IMMUNITY, IMMUNIZATION, AND CURE. 63 



amount of serum than when the serum of immunized 

 animals is employed. From this it appears that the 

 quantitative relations are of significance with regard to the 

 reaction of Pfeiffer. In order to express these relations 

 mathematically, Pfeiffer establishes as a standard or a unit 

 for the serum that amount that is just necessary when in- 

 jected simultaneously into the peritoneal cavity to destroy 

 ten times the minimal lethal amount of living bacteria. At 

 least 0.05 cu. cm. (of goat-serum, however, only 0.2 

 cu. cm.) of the serum of a normal animal are necessary for 

 this purpose ; whereas only one-tenth of a milligram of the 

 serum of a highly immunized goat, for instance, suffices. 

 With these quantitative limitations the reaction of Pfeiffer 

 may be considered specific. Cholera-serum manifests its 

 lysogenic activity in the peritoneal cavity of guinea-pigs 

 only against cholera-vibrios ; typhoid-serum only against 

 typhoid-bacilli, etc. The serum of individuals who have 

 recovered spontaneously from typhoid fever or cholera like- 

 wise exhibits the phenomenon of Pfeiffer. The standard 

 equals about o.oi. 



The reaction of Pfeiffer may be admirably employed for 

 purposes of differential diagnosis. With its aid it is possi- 

 ble to differentiate true typhoid-bacilli and true cholera- 

 vibrios from the great horde of microorganisms resembling 

 typhoid-bacilli and cholera-bacilli respectively. It need 

 scarcely be emphasized that precisely in this method of 

 differential diagnosis the quantitative relations must be 

 most carefully considered. 



3. AGGLUTINATION (GRUBER'S REACTION). The serum of 

 animals immune to cholera, typhoid fever, coli-infection, 

 etc., or of human beings that have recovered from typhoid 

 fever or cholera, behaves in a peculiar manner when added 

 in small amount to typhoid-bouillon, cholera-bouillon, coli- 

 bouillon, etc. The bacteria lose their motility, collect in 

 masses, and sink to the bottom of the test-tube as a floccu- 

 lent precipitate ; whereas the supernatant fluid remains per- 

 fectly clear. General attention was directed to this reaction 

 of immune serum through the labors of Gruber and Durham, 

 and somewhat later of Pfeiffer and his pupils, although a 

 number of investigators, and more particularly Bordet, had 

 described it previously, without, however, considering it of 

 special importance. Gruber introduced the term aggluti- 

 nation to describe the phenomenon. The reaction is ob- 



