating abscess 



*E. L. Opie, Lecture before the Harvey Society, New York, 

 Feb., 1910. The Harvey Lectures, J. B. Lippincott Co. 1910. 



