48 THE PRINCIPLES OF IMMUNOLOGY 



the law of multiple proportions. If this were true, how- 

 ever, the L + dose which in combination with the antitoxin unit 

 kills a pig in four days should contain one unit more of toxin 

 (i M.L.D.) than the L dose which just fails to produce symptoms. 

 Reference to the experiments offered to illustrate the determination 

 of M.L.D. , L dose, and L + dose show that the M.L.D. of the toxin 

 was 0.004 c - c -> the L dose of toxin was 0.40 c.c. or one hundred times 

 the M.L.D., and the L,. dose 0.48 c.c. or one hundred and twenty 

 times the M.L.D. The difference between L and L + doses is, there- 

 fore, twenty times the M.L.D. instead of exactly equal to it. This 

 has been interpreted to indicate that some body or bodies, other than 

 the toxin, has combined with the antitoxin, thus limiting its ability 

 to combine with toxin. Ehrlich, after numerous experiments and 

 hypotheses, reached the assumption that the toxic broth contains 

 two bodies other than toxin, which he named toxon and toxoid. The 

 toxon is a body with a smaller degree of affinity for the antitoxin 

 than has the toxin. In a determination of the L dose the antitoxin 

 neutralizes both toxin and toxon, so that no symptoms appear, but 

 if more toxin be added to the mixture it combines with antitoxin, 

 displacing the more loosely combined toxon. Finally, after suffi- 

 cient addition of toxin the antitoxin is fully saturated, and any addi- 

 tional toxin will be free, and if in sufficient quantity (i M.L.D.) 

 will lead to the death of the experimental animal. In more detail 

 the 20 M.L.D.'s necessary to make the difference between the L and 

 L + doses were so used that 19 M.L.D.'s were employed to displace a 

 proportionate amount of toxon and toxoid from combination with 

 the antitoxin unit, and the remaining i M.L.D. sufficed to kill the 

 pig in four days. If more than i M.L.D. were present in excess 

 death would ensue after a shorter period, and if less than i M.L.D. 

 were present death would occur later than four days or not at all. 

 It is believed, on the basis both of experimental and clinical obser- 

 vation, that toxon is responsible for the late paralyses of diphtheria. 

 The conception of the toxoid is based on the Ehrlich assumption 

 that the toxin molecule has a toxic fraction or " toxophore group " 

 and a combining fraction or " haptophore group." A toxin will 

 retain its binding power for antitoxin for a considerable length of 

 time with little change in the L + dose, but with marked deterioration 

 of toxic power and corresponding reduction of the M.L.D. This is 

 interpreted as meaning that the toxic fraction is labile and the com- 

 bining fraction much more stable. The toxoid, then, is the toxin 

 molecule so altered that its toxic part is reduced and the combining 

 part practically intact. As can readily be seen, this can account 

 also in part for the discrepancy between L and L + dose. The 

 discrepancy between L and L + dose in fresh toxic broth is be- 

 lieved to be due to the presence of toxon rather than toxoid, be- 

 cause too short a time has elapsed to account for toxoid formation. 

 As the toxic broth becomes older the discrepancy becomes greater, 

 even after a relative equilibrium has been established, and the differ- 



