IMMUNITY. 



219 



has are of many kinds, with affinities for other molecules of 

 different kinds. Each kind of receptor will then have an 

 affinity for a molecule of a particular kind, which it may be 

 said to "fit," as a key fits in a lock, although this expression 

 must not be taken in a literal sense. A receptor to which 

 tetanus toxin might become attached would not "fit" diphtheria 

 toxin. In order that 

 toxins may be able to 

 combine with the recep- 

 tors, their structure must 

 be nearly like that of the 

 food molecules which 

 the receptors are adapted 

 to receive. 



Secondly, soluble 

 toxins are to be looked 

 upon as definite chemical 

 bodies excreted by bac- 

 teria, and containing two 

 essential groups of 



atoms. One group is 

 the haptophore, by means 

 Of Which the toxin may 

 be linked with the re- 



FIG. 50. Receptors of the first order 

 j^, th ^.-(Journal of the American 

 Medical Association. 1905. P. 955.) 



a . Cell receptor, b. Toxin molecule, c. 

 Haptophore of the toxin molecule, d. Toxo- 

 phore of the toxin molecule, e. Haptophore 

 ceptors of the molecules of the cell receptor. 



of the cell. The other 



group is the toxophore, which is capable of destroying the 

 protoplasmic molecule, after attached to the receptor of the 

 latter by the haptophore. 



These relations have been represented schematically. In 

 Fig. 50 a portion of a cell is shown, with receptors. A m )lecule 

 of toxin, b, is attached by its haptophore, c, to the haptophore 

 of the cell receptor, a. A free cell receptor is also shown with 

 its haptophore, e, capable of uniting with any toxin molecule 



