258 MICROBES AND TOXINS 



quence of discovering the effect remaining in the animal body 

 from a previous infection. 



It has been used like agglutination to diagnose both a past 

 infection and the nature of a bacterial strain. There is no 

 reason to use it in the diagnosis of typhoid fever, Widal's 

 reaction being much more convenient. It is very useful in 

 those cases where agglutination cannot be employed. 



The Sero-diagnosis of Syphilis : Wassermann 

 Reaction. — Wassermann's reaction is simply the application 

 of the Bordet-Gengou reaction to discover the syphilitic 

 antibody in the serum of patients affected by this malady. 

 In practice it is rather delicate, and demands an experienced 

 worker. In theory nowadays it is regarded as a precipitating 

 reaction between certain colloid substances of the serum to be 

 examined and the colloids of an organ extract which is 

 employed as " antigen." 



The reaction is not strictly specific ; in leprosy, in trypano. 

 soma infections in animals, perhaps in scarlatina, modifications 

 of the serum often occur which give a positive reaction. The 

 method is nevertheless capable of clinical application, since 

 the physician has only to decide between a trypanosoma 

 infection, such as sleeping sickness, which scarcely exists in 

 Europe, leprosy, which produces characteristic lesions, scarlet 

 fever, which is easy to diagnose, and syphilis, which is 

 extremely common ; the selection is therefore easy. A 

 laboratory reaction which, taken by itself, does not give an 

 absolute diagnosis, becomes nevertheless an absolutely certain 

 sign when taken along with the sum-total of clinical and 

 biological indications. 



As a matter of fact, the Wassermann test, if carried out with 

 the best technique, is positive in about 90 per cent, of the 

 cases of primary syphilis, in 100 per cfent. in secondary syphilis, 

 and in 50-60 per cent, of late tertiary cases (Citron's figures) ; 

 the proportions are rather lower in the statistics hitherto collected 

 by the Pasteur Institute. The reaction becomes more definite 

 the more acute the infection and the more active the virus. 



There is some doubt as to whether it indicates syphilis in 



