Isolation of a Thermolabile Serum Protein which Precipitates -Globulin Aggregates and Participates in Immune Hemolysis.

A protein of normal human serum capable of precipitating soluble aggregates of γ-globulin was isolated and found to be a euglobulin with an S° 20.w = 11 S and the approximate electrophoretic mobility of γ-globulin. The activity of this protein proved thermolabile and independent of Ca++ and Mg++. Serum rendered deficient in 11 S component showed drastically reduced hemolytic activity despite virtually unchanged titers for the classical components of complement, and regained full hemolytic activity upon addition of microgram amounts of preparations of isolated 11 S. The possible significance of this component for the early steps in complement reactions was discussed.