426 HUMAN BIOLOGY 



charge and, as far as agglutination is concerned, Northrup 

 has shown that there is both a decrease of potential (below 

 13 milHvoIts) and a reduction of cohesive force by the 

 combined action of the serum constituents and the salts. 

 There is undoubtedly a surface change in the bacteria 

 as a consequence of their union with the specific serum 

 constituent which profoundly alters their relationship to 

 the environment. But this we know empirically rather 

 than by any clear understanding of the mechanism. 



It is therefore plain that the specific response of the 

 cells stimulated by contact with the bacterial antigen has 

 resulted in the formation of substances which render the 

 cell itself more sensitive to the antigen. Free in the cir- 

 culation, these antibodies can unite with the homologous 

 bacteria and thereby change them. By agglutination, the 

 bacteria are caught in the finer capillaries and more easily 

 ingested by endothelial phagocytic cells, such as the Kupffer 

 cells in the liver, and functionally similar cells in other 

 organs. At the same time, an increased susceptibility to the 

 destructive action of the complement or alexin, and an 

 increased susceptibility to phagocytosis (processes quite 

 actively defensive) are initiated. 



It must not be assumed that by the analysis of the relation- 

 ship of antigen to the production of circulating antibodies 

 we have covered the entire story of the adjustment of the 

 body to infection. It is quite clear from numerous observa- 

 tions that in addition to this mechanism there is also a 

 more deep-seated resistance of the cells of the tissues, and a 

 capacity for bacterial destruction by such cells, which cannot 

 be brought into relationship with antibody production. 

 There are, in naturally immune animals, cellular activities 

 like inflammatory response and phagocytosis, which seem 

 to go on independent of the presence of antibodies. And 

 in animals that have been immunized and allowed to rest 

 until all antibodies have disappeared there remains a 

 powerful capacity on the part of the tissues to respond to 

 infection in which, again, no cooperation of antibodies can 

 be demonstrated. Moreover, animals that have been non- 

 specificially rendered resistant by the injection of broth or 

 ptopene solutions into the peritoneal or pleural cavity 



