50 IMMUNITY 



amount of toxin. If ten times the amount of anti-toxin is used it 

 will exactly neutralize ten times the amount of toxin, and the mix- 

 ture becomes inert. Again, the union of the two substances follows 

 well known chemical laws, whereby chemical union takes place 

 more rapidly in concentrated than in dilute solutions, and when the 

 solutions are warm. If the mixture of toxin and anti-toxin is heated, 

 it, instead of being neutral, becomes toxic again. This toxicity can 

 be neutralized again by the addition of fresh unheated anti-toxic 

 serum (reactivation) . 



The production of bacteriolysins, cytolysins, agglutinins, precipi- 

 tins, and anti-toxins are manifestations of the activity of the immun- 

 ized organisms. To further understand this activity, Ehrlich's 

 side-chain theory of immunity must be comprehended. This is 

 known as the chemical theory. To understand it fully some con- 

 sideration must be given to the study of the toxin molecule. Ehr- 

 lich believes that each molecule of toxin is made up of two groups 

 of atoms, constituting what is known in chemical nomenclature as 

 lateral chains. 



Many molecules are made up of a central body and lateral chain 

 of atoms which are free to combine with other groups of atoms with- 

 out disturbing the central body. 



The benzol ring is very suitable for the demonstration of the 

 relationship of the side-chain to the central body. 



H 



I 



H C X ^C H 

 H-C ,C-H 



H 



BENZOL. 



The benzol molecule C 6 H is here represented graphically as a 



