<)() THE BLOOD 



animal, will protect the latter from subsequent injections 

 of diphtheria toxin. Something has been developed in the 

 blood of the immunized animal which can neutralize the toxin. 

 This is called antitoxin. To explain the production of anti- 

 toxin, the side-chain theory supposes that the biogen of the 

 animal tissues consists of a central nucleus or functionating 

 centre, attached to which are innumerable arms or side chains 

 which are of various combining powers at their free ends. 

 These side chains are concerned, physiologically, in the absorp- 

 tion of food molecules, which must, therefore, be likewise 

 provided with side chains of definite combining powers. It 

 is only by becoming locked to the functionating centre of a 

 cell, by means of . side chains, that food molecules can be incor- 

 porated with the protoplasm of the cell and its potential energy 

 made available to the tissues. 



A molecule of toxin possesses besides the actually toxic 

 portion, known as the toxophoric group, a combining side 

 chain known as the haptophoric group, and if it happens that 

 the latter fits one of the side chains of the tissue cells, then 

 will the toxin unite -with the cell and produce the symptoms 

 of the disease. In order to explain how the repeated injection 

 of toxin in increasing dosage leads to the production of anti- 

 toxin it is imagined that the union of tissue side chains with 

 toxin molecules leads to an overproduction of similar side 

 chains which are thrown off from the cells producing them 

 and which float free in the blood. This assumption is based 

 upon an observation of Weigert's, who showed that when a 

 tissue is injured the organism not only makes good the defect, 

 but reproduces the lost part in overexcess. The liberated 

 side chains are the antitoxin. Side chains of the kind just 

 considered are relatively simple, possessing but one combining 

 group. They are known as receptors of the first order. In 

 the case of hemolytic actions the disrupted biogen receptors 

 are more complex, inasmuch as there is formed, not merely 

 an inactive body as in the reactions between toxin and anti- 

 toxin, but a new compound provided with combining groups 

 not only for the erythrocyte but for the complement as well, 

 for the latter produces the actual lysogenic effect. These 

 are receptors of the third order. 



Receptors of the second order are to be found in those bodies 



