BY H. GREIG SMITH. 293 



' and suggested that the reason why pathogenic bacteria are not 

 so readily phagocytosed as non -pathogenic microbes may be that, 

 in the absence of specific cytases, there is much the same physical 

 relationship between the surface of the bacterium and the 

 phagocyte as there is between a drop of oil and water. A better 

 illustration is, perhaps, the relation between a particle of glass 

 and a drop of chloroform floating in water. The glass is not 

 englobed by the chloroform. If, however, the glass is coated 

 with a covering of shellac, it is quickly englobed. The coated 

 particle of glass appears to be analogous to the condition of 

 bacteria which have been agglutinated with active serum, for as 

 the clumped cells regain their motility we can infer that the pre- 

 cipitate coating the bacteria is slowly soluble, t and is on this 

 account capable of being absorbed by the phagocytes. By thus 

 covering or preparing the bacteria, agglutination may play a 

 much more active part in immunity than is generally supposed. 



At the same time I suggested the function of precipitins. 

 These are formed in the body-fluids of animals when foreign 

 substances, such as soluble albuminoids, are introduced. When 

 added to solutions of the same albuminoids, they cause the 

 formation of precipitates. We must assume that the alien sub- 

 stance is not wanted in the body-fluids of the inoculated animal 

 and that it will endeavour to get rid of it. The method adopted 

 by the animal is by the production of specific precipitins which, 

 in conjunction with the salts of the serum, coagulate the foreign 

 albuminoid or other substance. The precipitated particles are 

 englobed by the phagocytes like any other digestible particle 



t This was the line of thought, but it may be wrong. The precipitate 

 formed by the union or interaction of the albuminoid and its specific pre- 

 cipitin is soluble in an excess of the albuminoid (Michaelis, Biochem. Centrb. 

 iii. [April, 19051 693). The reassumption of motility is, therefore, probably 

 due to the exudation of an excess of agglutinable substance from the bac- 

 teria. This would only occur when all the agglutinin present had been fixed 

 or precipitated. 



