BACTERICIDAL PROPERTIES OF BLOOD SERUM 139 



ployed, since, as we shall see, protection of animals by bactericidal 

 or bacteriolytic sera does not follow the law of multiple proportions 

 as in the case of the protection against toxins by antitoxins. While 

 the dose of bacteria chosen should be considerably above the minimal 

 lethal dose for an animal of the weight used, it should nevertheless 

 be remembered that the bactericidal serum does not possess antitoxic 

 properties against the poisons liberated or produced as the bacteria 

 undergo dissolution, and at best the protection by bacteriolysis is 

 limited to a very definite maximum of bacteria, beyond which no 

 further increase of serum quantity will avail. The following table 

 will illustrate an experiment of this kind in which, in a series of 

 guinea pigs, the bacteriolytic protective power (titre) is determined 

 by comparative tests. 13 



PFEIFFER PHENOMENON 



"The bacteria may be measured for such an experiment by standard loopfuls ( 1 loop be- 

 ing equal to 2 milligrams), or by volume in emulsion with salt solution. 



Pfeiffer has established a system of standardization for the meas- 

 urement of sera by this technique. He speaks of one immunity unit 

 as the smallest amount of such a serum which is capable of causing 



13 For extensive discussion of the technique of such tests see Boehme in 

 Kraus u. Levaditi Handbuch, etc., Vol. 2, p. 366. The scheme of presenta- 

 tion of our example is taken from that used by him. See also Pfeiffer, 

 Zeitschr. /. Hyg., Vol. 19, 1895, p. 77. 



