Immunity acquired by natural means 439 



Tiatural that this substance should be sought in the blood of patients 

 who were suffering, or had recovered, from typhoid fever. Bordet and 

 Gengou 1 easily demonstrated, by the method described in Chapter ix, 

 the existence of typhofixative in the blood serum of two individuals 

 convalescing from this disease. 



Widal and Le Sourd 2 extended this discovery to the blood taken 

 during the course of the disease from typhoid fever patients. The 

 ten cases studied by them all gave a positive result, whilst all the 

 samples of blood from persons suffering from various other diseases 

 possessed no typhofixative. As yet we do not know whether this 

 substance persists for any length of time after recovery or not. In 

 this respect we have much more information concerning another 

 humoral property of typhoid patients, specific agglutination. Guided 

 by the fact that, even during the course of the disease, the blood of 

 persons suffering from typhoid fever acquires protective properties, 

 Widal sought to find out whether the agglutinative power of the 

 fluids of the body appears equally early. We know that his studies 

 gave a positive answer, and that the blood of typhoid patients may 

 have agglutinative properties from the first day of the disease. This 

 fact was made use of by Widal to establish the serum diagnosis of 

 typhoid fever, a method now generally used in clinical medicine. 

 The question which most interests us at this moment is whether this 

 acquired agglutinative property persists for any length of time after 

 the recovery of the patient, and whether it can be employed as the 

 measure of immunity obtained. 



In certain cases the serum was found to be fairly strongly aggluti- 

 native for a considerable period after recovery had taken place. But 

 these cases are rare, and the agglutinative power, like the protective 

 power of the blood, usually begins to decrease very soon after 

 recovery. Bensaude 3 observed that the former disappeared between [*6i] 

 the 10th and 95th day of apyrexia. Widal and Sicard 4 have noted in 

 certain of their cases the complete disappearance of the agglutinative 

 power of the blood, which took place in one case on the 18th, in 

 another on the 24th day of defervescence. In many convalescents, 

 fifteen to thirty days after the commencement of apyrexia, the agglu- 

 tinative power begins to be attenuated. 



1 Ann.de Vlnst. Pasteur, Paris, 1901, t. xv, p. 289. 



* Bull, et mem. Soc. meet. d. hop. de Paris, 1901, 20 juin, p. 624. 



3 "Le phenomena de 1'aggluti nation des microbes," Paris, 1897, p. 76. 



4 Presse med., Paris, 1896, No. 83. 



