THE ORIGIN OF ANTITOXIN THE SIDE-CHAIN THEORY 99 



take place ; and Weigert points out that this always goes on to 

 excess, more new material being formed than is necessary to 

 replace the amount lost. The application of this theory to the 

 mode of new formation of receptors is sufficiently obvious. 



We will now consider certain phenomena in the light of the 

 side-chain theory. 



The occurrence of antibodies in the blood of normal animals is 

 susceptible of a ready explanation. The receptors are, in general, 

 united to the cell, but it is easily conceivable that a few may be 

 desquamated accidentally and escape into the blood. It must be 

 remembered that the actual amount (weight or bulk) of these 

 antibodies in the normal blood is probably always infinitesimal, 

 although the effects of this small amount may be striking. Thus 

 the agglutination of typhoid bacilli at a dilution of i to 100 

 (which often occurs with the blood of normal horses) indicates 

 the presence of a very small amount of specific antibody, as is 

 apparent from the fact that the blood may be made to clump at 

 i to 1,000,000 without being altered chemically in any way that 

 can be detected. If an exceedingly minute proportion of all the 

 receptors are shed into the blood, it will probably account for all 

 the antibodies found in that situation under normal conditions. 



The fact that toxoids produce antitoxin 1 is also explicable. We 

 must regard the receptor which is occupied with toxoid as being 

 thereby rendered useless for the protoplasm, although the latter is 

 not poisoned as a result of the union. 



Next arises a most important question, and one which is ap- 

 parently satisfactorily answered on the side-chain theory the 

 fact that certain substances (bacterial toxins in particular) give 

 rise to the production of antibodies, whilst others, such as. the 

 alkaloids, glucosides, mineral poisons, poisons of simple con- 

 stitution, such as alcohol, etc., do not. (This may be taken as 

 fairly proved, certain researches which go to prove the existence 

 of antibodies to morphine, alcohol, etc., being inconclusive.) 



The explanation is founded on differences in the way the 

 various substances form combinations with the protoplasm. This 

 latter has, as an integral part of its constitution, certain receptors 

 which have a specific combined affinity for proteids, and we must 

 imagine the union of proteids, whatever be their nature, as taking 

 place by a direct union with these receptors. Other substances, 



1 According to Ehrlich. The fact is not universally admitted, though all 

 agree that they will produce immunity. 



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