Detection by ELISA of shed cell-surface molecules in serum

Monoclonal antibodies OKT9 and OKT9A, which recognize distinct epitopes on the transferrin receptor, were used to develop a sandwich enzyme-linked immunosorbent assay (ELISA) for this molecule. A 115,000 dalton molecule was detected in serum which probably represents a proteolytic cleavage fragment of the 186,000 dalton (dimer of two 93,000 chains) found on the cell surface of replicating cells. Serum levels of the shed transferrin receptor in human sera may be studied by means of this assay.