128 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



having made a suitable chemical connection with 

 chemical groups in the cells, the poison proceeds to 

 do damage to the cells. It is in this way that the 

 body structures are injured in the course of disease 

 by the action of bacterial poisons probably the 

 most potent and widespread way of damaging the 

 body. If, however, the doses of the bacterial poisons 

 are not too large, there arises another effect of a very 

 different kind, a most singular, profoundly conserva- 

 tive effect. This is nothing less than the formation 

 of protective substances by the somewhat damaged 

 cells, substances which, owing to chemical groups 

 which they contain, are able to unite with some of 

 the poison that may reach the cells, and thus anchor 

 it fast and neutralize it. Substances of this kind are 

 called antibodies, and when such antibodies are cast 

 off into the blood, as they ultimately are when 

 they have been elaborated in excess, they receive 

 the name of antitoxins. These antitoxins, or free 

 antibodies, have thus the power of intercepting 

 poisons by combining with them and hence screen- 

 ing the vulnerable cells from the damage which they 

 would suffer if not so protected. The number of 

 antigens, or substances capable of exciting the forma- 

 tion of antibodies, is considerable. The venoms of 

 different snakes, the poisons formed by the bacilli 

 of lockjaw and of diphtheria, are among the best 

 examples of antigens from whose action may be 

 derived antitoxins suitable for the treatment of hu- 

 man injuries due to these poisons. And it should 



