IMMUNITY. 155 



The side-chains are therefore preferably called " receptors" 

 The numerous receptors which a molecule has are of many 

 kinds, with affinities for other molecules of different kinds. 

 Each kind of receptor will then have an affinity for a mole- 

 cule 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 receptors their 

 structure must be nearly like that of the food molecules 

 which the receptors are adapted to receive. 



FIG. 48. 



\ Antitoxin 

 Body cell > Antitoxin 



Diagram to Illustrate Side-chain Theory (modified from Aschoff). 



Secondly, soluble toxins are to be looked upon as definite 

 chemical bodies excreted by bacteria, and containing two 

 essential groups. One group is the haptophorc, by means of 

 which the toxin may be linked with the receptors of the 

 molecules of the cell. The other group is the ioxophore, 

 which is capable of destroying the protoplasmic molecule, 

 after being attached to the receptor of the latter by the 

 haptophore. 



These relations may be represented schematically though 

 in a very crude manner. In Fig. 48 a portion of a cell is 

 shown, with receptors. Molecules of toxin, with hapto- 

 phores and toxophores are near by or attached. At the 



