108 INFECTION AND IMMUNITY. 



be as short as a few hours only,, in hydrophobia it 

 is on rare occasions as long as six months, but 

 more commonly two to four weeks. In a rather 

 large group of diseases the onset follows in from 

 one to two weeks after exposure, so that there 

 would seem to be a tendency to some general law, 

 the basis of which is not known. 



It has been suggested that it may have a relation 

 to the anaphylactic reaction (Rosenau and Ander- 

 son) ; namely, that the first micro-organisms intro- 

 duced "sensitize" the body in some way not yet 

 understood, and when sensitization has taken place 

 (seven to fourteen days) the body then has a 

 greater susceptibility to the organism and yields 

 readily to infection (see "Anaphylaxis"). 



Occasional individuals show a susceptibility to a 

 first injection of horse serum (e. g., diphtheria 

 antitoxin) in that they develop an urticarial rash, 

 adenopathy, effusions into the joints, and perhaps 

 fever, in from eight to twelve days after the intro- 

 duction of the serum. Von Pirquet noted that 

 when a second injection of the serum was given 

 after an interval of two weeks or more similar 

 symptoms would develop within a few hours rather 

 than after several days; and there is a great deal 

 of constancy in this when susceptible individuals 

 are concerned. He arrived at the conclusion that the 

 horse serum as such is not toxic for man, but that 

 as a consequence of the injection antibodies to the 

 serum are formed and that the intoxication comes 

 about as a result of some kind of chemical reaction 

 which takes place between the antibodies and the 

 serum. The conditions in immunization render 

 this explanation extremely probable. After the 

 first injection of serum antibodies to the serum 



