EHRLICH'S THEORY OF IMMUNITY 201 



plasm into the blood as free atom groups, which are then capable 

 of uniting in the blood with the poison molecules. In this 

 way, chemical union of antigen and anti-body takes place out- 

 side or apart from protoplasm, and when thus united, the poison 

 is made harmless, because of its inability now to unite with any 

 protoplasmic group its valencies have been satisfied. Fur- 

 thermore, Weigert has shown that the quantity of protective 

 substances in the blood (anti-bodies) is out of all proportion to 

 the quantity of toxin which stimulated the reaction. In other 

 words, hyper-regeneration follows such toxic injuries to the 

 protoplasm. Thus, in the case of diphtheria it has been shown 

 that one unit of diphtheria toxin is sufficient to produce 100,000 

 units of anti-body. Immunity, therefore, is explained by Ehr- 

 lich as the condition whereby the blood is loaded up with free 

 chemical substances, which unite with and render harmless 

 the specific poison of a disease-causing parasite. It is a most 

 pregnant theory, and has been developed with surprising 

 ingenuity to satisfactorily account for all of the complications 

 connected with zymotic diseases. One only of these compli- 

 cations will be given here, as the subject of immunity is vast 

 and perplexing. The case of opsonin formation and action is a 

 relatively simple adaptation of the theory. Some bacteria 

 in the blood, e.g. } tubercle bacilli, are relatively unharmed by the 

 phagocytes under normal conditions, but if immune serum be 

 added, the bacteria are immediately devoured, or in some cases 

 dissolved without being engulfed. According to Ehrlich's 

 theory, there is no chemical attraction or proper grouping of 

 atoms in the bacterial cell to enable the protecting substances to 

 unite with them. With the addition of immune serum, however, 

 the union is effected the substances contained in it being able 

 to unite with both the bacteria and the phagocyte. In such a 

 case, the phagocytes or dissolving anti-bodies form the com- 

 plementj the molecules of immune serum form the connecting 

 links, and are known as amboceptors. Hence, without the am- 

 boceptors, the anti-bodies in the blood are unable to unite 

 with the toxins, any more than pepsin can digest proteins in an 

 acid-free medium. 



