IMMUNITY. 183 



by the analogies presented by the graphically written formulae 

 of some complex molecules. It is necessary to conceive of mole- 

 cules made of an immense number of atoms, and bristling with 

 projecting side-chains. The function of the side-chains is to 

 become attached to other organic molecules with which they 

 have affinities. In this manner they aid in absorbing the sub- 

 stances essential for the nutrition of the protoplasm of cells. 



The side-chains are therefore also called "receptors" — a 

 more appropriate name. 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 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 Uteral sense. A receptor 

 to which tetanus toxin might become attached would not "fit" 

 diphtheria toxin. In order that toxins may be able to com- 

 bine with the receptors 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 bacteria, and containing two 

 essential groups of atoms. One group is the haptophore, by 

 means of which the toxin may be Hnked with the receptors of 

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

 which is capable of destroying the protoplasmic molecule, 

 after being attached to the receptor of the latter by the hapto- 

 phore. 



These relations have been represented schematically. In 

 Fig. 48 a portion of a cell is shown, with receptors. A molecule 

 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 

 that may be present. The toxin molecule, b, has its toxophore 

 group represented by the fringe-like end, d. If the cell receptor 

 becomes detached from the cell, its haptophore, e, may unite 



