46 TOXINS AND TOXOIDS 



found that it will have fallen off in potency ; for example, it may 

 take -!Q c.c. to kill a guinea-pig. It might be supposed that this 

 was due to a complete destruction of half the toxin, but this is not 

 the case. If it were so, we should find that to neutralize i c.c. 

 ( = 50 lethal doses) we should require J c.c. of antitoxin, since the 

 latter has not altered in potency. As a matter of fact, we find 

 that we still require i c.c. of antitoxin ; in other words, the 

 diminution of the toxic power of the solution has not been accom- 

 panied by a diminution in its combining capacity for antitoxin. 

 The explanation given by Ehrlich, and fully proved by analogy 

 with numerous other similar phenomena, is that part of the toxin 

 has altered into a substance which retains its power of uniting 

 with antitoxin (and, as we shall show later, with the tissue cells), 

 but which has been deprived of its toxicity. Toxin which has 

 undergone this change is called toxoid. Haemolysin also appears 

 to undergo a similar change into haemolysoid, and the rapid loss of 



>.w 



I 



FIG. 5. A MOLECULE OF TOXIN WITH ITS HAPTOPHORE (a) AND 

 TOXOPHORE (b) GROUPS. 



On the right a similar molecule, which has lost its toxophore group, and 

 become converted into toxoid. 



activity which tetanolysin undergoes is very probably due to a 

 change into that substance. 



The alteration of the toxin to toxoid can be best explained 

 by supposing that the power of entering into combination and the 

 power of intoxication reside in two different parts which we may 

 regard as groups of atoms of the molecule of toxin, and by 

 further supposing that the combining group is a relatively stable 

 one, and that the toxic group is easily destroyed. In the very 

 convenient nomenclature introduced by Ehrlich, and now uni- 

 versally adopted, the group of atoms which has the power of 

 entering into chemical combination with the living protoplas 

 or with antitoxin is called the haptophwe group, whilst the portion 

 on which the toxic action depends is called the toxophore group. 

 The change of toxin into toxoid, or of haemolysin into haemolysoid, 

 consists in a destruction of the toxophore group, with retention of 

 the more stable haptophore group (Fig. 5.) From what has been 

 said as to the dependence of the phenomena of intoxication on a 



