PATHOGENIC PROPERTY OF MISRO-ORGANISMS 211 



enzymes in this respect. They differ from enzymes in that 

 they are used up in combining with tissue. Thus tetanus toxin 

 may be completely neutralized by the addition of brain tissue, 

 and either diphtheria or tetanus antitoxin may be quantitatively 

 neutralized by its specific antitoxin. Ehrlich in his study of the 

 reactions of diphtheria toxin showed that on standing it loses 

 much of its poisonous property without any diminution in its 

 ability to combine with diphtheria antitoxin, and to this less 

 poisonous substance he gave the name toxoid. From this observa- 

 tion he concluded that the toxin molecule contains at least two 

 very definite g,tomic groups. One of these is comparatively 

 stahle and serves for attachment of the toxin molecule to the 

 cell attacked by it, and is called the haptophorous group or 

 simply haptophore. The other recognizable chemical group 

 disintegrates more readily and is that which bears the poisonous 

 property. To this he gave the name of toxophorous group or 

 toxophore. In their reactions toxins behave in part like feebly 

 dissociated chemical compounds, as has been shown by Arrhenius 

 and Madsen, but the reactions by which they combine are only 

 slightly or not at all reversible and, moreover, take place in 

 variable proportions. Bordet very aptly compares the reactions 

 of toxin to the union of a dye with a stainable material. Bacteria 

 also produce poisons which are part of their own body substance, 

 and set free only upon their death and disintegration. These 

 are spoken of as intracellular toxins. Injurious substances may 

 also be produced from the tissue of the host by a secondary action 

 outside the cell of the parasite, but these secondary products 

 play a very minor r61e. 



Defensive Mechanisms.^ — The defensive armor of parasites 

 seems also to be in part physical and in part chemical, and perhaps 

 we may regard the physiological adaptation to slow growth as a 

 defensive mechanism because it tends to avoid exciting the 

 opposition of the host. The physical structure seems to be 

 protective in case of the waxy bacteria (tubercle and leprosy) 

 and the capsules of other bacteria may serve'a similar purpose 



