EHRLICH'S THEORY 373 



the body towards a toxin of complex and unknown struc- 

 ture. When an animal is repeatedly injected with a non- 

 lethal dose of diphtheria toxin it becomes immune to a 

 dose of the toxin many hundred times the strength of what 

 would have been originally a fatal dose. Further, the serum 

 of the animal thus artificially immunised, when injected 

 into a normal animal, confers upon the latter an immunity. 

 This is the basis of the modern treatment of diphtheria and 

 of tetanus. As when hippuric acid is administered, the 

 body has produced a protective substance — an antitoxin — • 

 and has produced this antitoxin in excess. The difference 

 between the two cases is that no toxin or combination of 

 toxin with antitoxin can be detected in the urine. It 

 therefore appears that in the two cases the mechanism of 

 defence is essentially the same, the apparent difference 

 between them being explained by the fact that in one case 

 the molecules concerned, being small, diffuse through the 

 kidney, while in the other case the molecules, being large, 

 remain in the blood and accumulate there. 



Ehrlich has given a graphic representation of the forma- 

 tion of antitoxin. He conceives the cell protoplasm as 

 having a number of different unsatisfied aflfinities which he 

 calls receptors. To one of these receptors a particular 

 toxin fits as a lock fits a key, and when it is thus fixed it 

 kills the cell. It is quite clear that a toxin can only kill a 

 tissue by entering into chemical combination with some 

 component of its structure. Tetanus toxin attacks the 

 nervous system. When an animal has died of tetanus the 

 toxin can be recovered from every tissue except nervous 

 tissue. It has combined with the nervous tissue to form a 

 permanent compound. According to Ehrhch, when a 

 non-lethal dose of toxin is administered, the tissue which 

 is susceptible to that toxin is stimulated to produce the 

 corresponding receptor in great numbers and to cast them 

 off into the body-fluids. The result is that when a second 

 dose of toxin is given, the molecules combine with the free 

 receptors and the cell protoplasm is unaffected (Fig. 71). 



