TOXINS AND ANTITOXINS 173 



p;r()up (toxoid is the muuc given to such altei-ed toxins), tiie cell can- 

 not then be injured by the corresponding; active toxin, showing that 

 the toxin nuist first become united to a cell receptor by its haplophorc 

 group before the toxophore group can cause an injury. 



Animals that are naturally immune to toxins may owe their im- 

 munity to the fact that their vital tissues contain no substances with 

 a chemical affinity for the toxin, and hence the toxin cannot unite 

 with them to cause harm. (In Ehrlich's terminology, the cells con- 

 tain no receptors for the toxin.) The toxin may not combine with 

 any tissue element at all in such immune animals, and may circulate 

 for some time harmlessly in the blood, or it ma}^ combine with some 

 organ where it does little harm, e. g., tetanus toxin is said to combine 

 chiefly in the liver of some animals, and therefore it does not harm 

 their nervous system. 



According to this theory, the antitoxin consists of cell receptors 

 that have been produced in excess and secreted by the cells into the blood. 

 In the blood they combine with any toxin that 'may have been intro- 

 duced, and by saturating its affinities render it incapable of uniting 

 with the cells. As the toxin harms cells only after it has been chemi- 

 cally united to them, it is rendered harmless when its affinities for the 

 cell (the haptophore groups) are saturated by cell receptors in the blood 

 stream. The process of immunization consists in injuring the body 

 cells to such a degree that they are stimulated to regenerate the 

 receptor groups with which the toxin combines; these receptor groups 

 are produced in excess, and not only replace those combined by the 

 toxins, but the excessive groups escape free into the blood. Hence 

 the serum of an immunized animal is antitoxic because it contains free 

 cell receptors that can unite with the toxin. An important point is 

 that the receptors liberated by all animals which have been immunized 

 wdth a given toxin seem to be the same — horse serum, or sheep serum, 

 or goat serum will neutralize diphtheria toxin if the animals have been 

 made immune to this toxin; and, furthermore, their serum when intro- 

 duced into the body of an entirely different animal, e. {7., a guinea-pig, 

 will neutralize diphtheria toxin within its body. Equally important 

 is the fact that the antitoxin for one toxin will not neutralize any other 

 toxin; e. g., diphtheria antitoxin will not neutralize tetanus toxin, or 

 conversely. This means that diphtheria toxin is attached to chemical 

 groups of the body cells (receptors) which are quite difTerent from the 

 groups to which tetanus toxin unites, and hence different receptors 

 are thrown out in immunizing against each. True toxins have been 

 designated monovalent antigens^ since animals immunized with a puri- 

 fied toxin produce onl}^ the one antibody, the antitoxin, whereas many 

 protein antigens produce precipitins, Ij^sins, agglutinins and other 

 antibodies; presumably this is because of the relatively small size of 

 the toxin molecule, which limits the number of its antigenic radicals 

 (Pick). Or it may well be that the immune body for antitoxin is 



