INFECTION AND IMMUNITY. 605 



activity of similar, unimpaired atom-groups surrounding 

 those more directly influenced by the intoxicant during 

 the process of immunization (see page 601); and that 

 this excess of such "receptors," although physiologically 

 useless, being of no known service to normal function, 

 circulates unchanged in the blood, and serves, through 

 specific combining affinity for the poison against which 

 the animal has been rendered immune, to protect the 

 normal tissues from its hurtful action. 



According to the nature of the irritant from which 

 the animal has been immunized, the "receptor" is con- 

 ceived to be either of simple or complex construction, 

 and its protective function to be performed in either a 

 comparatively simple or in a more or less complicated 

 and roundabout manner. 



As a result of his studies of toxins, Ehrlich reached 

 the conclusion that they are composed of at least two 

 functionally distinct atom-groups : the one, a " hapto- 

 phore " group, characterized by its combining tendencies ; 

 the other, a "toxophore" group, distinguished for its 

 intoxicating powers ; and that for the exhibition of its 

 hurtful characteristics a toxin molecule needs to be first 

 anchored, so to speak, to the susceptible tissue by the 

 " haptophore " group, after which its intoxicating char- 

 acteristics are exhibited by the " toxophore " group. He 

 conceives the " receptors " to be likewise provided with 

 " haptophore " groups that pair with the corresponding 

 " haptophores" of the poison to which the animal is 

 susceptible or from which it has been immunized. Where 

 immunization has been induced against such relatively 

 simple substances as toxins, ferments, and certain cell 

 secretions, the " receptors " and their functions are com- 

 paratively simple L c., the single haptophore of the 



