BACTERIA RESISTANCE. 103 



to 



If an immune serum contains in 1 c.c. one immunizing 

 unit (IE), i. <?., neutralizes 1 c.c. DTN, thenit represents 

 a normal antitoxin (DAN). 



To determine the strength of an immune serum, 1 c.c. 

 of normal toxin is mixed with increasing quantities of the 

 serum ; the quantity of the serum which suffices to neu- 

 tralize it for example, 0.1 c.c. contains one immunizing 

 unit, or the serum contains 10 IE to the c.c., and is then 

 ten times DAN, and is represented thus, DAN 10 . 



To cure a sick man, usually 600 to 1800 IE are em- 

 ployed, which are contained in 2, 4, or 6 c.c. ; then it is a 

 DAN 300 that is used. 



Recently a dried DA has been produced, of which 1 gm. 

 contains as much as 5000 IE; of this about 0.125 gm. 

 suffices for a single healing dose. 



2. After Ehrlich. Ehrlich has recently introduced in 

 the institute for testing serum, as a standard for deter- 

 mining values, a very durable dry antitoxin, which con- 

 tains 1700 IE in 1 gm. A test-toxin is prepared corre- 

 sponding to this antitoxin, and with this toxin the strength 

 of the unknown serum is titrated. For the crude estima- 

 tion of the working value of a serum, a toxin can be pre- 

 pared corresponding to a higher serum of guaranteed 

 strength of IE, and the unknown serum be titrated with 

 this toxin. Because of the numerous cautions to be ob- 

 served, serviceable results can be obtained only by experts. 

 Compare Ehrlich, Klin. Jahrbuch, Bd vi. 



B. Bacteria Resistance (Specific Bacterial Immunity). 



While the antitoxins antagonize the toxins of the bacte- 

 ria in an active manner, they are able, according to older 

 observations not at all, according to more recent observa- 

 tions 1 only to a slight extent, to kill bacteria i. e. , to act as 

 bactericides. On the contrary, in a second group of infec= 

 tious diseases (typhoid, cholera, swine erysipelas) the 

 immunity depends upon the bactericidal action of the 



1 According to van de Velde (C. B. xxn, 527), strong antidiphthe- 

 ria serum also possesses considerable bactericidal action; similar double 

 action is presented also by various other immune sera; for example, 

 antipyocyaneus serum. Compare also Emmerich and Low, p. 110. 



