368 BA CTERIOL OGY. 



it remains chemically unaltered, has a definite poisonous 

 strength with a definite value in neutralizing antitoxin. 

 This neutralization he believes to be a chemical union, 

 in which two hundred fatal doses of toxin for a 250 

 grammes' weight guinea-pig combine with one unit of 

 antitoxin. The toxin is, however, an unstable com- 

 pound, and begins to change almost immediately into 

 substances which are not, at least acutely, poisonous, 

 but which retain their full power to neutralize anti- 

 toxin. These substances, according to Ehrlich, fall 

 into three groups. The first has more affinity for 

 combining with the antitoxin than the toxin itself 

 (protoxoids). The second has the same affinity (syn- 

 toxoids). The third has less affinity (epitoxoids). 



According to him, if a mixture of toxoids and toxin 

 is added to antitoxin, the protoxoids first combine with 

 the antitoxin, then the syntoxoids and the toxin com- 

 bine in equal proportions, so long as the supply lasts, 

 with the amount of antitoxin remaining, or, if there 

 is a surplus, with enough to satisfy them; finally, if 

 any antitoxin remains, the epitoxoids unite with it. 



If to a mixture in which all three toxoids, as well as 

 toxin, have united with antitoxin, some additional toxic 

 culture bouillon be added, the new protoxoids displace 

 first the epitoxoids, and then, if free protoxoids remain, 

 the toxin and the syntoxoids from their antitoxin, and 

 thus liberate as well as add free toxin to the solution. 



Ehrlich gives an interesting theory to explain the pro- 

 duction of antitoxin in the blood. This he does upon 

 the supposition that, when absorbed, the toxin combines 

 with a portion of certain selected cells, and that this 

 portion, by its union with toxin, becomes at least 

 physiologically dead. The cell replaces this dead 



