ANTIGEN-ANTIBODY REACTIONS 44?> 



produce any reaction in the guinea-jjig under standard 

 conditions. It would be expected that L+ — L^ = l MLD, 

 but this is not the case, the difference being of the order 

 of 20 to 50 MLD. Ehrlich explained this phenomenon as 

 being due to the presence in toxin (T) of an epitoxoid (E) 

 which is non-toxic and has a less affinity than, but the 

 same combining power as toxin for antitoxin. If it is 

 assumed, for purposes of illustration, that crude toxin 

 contains equal parts of toxin and epitoxoid there will be 

 nT-f nE units of the mixture. If nA units of antitoxin 

 were added to it, the non-toxic mixture nTA-f nE would 

 result. This corresponds to the L^ dose. The addition 

 of 2nA units of antitoxin would yield the mixture 

 nTA+nEA, also non-toxic. If more crude toxin, say 

 1 unit, is added, then the true toxin, having a greater 

 affinity for antitoxin than the epitoxoid, v,'ould turn 

 some of the latter out of combination : — 



nTA + nEA + T-rE > (n + l)TA-f-(n— 1)EA + 2E. 



The addition of more and more toxin will turn out more 

 and more epitoxoid and combine with the resulting anti- 

 toxin until there is no epitoxoid-antitoxin complex left, 

 the mixture remaining non-toxic ; after that free toxin 

 can accumulate and the mixture becomes toxic, corre- 

 sponding to the L^ dose. Thus the L+ dose will be 

 bigger than the L^ dose according to the proportion of 

 epitoxoid in the crude toxin preparation. 



Another phenomenon first observed in connection 

 with toxin-antitoxin reactions, and since found to occur 

 with all antigen-antibody reactions, is the Danysz phen- 

 omenon. The amount of toxin neutralised by a given 

 amount of antitoxin depends on the way in which the 

 reagents are mixed. If equivalent amounts of toxin and 

 antitoxin are mixed rapidly the product is non-toxic : — 



nA + nT > nTA. 



