A monoclonal IgG4 (lambda) with factor V inhibitory activity.

A monoclonal IgG4 (lambda) with inhibitory activity to human coagulation factor V was isolated from the serum of a patient with a fatal hemorrhagic diathesis by using a combination of DE-52 ion exchange chromatography and isoelectric focusing techniques. Using the criteria for defining a monoclonal immunoglobulin of restricted mobility on protein electrophoresis, immunoelectrophoresis, and isoelectric focusing, as well as neutralization with class, subclass, and light chain type antisera, we are the first to demonstrate a factor V inhibitor as a monoclonal IgG4 (lambda) detectable in serum or plasma.