CELL RECEPTORS. 157 



of an animal do not possess groups or receptors 

 which are capable of forming a chemical union 

 with the toxin, the latter would be unable to pro- 

 duce injury, i. e., the animal would be immune 

 even in the absence of all bactericidal or antitoxic 

 properties. This condition, however, is not one 

 which is capable of satisfactory demonstration, 

 at least at present, but the conditions point irre- 

 sistibly to its existence in some cases. 



We are accordingly led to the conclusion that 

 immunity to toxins is not in all cases antitoxic, 

 in the sense that the serum contains demonstrable 

 antitoxin; and likewise that immunity to bacteria 

 is not in all cases antibacterial, in the sense that 

 the serum contains substances which are able to 

 kill the bacteria in test-tube experiments. Non- 

 susceptibility and phagocytosis may be of impor- 

 tance in resistance of this type. 



There is another factor, however, which may importance 

 throw light on the type of natural immunity just 

 considered. We know that tetanus toxin causes 

 tetanus through its power of uniting with the 

 nerve cells, and we may consider that tetanus is 

 a very fatal disease, primarily because of the vital 

 nature of the tissue which it attacks. Now, if the 

 toxin, instead of uniting with the cells of a vital 

 organ, were to combine with cells of less impor- 

 tance to the economy, as, for example, the cells 

 of the subcutaneous tissue, it is probable that we 

 should have no tetanus. In some of the lower ani- 

 mals there is reason to believe that the toxin of 

 tetanus does unite with such tissue (Metchnikoff). 

 Roux and Borrel believe that the greater degree of 

 immunity to tetanus which the rabbit has over the 

 guinea-pig is due largely to the fact that the rab- 



