INTERREACTIONS OF TOXIN AND ANTITOXIN 



73 



combine with one -fourth more antitoxin than is necessary to 

 neutralize it. 



To account for this Ehrlich supposed that there are really two 

 substances present in the broth in which diphtheria bacilli have 

 been grown. There is the true toxin, which brings about local 

 inflammatory oedema, often going on to necrosis and causing local 

 alopecia, and causing acute death, and toxon, which produces only 

 soft and transient oedema locally and subsequent paralysis. Both 

 these substances combine with antitoxin, but the toxin has the 

 greater affinity for that substance, and when the total neutralizing 

 dose of antitoxin is added in successive small amounts, the whole 

 of the toxin is neutralized first, leaving the toxon free, and this 

 takes place when three-fourths of the whole amount of antitoxin 

 has been added. Ehrlich represents this result in the form of a 

 spectrum, thus : 



25 50 75 100 



FIG. 8. SIMPLE SPECTRUM OF TOXIN. 



The rectangle represents the L dose of toxin i.e., in this simple 

 case i c.c. of the solution. The portion with the greatest affinity 

 for antitoxin is placed at the left hand of the " spectrum "; in 

 this case it is represented by the toxin. On the right are the 

 substances with the least affinity for antitoxin in this case the 

 toxon. 



Further investigation shows that the process is not usually so 

 simple as this. In certain samples of toxin we find that the 

 addition of small quantities of antitoxin causes no alteration in the 

 toxicity of the L dose. Thus, in a case of frequent occurrence 

 it happens that we may add ^ c.c. of normal antitoxin before 

 any loss of toxicity occurs ; i c.c. of the normal toxin will kill 

 100 guinea-pigs, and i c.c. of the same toxin + |- c.c. of normal 

 antitoxin will still kill 100 guinea-pigs. To explain this, Ehrlich 

 supposed that the solution contains a third substance, prototoxoid, 

 which is entirely devoid of lethal activity, but which has a power 

 of combining with antitoxin even greater than that which toxin 

 possesses. Thus, on the addition of small amounts (up to -J- c.c.) of 

 the antitoxin, this inert substance will seize on the antibody, unite 



