THE ORIGIN OF ANTITOXIN THE SIDE-CHAIN THEORY 103 



equally important, is that the proteid molecules shall be " diges- 

 tible " by the cell. If the first requisite fails, the proteid molecule 

 fails to unite with the cells ; it may remain for long periods in the 

 blood (as is the case with tetanus toxin in the blood of Oryctes, 

 where it remains months after injection), or may probably be 

 eliminated with the excretions, or be otherwise dealt with. In 

 this case no antibody can be formed. 



When the injected proteid molecule finds suitable receptors and 

 unites with them, but when the resulting compound is not suit- 

 able for the nutrition of the cell, and cannot be " digested " by it, 

 the case is different. The receptors which are occupied by the 

 molecules of proteid are useless for the cell, and the conditions 

 which we have discussed in dealing with the action of toxins are 

 reproduced, although there may be no toxic action ; the receptors 

 are useless and have to be regenerated, and under suitable circum- 

 stances may be produced in excess and form antibodies. The 

 criteria of the suitability of any animal for the production of an 

 antibody to a given proteid are therefore three in number : 



1. This proteid must not exist naturally in the blood of the 

 animal. 



2. It must possess haptophores which " fit " the receptors of 

 the animal cells. 



3. The compound thus formed must be " indigestible " by the 

 cell and useless for its nutrition. 



These are the theoretical criteria, which are derived from a 

 study of the side-chain theory, and, on the whole, experimental 

 research agrees with them, and thus corroborates the theory to an 

 extraordinary extent. For example, we need scarcely refer to 

 criterion i : we know that precipitins do not occur normally in 

 the blood of any animal to the proteids which circulate in that 

 fluid; it is only the injection of a foreign proteid which causes 

 their production. Further, the more closely allied are two animal 

 species, the less the production of precipitin when serum from the 

 one is injected into the other. Thus, in order to prepare a potent 

 antihuman serum, the blood of a man must be injected into a 

 rabbit, fowl, etc., not into a monkey. If it is true, as appears to 

 be the case, that the serum of an animal causes the production of 

 precipitins when injected into another of the same species, this 

 does not affect the argument, for these precipitins only occur in 

 minute amount, and simply show that the fluids and tissues of 

 different animals of the same species are not in reality identical. 



