NATURAL. AND IMMUNE ANTIBODIES 145 



measuring the strength of immune serum. This he did by select- 

 ing a number of guinea pigs of uniform weight and inoculating a 

 series with varying amounts of heated immune serum mixed with 

 two milligrams of virulent bacteria, an amount sufficient to insure 

 death in normal guinea pigs when inoculated alone. The least 

 amount of immune serum that dissolved the bacteria and pro- 

 tected the animal was called an ''immunity unit." 



Bordet's Explanation of Lytic Mechanism. — It remained, 

 however, for Jules Bordet (1898) working in Pasteur's laboratory 

 to investigate carefully and explain the phenomena described by 

 Pfeiffer, His results may be briefly summarized as follows: 



1. He confirmed Pfeiffer 's observation that normal guinea pigs 

 could be protected by injecting them with immune serum. This 

 is now called passive immunity as contrasted with active immunity 

 enjoyed by animals that recover from an infection. 



2. He further showed that fresh unheated serum from cholera 

 immune guinea pigs would specifically kill and dissolve cholera 

 vibrios in a hanging drop preparation under the microscope. He 

 found normal guinea pig serum to be only slightly potent and not 

 very specific. 



3. In addition, he observed that the serum from immune guinea 

 pigs, when heated to 55° C. for one-half hour and mixed with 

 vibrios in hanging drop preparations, would frequently clump the 

 organisms (agglutinate them), but would not kill or dissolve them. 

 However, if he added a little fresh normal serum, which by itself 

 had no germicidal properties, to the hanging drop of bacteria and 

 immune serum, the vibrios were quickly killed and dissolved. 

 Pfeiffer had been unable to render heated immune serum potent, 

 l)ut Bordet definitely proved it possil)le. Thus Bordet showed that 

 fresh serum from nonimmune as well as immune animals contains 

 a thermolabilc or heat sensitive substance (called complement by 

 Ehrlich) whicli will act upon and destroy bacteria suspended in 

 heated immune serum. 



Bordet concluded that some specific thermostable (heat resistant) 

 substance in the immune serum rendered the cholera vibrios sus- 

 ceptible to the digestive action of complement. The chemical sub- 

 stance in the immune serum that renders bacteria sensitive to 

 complement has been called by various names such as protective 



