150 PHYSIOLOGY FOR DENTAL. STUDENTS. 



ferments in many of its actions (see p. 34). A peculiarity in 

 the action of the toxins is that a relatively long period elapses 

 between the injection of the toxin and the reaction of the body, 

 whereas in the case of the alkaloids or vegetable poisons, the re- 

 action appears very quickly. 



Antitoxin. In spite of the very poisonous character of the 

 toxin molecule, the body is provided with a means of defense 

 against it, and is able to make itself still further immune to the 

 action of the toxin. Thus, if somewhat less than the fatal dose 

 of diphtheria or tetanus toxin be injected into the body, certain 

 symptoms will follow, and the animal will react to the toxin in 

 such a way that a subsequent injection can be made larger with- 

 out proving fatal. If successively increasing doses are given, the 

 animal after some weeks will be able to withstand very large 

 doses of the toxin. In other words, the body develops an im- 

 munity towards the toxic agent; it produces an antibody which 

 neutralizes the poison of the toxin. To this body we give the 

 name of antitoxin. Since these antibodies are found in solution 

 in the blood, it is possible to withdraw the blood from such an 

 immune animal, and inject it into a non-immune animal, thus 

 rendering the latter immune to the toxin. It is this principle 

 that is used in the preparation of diphtheria and tetanus anti- 

 toxins. The exact nature of the combination of the toxin and the 

 antitoxin cannot be learned from chemical studies, but Ehrlich 

 has given to the phenomenon a biological explanation based on 

 the various known reactions of the bodies. 



Ehrlich 's Side Chain Theory of Immunity. Briefly summar- 

 ized Ehrlich 's theory is as follows: Each toxin molecule is 

 made up of a central nucleus of chemical radicles similar to those 

 found in organic compounds. To the main body of this mole- 

 cule are attached at least two other radicles, or side chains. One 

 of these has a great affinity for certain chemical constituents of 

 the tissues of susceptible animals, and unites the toxin molecule 

 to the tissue cell. This chain is known as the haptophon <jronp. 

 The other side chain, the toxophorc group, exerts the injurious 

 effect upon the tissue after the haptophore group has joined the 

 toxin to the cell. For example, tetanus toxin owes its effect to 



