REACTION OF THE HOST TO INFECTION 2IQ 



joint cavities, and in the cerebrospinal fluid. 1 Parasites in 

 these locations are less readily influenced by antibodies circulating 

 in the blood, so that localized infections may continue in these 

 regions in spite of a considerable concentration of antibodies in 

 the body generally. 



Allergy. Allergy is a term invented by Von Pirquet to 

 designate the condition of altered reactivity on the part of the 

 body which comes about as a result of infection. A few of the 

 phenomena which may be included under this term have been 

 considered above in this chapter. Many of these alterations in 

 bodily function are manifestly of advantage to the host in limiting 

 the activities of the parasite, neutralizing its poisonous products, 

 and even in destroying and removing the parasite itself. Some of 

 them, such as specific precipitation, seem to serve no important 

 purpose, while others, such as cytolysis and proteolysis actually 

 lead sometimes to results very harmful to the host, although 

 their usual effect is favorable. Many of the recognized weapons 

 which the body employs in its battle against parasites are still 

 imperfectly understood, and there are doubtless many factors 

 involved in this relation which are not yet capable of definite 

 recognition. Of those agents mentioned above, the phagocytes 

 are ready for immediate defense as soon as the body is invaded 

 by the parasite. Hyperplasia and encapsulation require more 

 time, probably one to four weeks. The chemical antibodies, 

 antitoxins, agglutinins, cytolysins and opsonins, although possibly 

 present in small amounts in the normal body fluids, become 

 definitely increased in from eight to twelve days after the entrance 

 of the parasite, an interval approximately equal to the incubation 

 period of some infectious diseases. These various agents have 

 much to do in determining the manifestations and course of the 

 disease as well as the final outcome, and as we shall see, they also 

 play a part in immunity. 



1 See Flexner, Harbin Lectures, Journ. of the State Medicine, March, April, May 

 1912. 



