REACTION OF THE HOST TO INFECTION 219 



ture in broth or salt solution, is added. The reaction may be 

 observed by mixing small quantities (loopfuls) on a large cover- 

 glass and studying the mixture microscopically as a hanging 

 drop, or by mixing larger quantities in small tubes and incubating 

 them at 37° C. Control specimens free from serum and contain- 

 ing normal serum should be set up at the same time for compari- 

 son, as many bacteria may be agglutinated somewhat by normal 

 serum in a dilution of one to ten, and sometimes the organisms 

 in the culture, especially if it is too old, may be already grouped 

 together somewhat or may spontaneously clump during the ex- 

 periment. Some practice is necessary before one can estimate 

 agglutinins reliably and, on the whole, accuracy is more easily 

 attained with the macroscopic test. For agglutination tests 

 requiring only moderate accuracy, dried blood may be used, the 

 dilutions being prepared by comparison of colors with an empirical 

 standard. 



Bactericidal Substances, Alexin.^ — Nuttall (1886) showed 

 that normal blood is capable of killing bacteria and that this 

 germicidal property is destroyed by heating the blood to 55° C. 

 for thirty minutes. Buchner confirmed these observations and 

 showed further that the germicidal property is resident in the 

 serum and not exclusively in the cells of the blood as taught by 

 Metchnikoff. To this germicidal substance Buchner gave the 

 name alexin, and he ascribed the normal resistance to infection 

 exhibited by the healthy animal, as well as the heightened resist- 

 ance of the immunized animal, to this substance. It will, have 

 been noted that, historically, these discoveries followed Metch- 

 nikoff's first observations on the phagocytes, and preceded 

 the discovery of antitoxins, agglutinins and precipitins, and 

 thus presented the first proof of the existence of soluble anti- 

 infectious agents. These bactericidal substances are now con- 

 sidered to be identical with the bacteriolysins and will be con- 

 sidered with them under the more general heading of cytolysins. 



Cytolysins. — Pfeiffer (1896) found that guinea-pigs, when 

 injected repeatedly with non-fatal doses of cholera germs, reacted' 



