BACTERIA SESISTANCE. 103 



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

 unit (IE), — i. e. , neutralizes 1 c.c. DTN, — then it 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'". 



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^oo 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^ 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 



^According to van de Velde (C. B. xxil, 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. 



