BHRLICH 8 rHBORY OF IMMUNITY. 15 



In isss Ifericourt and Richet, and in iss'.i Hal>es and Lepp re- 

 ported investigations which showed that by injections of blood 

 serum from animals having acquired immunity to certain diseases, 

 immunity to these same diseases can be conferred to other ani- 

 mals. Soon after this Behring and Kitasato reported successful 

 immunization of rats to tetanus by means of injections of blood 

 serum from rabbits immunized to tetanus. From the work of these 

 investigators there developed the humeral theory of immunity. 



Behring and Knorr found that the toxic product of the meta- 

 bolism of the tetanus bacillus, could, without the presence of the 

 bacillus, be used in immunization. The serum thus produced, they 

 found would protect against the disease and the injection of the 

 by-products of the tetanus bacillus. They assumed from this 

 that toxin is neutralized by the immunizing substance in the m- 

 mune serum, as a base is neutralized by an acid. 



Numerous theories have been advanced to explain the facts 

 observed and reported, but of all these theories one stands out 

 prominently Ehrlich's side chain or receptor theory, advanced 

 by Ehrlich in 1897. 



According to this theory, every living cell consists of a domi- 

 nating nucleous (Leistungskern) and of side chains or receptors. 

 The paradigm of this picture is to be found in the benzol ring 

 with its side chains. The side chains or receptors of a cell are of 

 many different kinds and serve usually to anchor and assimilate 

 the food stuffs. At times, however, by means of the receptors 

 the cells are combined with substances, not foods but cell poisons. 

 The combination of the receptors of the cell with the receptors of 

 foods differs from the combination of receptors of the cell and re- 

 ceptors of toxins. The combination of the receptors of the cell 

 with the receptors of foods is so loose that after undergoing certain 

 changes which are of value in assimilation of the food and the 

 nourishment of the cell the food stuff is again eliminated without 

 having injured the receptor. Toxins on the other hand, combine 

 so firmly with the receptor of the cell that they cannot ajjain 

 be separated. As a result of this, receptors anchoring toxins 

 are lost to the cell. 



When a certain limit of anchoring of receptors by toxins has 

 taken place the cell peri hes, and if a sufficient number of cells 

 which have vital functions to perform are destroyed, death of 



