Contagious (^Zymotico Diseases. 2 



of chemical affinity and each must find its proper affinity to be- 

 come effective, as a halogen or acid must meet with a base in 

 order to form a salt. But just as a given acid and a given base 

 may have no mutual chemical affinity, so a potent toxin may be 

 present and yet prove harmless for lack of the intermediate 

 bodies which are necessary to establish a. true affinity with the 

 cell. The simile has been offered of a lock with its wards, the 

 key fitting those wards, and the hand that turns the key, and if 

 the reader will eliminate the mechanical elements of the picture 

 and replace them by chemical affinities, the interdependence of 

 each element on the adjacent one in order to establish a toxic 

 action in the cell, will be fairly represented. The current dia- 

 gram in which each element is represented as having on one end 

 a mechanical conformation which will fit a corresponding struc- 

 ture on the adjacent element, and can unite with no other, and 

 at the other end a special form which will fit that of another 

 element and no other, is liable to mislead certain minds. A 

 special element in the chain is fitted to connect with two other 

 elements differing from itself and from each other, but, the con- 

 nection is established through chemical attraction and not by 

 gross mechanical adaptation. 



The absence of these affinities may in part explain why certain 

 animals and certain genera are invulnerable by certain microbes 

 and their toxins, — why given diseases are peculiar to given species 

 of animals. 



Detachment of Receptors. Under many infections by the af- 

 finity for the microbe or its toxins, the leucocytes (the natural 

 defensive cells) accumulate in the seat of the microbian invasion 

 and even in the infected blood, the encrease being due not only to 

 their congregating, but no less to direct multiplication. Their 

 nuclei enlarge at the expense of the surrounding protoplasm and 

 nuclein escapes into the surrounding liquid. According to 

 Ehrlich multitudes of receptors are cast off from the surfa.ce of 

 each cell and meeting in the serum with the haptophores of the 

 toxins, unite with these, satisfying their affinity and thus ward 

 off their union with those receptors which are still attached to the 

 cell, and in this way they protect the life and integrity of the 

 cell. This contributes to abort or cut short the disease, and the 

 overproduction of these receptors, once started, tends to continue 



