368 BACTERIOLOGY. 
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 autitoxin 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. 
Ehriich 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 
