138 CHEMISTRY OF IMMUNITY AGAINST BACTERIA 



Therefore it is not the haptophore that causes the harm to the 

 cell, but there must be some other group with this particular 

 function. To the group that produces the harm the name toxo- 

 phore is given. If all the receptors of a cell are combined by 

 toxin molecules that have lost their toxophore group (toxoid is 

 the name given to such altered toxins), the cell cannot then be 

 injured by the corresponding active toxin, showing that the 

 toxin must first become united to a cell receptor by its hapto- 

 phore group before the toxophore group can cause an injury. 



Animals that are naturally immune to toxins may owe their 

 immunity to the fact that their vital tissues contain no sub- 

 stances with a chemical affinity for the toxin, and hence the 

 toxin cannot unite with them to cause harm. (In Ehrlich's 

 terminology, the cells contain no receptors for the toxin.) The 

 toxin may not combine with any tissue element at all in such 

 immune animals, and circulate for some time harmlessly in the 

 blood ; or it may 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 introduced, and by saturating its affinities render it 

 incapable of uniting with the cells. As the toxin harms cells 

 only after it has been chemically 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 with 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 introduced into the body of 

 an entirely different animal, e. g., 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 



