The direct version of the Coombs test would be performed to determine if the infant has maternal IgG (anti-Rh) already bound to the erythrocytes in the cord Blood. IgG antibodies are relatively small. When they coat large paicles or cells, such as erythrocytes, they do not readily agglutinate them. In the direct Coombs test, anti- immunoglobulin (antibody against the Fc region of human IgG) is directly added to the infant's erythrocytes. The anti-immunoglobulin (also known as Coombs reagent) spans the distance between cells to form an agglutinating lattice that can be visualized in the test tube.