230 DISEASES FROM HARMFtX ORGANISMS 



isms which produce them, or both, are attacked by chemi- 

 cal means, and to this is given the name of the immun- 

 ity-reaction. 



The Action of Phagocytes. — Probably not aU of 

 the white blood-corpuscles exert the so-called phagocytic 

 action. In those that do, the action consists in wander- 

 ing about by amceboid motion (see Chap. XIII), the 

 direction of the motion probably being influenced by 

 chemical substances given off by the invading organisms. 

 Whenever contact is made with a microorganism — a 

 bacillus of typhoid, for instance — the phagocyte presses 

 closely against the bacillus and by movements of its pro- 

 toplasm engulfs it completely. Once within the body of 

 the phagocyte the organism is digested and destroyed. 

 It is an interesting fact that if phagocytes are in the pres- 

 ence of large numbers of a particular kind of organism, 

 such as the typhoid bacillus, the number of organisms a 

 single phagocyte will engulf is quite definite, so definite in 

 fact that it can be used as an index of the efficiency of the 

 body in defending itself against typhoid, since evidently 

 the more organisms each phagocyte will engulf the more 

 readily will the body rid itself of all the organisms which 

 have invaded it. A further curious fact about the action 

 of phagocytes, and a fact of very great importance in the 

 whole process of overcoming infection, is that if the 

 typhoid bacillus, to use the same illustration, becomes 

 established in the body and begins to multiply so that 

 poisons begin to be poured out and the symptoms of 

 disease begin to show themselves, a chemical reaction 

 goes on in the cells of the body, producing a material 

 which affects the bacteria in such a way as to cause each 

 phagocyte to engulf many more of them than formerly. 

 The details of this action are not at all clear, but of the 

 fact and its value as a means of overcoming disease 

 there can be no doubt. The protective inoculation 

 against typhoid, which reduced the occurrence of that 

 disease to such an extent that from being a frightful 



