26 APPLIED IMMUNOLOGY 



tinins and precipitins. Receptors of the third order 

 possess two combining groups, and are termed ambo- 

 ceptors. The first of these anchors the antigen to the 

 cell and is known as the cytophilic group, while the 

 second combines with the complement of the blood- 

 serum and is known as the complementophilic group. 

 Thus, for the action of receptors of the third order, an 

 outside substance, the complement of the blood-serum, 

 is necessary. Among these are cytolysins (bacterioly- 

 sins, h^emolysins, opsonins, etc.). It is mmecessary 

 to go into the experimental evidence that has been 

 brought forward in support of Ehrlich's theoiy of im- 

 munity, but numerous investigators have fully con- 

 firmed his views by experiments. 



It is well known that poisons of known composi- 

 tion, such as alkaloids, glucosides, etc., when intro- 

 duced into the body do not produce antibodies, thus 

 differing essentially from true antigens. The true 

 antigens are closely related to foodstuffs and conse- 

 quently have an intimate chemical relation with the 

 body-cell, which provides special receptors for them. 

 The poisons, on the other hand, not having this spe- 

 cial chemical affinity, are not closely bound to the cell- 

 substance, but become physically stored up. Many 

 of these substances can be recovered from the body by 

 extraction, which could not occur if chemical union 

 had taken place. 



