DEFENSES OF THE BODY 129 



be noted that in each case the action of the antigen 

 is specific. The antitoxin formed by the action of 

 the diphtheria poison or toxin will neutralize diph- 

 theria poison, but not lockjaw, and the converse is true. 



Whether the antibodies and the opsonins are sub- 

 stances of the same chemical type, or distinct, is 

 unknown and need not concern us in this general 

 discussion, but it seems likely that different investi- 

 gators, approaching the problem of immunity from 

 different standpoints, have discovered identical or 

 similar substances under different names. 



There is still another physical defense of great 

 importance to the maintenance of life, but of a nature 

 wholly different from any that has been mentioned. 

 This is the protective action of the microorganisms 

 that occupy the digestive tract. It is well known 

 that throughout life, from a few hours after birth, 

 the human intestine swarms with bacteria. It is 

 less well known that in healthy people these inhab- 

 itant bacteria possess a clearly defined biological 

 character which varies within not very wide limits 

 for the same individual and even for different 

 persons, provided the diet remains reasonably con- 

 stant. There is not the least doubt that in some 

 way a rather intricate way these normal and 

 dominant flora of the intestine exert a protective 

 action on the entire body. This protective action 

 is complex in operation, but consists at least in part 

 of a preemptive effect on the intestinal domain, by 

 which other types of bacteria are in large degree 



