Immune labeling of the D and E regions of human fibrinogen by electron microscopy.

Human fibrinogen was digested with trypsin to yield core fragments D and E, and antibodies were made against the isolated fragments. The Fab' fragments derived from these antibodies were mixed with native fibrinogen, resulting in the formation of soluble immune complexes. These were rotary shadowed with platinum or negatively contrasted with uranyl acetate and examined by electron microscopy. Fab' from anti-D immunoglobulin was found to be attached to the outer nodules of fibrinogen with a frequency of 79% prior to affinity purification and 91% afterward. Fab' from anti-E immunoglobulin attached to the central nodule with a frequency of 78% prior to affinity purification and 82% afterward. The evidence clearly identifies fragment D produced by plasmin or trypsin digestion of fibrinogen with the outer nodules and the single fragment E, with the central nodule.