472 IMMUNITY 



'9 gr. ; difference = '36 gr., i.e. 21*9 M.L.D. This, in brief, is 

 what is known as the " Ehrlich phenomenon," and it has been 

 explained by him as the result of the presence of toxoids (vide 

 p. 171), i.e. toxin molecules in which the toxophorous group has 

 become degenerated. He distinguishes three possible varieties 

 of such bodies according to the affinity of the haptophorous 

 group, namely prototoxoid with more powerful affinity than the 

 toxin molecule, epitoxoid with less powerful affinity, and syntoxoid 

 with equal affinity. The presence of epitoxoids would manifestly 

 explain the above phenomenon. The L dose would represent 

 toxin + epitoxoid molecules all united to antitoxin molecules and 

 the addition of another M.L.D. of toxin would not result in 

 there being a free fatal dose, but in the added toxin taking the 

 place of epitoxoid. Several lethal doses would need to be added 

 before the mixture was sufficient to produce a fatal result ; that is, 

 L t - L would equal several M.L.D.s. Ehrlich observed another 

 fact strongly in favour of the existence of toxoids, namely that 

 in the course of time the toxin might become much weakened, 

 so that in one case observed the M.L.D. was three times the 

 original fatal dose, and still the amount of antitoxin necessary to 

 neutralise it completely was the same as before. Ehrlich also 

 investigated the effects of partial neutralisation of the L amount 

 of toxin, that is, he added to this amount different fractions of an 

 immunity unit and estimated the toxicity of the mixture. He 

 found by this method that the neutralisation of the toxin did 

 not take place gradually, but as if there were distinct bodies 

 present with different combining affinities the graphic repre- 

 sentation of the mixture not being a curve but a step-stair line. 

 Thus he distinguished proto-, deutero-, and trito-toxins (with 

 corresponding toxoids). It will thus be seen that Ehrlich regards 

 the combination toxin-antitoxin to be a firm one, and that the 

 neutralisation phenomena are to be explained by the complicated 

 constitution of the crude toxin. 



The chief criticism of Ehrlich's views has come from the 

 important work of Madsen and Arrhenius. Their main con- 

 tention is that the toxin-antitoxin combination is not a firm one 

 but a reversible one, and is governed by the laws of physical 

 chemistry. For example, in the case of a mixture of ammonia 

 and boracic acid in solution, there is a constant relation between 

 the amounts of each of the substances in the free condition and 

 the amounts in combination, the combination is reversible, so 

 that if some of the free ammonia were removed a certain amount 

 of the combined ammonia would become dissociated to take its 

 place ; further, if to the mixture, in a state of equilibrium, 



