INTERREACTIONS OF TOXIN AND ANTITOXIN 



79 



partial neutralization. If, however, we now keep this atrtitoxin 

 for some time, especially if it is exposed to warmth, light, air, 

 or certain chemical substances, we find a great change. The 

 L dose is unaltered : i c.c. is still exactly neutralized by i unit 

 of antitoxin, but we find that this amount is now much less lethal, 



Prototoxoid 



Epitoxoid 



50 parrs 



loo parrs 

 FIG. 17. 



SO parrs 



and the minimal lethal dose may have risen from o'oi c.c. to 

 O'O2 c.c., or higher. 



If the cause for this increase in the lethal doses is investigated 

 by the partial neutralization method described above, it will be 

 found that the results obtained are such as will be readily ex- 

 plicable on the assumption that some of the molecules of toxin 



Profotoxoid. 



Toxoid (SO parts) 



Epiroxoid 



50 parts 



100 parts 

 FIG. 18. 



50 parrs 



have ceased to be poisonous, but have retained their combining 

 power unaltered; whilst the non - poisonous portions of the 

 spectrum are unaltered. Thus, to take the simple case described 

 above, and shown in Fig. 17, in which protoxoid, toxin, and 

 epitoxoid are present in the proportion of 50, 100, and 50. If we 

 keep this, we may find the lethal dose doubled i.e., ^ c.c. instead 

 of T<JTT c - c - But on working out the action of antitoxin, we may 

 find that the first -25 c.c. added removes none of the toxicity, and 

 the last -25 is equally without apparent effect. Thus the middle 

 5 c.c. is used up in neutralizing 50 lethal doses, and of this 

 fraction further investigation shows that each one-fiftieth part 

 neutralizes one lethal dose; the antitoxin, therefore, appears to 

 have fallen off in potency ; but we know that this is not the case. 

 The explanation is that half the molecules have been changed into 

 the non-toxic form described above. The spectrum of this altered 

 form is shown in Fig. 18. 



Thus, in a particular case Ehrlich found the minimal lethal 



