A new property of iso-agglutinins of the ABO blood group system.

component of a mixed population of erythrocytes is sensitized by treating the mixture with a specific iso-antibody. After washing, the sensitized minor population erythrocytes are treated with antiglobulin serum. After further washing, a suspension of sensitized, washed erythrocytes is introduced into the mixture to provide a source of globulin coated particles with which tile available antiglobin molecules may combine. Since the only available antiglobulin activity in tile suspension is confined to antiglobulin molecules coating the sensitized minor population erythrocytes, the effect of adding the “detector” suspension of sensitized washed cells is to produce a characteristic agglutinate which is thought to consist of a central, single erythrocyte of the minor population surrounded by a “shell” of detector erythrocytes. The linkage between the minor population cell and the detector cells is provided through the antiglobulin antibody which, under these circumstances, acts as a bivalent antibody molecule. A logical extension of these principles suggests that similar reactions may be produced by using iso-agglutinins of tile ABO system as lattice-forming bivalent antibodies. In such a system, a typical minor population would be Group A erythrocytes suspended in a diluting medium of Group 0 erythrocytes. The lattice-forming antibody would be anti-A and the detector cells would be Group A erythrocytes. The essential difference between the mixed agglutination