TOXIN AND ANTITOXIN 111 



brium" at which the conversion of toxin to toxoids has been reduced 

 to a minimum and the change of relationship between L and a T" 

 or M L D has practically ceased to go on. From the very begin- 

 ning of the growth of the culture in the incubator the process of 

 toxoid formation has probably occurred, and even freshly prepared 

 toxic filtrates therefore are not pure "toxins," especially since the 

 conversion of toxin to toxoid seems to diminish in velocity as time 

 goes on. 



^sTow in spite of the presence of such alteration products, in com- 

 paring the values L and L+ of any given toxin preparation, one 

 would naturally suppose that L+ minus L should be equal to one 

 M L D, or the quantity just sufficient to kill a guinea pig of 250 

 grams in 4 to 5 days. For we have seen that L just neutralizes 

 one antitoxin unit while L+ is the quantity which, in addition to 

 such neutralizing power, has an excess of toxin equal in action to 

 one minimal lethal dose. This, however, is not the case. Let us 

 illustrate this by a concrete case. One of Ehrlich's toxins on meas- 

 urement showed a minimal lethal dose or M L D of 0.0025 c. c. 



The L+ dose of this was . 25 

 while The L o dose of this was . 125 



The difference was .125 or 50 M L D instead of 1 M L D as we 



would suppose. 



Stated in words, this measurement means that after neutralizing 

 completely one antitoxin unit with the toxic filtrates, in order to 

 obtain death in a guinea pig in 4 days with such a mixture, it was 

 necessary to add, beyond the neutralizing quantity, 50 M L D, or 

 again as much as was necessary for neutralization. 



This last relation is merely coincidence, since it might have been 

 30 or 40 or 60 M L D just as well. The important point is the fact 

 that more than 1 M L D was necessary, and by this fact Ehrlich was 

 led to resort to an assumption which forms one of the basic princi- 

 ples of many of his explanations for serum phenomena, namely, the 

 assumption of differences in combining avidity or affinity. 



As applied to the present problem he reasoned as follows: 



It is conceivable that the toxoids resulting from deterioration of 

 toxin might possess three different degrees of affinity for the anti- 

 toxin. They might have a stronger, an equal, or a lesser affinity than 

 the toxin itself. If their affinity for antitoxin were equal to that of 

 toxin they would, of course, not influence the L+ dose itself; if 

 stronger than toxin their influence would be so exerted that toxin 

 would be forced out of combination with antitoxin, giving place to 

 the toxoid, and the effect would be the opposite from that experi- 

 mentally observed. If, however, their affinity for antitoxin were 

 weaker than that of toxin each additional toxin unit added to the 

 L dose would unite with antitoxin, replacing a corresponding quan- 



