CALVIN B. COULTER 407 



sediment alone. However, in the experiment recorded in the upper 

 curve of Fig. 1, an hemolysis corresponding to an activity of 78 per 

 cent of the total amount of complement originally present points 

 distinctly to a true binding of end-piece by the persensitized cells. ' 



It is generally believed that the protective and sensitizing effects of 

 one colloid upon another with reference to precipitation by a third 

 substance are due to a combination of some sort between the two 

 colloids. There is no evidence available on which to base a judgement 

 as to the nature of the combination between the cells and the serum 

 euglobuhns considered here, whether physical or chemical. However, 

 the fact that the presence of the sera displaces the optimum for ag- 

 glutination of the cells quite sharply to the reaction characteristic of 

 the flocculation of the euglobuhn added, or of that added last and in 

 largest amount as in the case of the guinea pig serum, suggests the 

 occurrence of a surface combination or condensation of the serum 

 protein upon the surface of the red cell. The conclusion is supported 

 by the observation of Porges^ that bacteria which have been treated 

 with such an excess of immune serum that agglutination does not 

 appear, show the salt precipitation limits of the serum euglobulin and 

 not those of the native bacteria. 



The cells in the experiments reported here were not permeable to 

 hemoglobin since hemolysis did not appear, and as will be mentioned 

 later were very little permeable to inorganic ions, so that a penetration 

 of the euglobulin into the interior of the cell does not seem possible. 

 It would be difficult to explain furthermore how the small amount of 

 serum protein relative to the mass of cells could give its own point of 

 optimum flocculation to a mixture of cells and serum in any other way 

 than as a surface deposition. 



The euglobulin appears to be the active protein of the serum in 

 combining with the cells. 



A phenomenon based, as far as one may judge, upon precisely the 

 same mechanism in the combination of a protein with a surface has 

 been observed by Loeb.* This author has found that collodion mem- 

 branes always acquire the characteristics of the protein with which 



' Forges, 0., Ccntr. Bakt., Orig. 1906, xl, 133. 

 'Loeb, J., J. Gen. Physiol, 1919-20, ii, 577. 



