BACTERICIDAL SERUMS. 251 



Bactericidal serums are not obtained with equal 

 readiness for all micro-organisms. We are most 

 familiar with those which are yielded by immuni- 

 zation or infection with the microbes of cholera, 

 typhoid, plague, the colon bacillus and related bac- 

 teria. Many other bacteria, as the pneumococcus, 

 streptococcus, tubercle bacillus and others, yield 

 neither antitoxins nor bactericidal substances. In- 

 asmuch as recovery from such infections is an ex- 

 pression of acquired immunity, no matter how 

 temporary it may be, it is evident that not all ex- 

 amples of acquired immunity can be explained on 

 the basis of the serum properties which we now 

 recognize. This will be referred to again in rela- 

 tion to phagocytosis (Chapter XVIII). 



Experiments of some importance have to do 

 with the ability of bacteria to absorb the homolo- 

 gous bactericidal substance from a serum when the 

 two are mixed in test-tubes. Hence, if natural 

 antibacterial immunity depends on the bacterioly- 

 sin which is present in the circulation, a large 

 mass of the bacterium when injected intravenously 

 should absorb or fix the bactericidal substances ; as 

 a consequence, serum which is drawn later should 

 show a great decrease in its bactericidal power for 

 the organism which was injected. Although re- 

 sults of this nature have been obtained by a num- 

 ber of competent investigators, they are not with- 

 out exception. In the same connection fatal infec- 

 tions should be accompanied by a decrease of the 

 natural bactericidal power of the serum for the 

 organism involved. This has been found to be true 

 in man in relation to plague, and in some animal 

 infections. 



