8 Ehrlich's Theory 



combines with the cell receptors, or " side chains," because of a special 

 affinity between the groups. At first the combination may be effected 

 within the cell substance, but owing to an increased demand upon the 

 cell for receptors fitted to the haptophors, receptors are produced in 

 increasing quantity, and are finally thrown off as free receptors into 

 the circulation. It is the free receptors which, circulating in the blood, 

 lead to its antitoxic character. The serum of an animal treated with 

 toxoid is antitoxic, for the reason that the receptors disarm the toxin 

 of its haptophorous group, and consequently do not permit the toxo- 

 phorous group to combine with the cell protoplasm. 



Toxoid is therefore to be regarded as a toxin molecule, the toxo- 

 phorous group of which has been destroyed. Similar or analogous 

 observations have been made upon other antibodies, such as tetano- 

 lysin (Madsen), snake venom (Myers), milk-curdling ferment (Morgen- 

 roth), etc. 



Immunity to the effects of a toxin would therefore appear to depend 

 upon the absence of certain haptophorous groups in the toxin, for failing 

 these the toxin does not become anchored to the cell, and the latter 

 remains uninjured. Whereas the haptophorous group immediately enters 

 into combination with its corresponding receptor, the action of the 

 toxophorous group may be delayed for weeks. If tetanus toxin is 

 injected into frogs, the animals being maintained at 20 C. (Courmont), 

 large amounts may be injected with impunity. Morgenroth has shown 

 that the haptophorous group of the tetanus toxin enters into combination 

 with the nervous substance of the frog, but that the toxophorous group 

 does not do so unless the temperature is raised. 



Antitoxins are not of themselves toxic. As stated, the evidence of 

 their combining with toxins is based upon animal experiment, a mixture 

 of suitable proportions of homologous toxin and antitoxin being without 

 effect upon an animal which is susceptible to the toxin alone. Much 

 more favourable are the conditions of experiments with other antibodies, 

 such as haemolysins, agglutinins, and precipitins for the reason that the 

 interaction of the substances can be observed in vitro. 



The study of the specific bacteriolysins 1 , begun by Behring and 

 Nissen (1890) on animals immunified with Vibrio Metchnikovi, but 

 brought especially into prominence liy Pfeiffer (1894) in his studies upon 



1 That a difference between the bacteriolytic power of minimi and immune serum 

 might exist was already indicated by me (1888, p. 388). The serum of a sheep immunified 

 against anthrax was found to be more bacteriolytic for anthrax bacilli than normal sheep's 

 serum. The desirability of further experiments in this direction was indicated by me at 

 the time. 



