EHRLICH'S THEORY OF IMMUNITY 197 



Thus in the case of diphtheria again it has been shown that one 

 unit of diphtheria toxin is sufficient to produce 500,000 units 

 of anti-body. Immunity therefore is explained by Ehrlich 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 complications 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. Bacteria are not touched 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 just as HC1 is necessary for pepsin to act on 

 proteids. With the addition of immune serum, however, the 

 union is effected the molecules 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 complement, 

 the molecules of immune serum form the connecting link and 

 are known as amboceptors. Hence without the amboceptors 

 the anti-bodies in the blood are unable to unite with the tox- 

 ins any more than pepsin can digest proteids in an acid-free 

 medium. 



It is not our purpose here to examine deeply into the secrets of 

 immunity the magnitude of the subject forbids anything more 

 than the briefest exposition of the phenomena of physiological 

 adaptation which immunity suggests. What this phenomenon 

 means to the organism can be imagined when the same individ- 

 ual becomes successively immunized to chicken-pox, whooping 

 cough, measles, mumps, smallpox, and half a dozen other dis- 

 eases. The fact of minute reactions bringing about great adap- 

 tations against disease is only one more instance of the marvel- 

 ous powers of adaptation which protoplasm manifests. 



