178 CHEMISTRY OF THE niML'MrY REACTIOXS 



time, or when placed under certain unfavorable conditions, the toxin 

 loses its poisonous properties without losing- its power to combine 

 with cells, as shown by the fact that immunization with such altered 

 toxin g-ives rise to the formation of antitoxin. Therefore it is not the 

 haptophore that causes the harm to the cell, but tliere must be some 

 other group with this particular function. To the group that pro- 

 duces the harm the name toxophore is given. If all the receptors of 

 a cell are combined bj^ toxin molecules that have lost their toxophore 

 group {toxoid is the name given to such altered toxins), the cell can- 

 not 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 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 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 hij the cells into the 

 Mood. 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 slieej) 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 bod.y of an entirely 

 different animal, e. g., a guinea-pig, will neutralize diphtheria toxin 

 within its body. Equally iinp:)rtaiit is the fact that the antitoxin for 

 one toxin will not neutralize any other toxin; e. g., diphtheria anti- 

 toxin will not neutralize tetanus toxin, or conversely. This means 

 that diphtheria toxin is attached to chemical groups of the body cells 



