The interfacial tension method has been applied to the study of the surface composition of mammalian blood cells and to certain other particles.

Unsensitized erythrocytes and stromata possess only a small margin of stability in the interface and pass readily into the oil phase.

Specifically sensitized erythrocytes and stromata possess much greater stability in the interface and pass into the oil only with considerable mechanical aid; characteristic deformations of the erythrocyte surface or the interface or both often result.

With special immune sera prepared by Landsteiner and van der Scheer the quantitative relations are such as to indicate that the increased polarity of the sensitized erythrocyte surface is due to combination of the red cell surface lipoids with hemolytic sensitizer. These results are corroborative of the conclusion of Landsteiner and van der Scheer that erythrocytes contain specific lipoid-soluble antigens.

The tentative conclusion is reached that with these anti horse-erythrocyte sera at least the agglutinins combine predominantly with the protein of the red cell surfaces.

Fresh human leucocytes are spread and disintegrated by the interfacial stresses. After heat injury over the condenser with substage lamp the leucocytes typically do not enter the boundary surface. They are pushed before the advancing interface and, if their further advance is obstructed, bend the interface backward to form peninsulas and vacuoles. This change after heating is in the opposite sense to that to be expected from denaturation of the proteins of the protoplasm.

Fresh oxalated rabbit platelets pass very easily into the oils. After heating over the substage lamp these elements also become less oil-miscible.

The interfacial tension relations of blood cells, bacteria, and several cell products are tabulated.

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