IMMUNITY 15 



the body these receptors are probably cast off and 

 regenerated. 



Eegenerative processes of the body, however, do 

 not, as a rule, stop short at simple replacement of 

 the lost elements, but usually tend to over-compensate. 

 The receptors eliminated by toxin absorption are 

 not, therefore, simply replaced in the same quantity 

 in which they w^ere lost, but are reproduced in excess 

 of the simple physiological need of the cell. Con- 

 tinuous and increased doses of the poison, conse- 

 quently, soon led to such excessive production of the 

 particular receptive atom groups that the cell in- 

 volved in the process soon becomes overstocked and 

 casts them off to circulate freely in the blood. These 

 freely circulating receptors represent the antitoxins. 

 These, by uniting with the poison before it can reach 

 the sensitive cell, prevent its deleterious action. 



In the immediately preceding paragraphs we have 

 dealt solely with immunity as it occurs where soluble 

 toxins play an important part, and in which anti- 

 toxins are developed in the immunised subject. How- 

 ever, there are many species of pathogenic bacteria 

 which stimulate little or no antitoxic substance w'hen 

 introduced into animals, and the resistance of the 

 immunised animal cannot, therefore, be explained by 

 the presence of antitoxin in the blood. 



As mentioned, normal blood serum has the powder 

 of killing certain pathogenic bacteria, and the sub- 

 stance which gives serum this bactericidal power can 

 be destroyed by heating the serum to a temperature 



