DEFENSES OF THE BODY 127 



leucocytes to take up bacteria, and hence a corre- 

 sponding decline in immunity. For example, there 

 are persons with persistent boils due to the pus- 

 forming staphylococcus, in which the opsonins for 

 these germs are reduced in activity. The activity 

 of these opsonins can, however, be raised by suitable 

 treatment, and simultaneously the boils disappear. 



The fact that the opsonins for staphylococci can 

 be raised in activity by injecting into the body of a 

 susceptible person very small, carefully adjusted 

 doses of the specific poisons formed by these germs 

 is an illustration of a principle of great importance 

 for the defenses of the organism. For it is now well 

 established that there are a variety of poisonous 

 substances which, on circulating in the blood, 

 stimulate the cells of the body to form substances 

 which are able to antagonize or neutralize any fur- 

 ther doses of the original poisons. Such poisons may 

 be called antigens, and the antagonizing substances 

 to which they give the stimulus of formation may 

 be called antibodies. The great investigator of 

 immunity Professor Paul Ehrlich maintains that 

 the antigens are absorbed by the cells of the body 

 through chemical mechanisms like those used in the 

 absorption of the foodstuffs. Consequently a poison 

 may operate as such because it presents a chemical 

 face like the chemical face belonging to nutritive 

 substances much as an enemy might be admitted 

 to a man's house because he wears the mask of a 

 friend. Having once gained admission, that is, 



