Further studies on the preparation of the immunosuppressive alpha-2 protein fraction from the serum and its assay in mice.

The method for the assay in mice of the α glycoprotein fraction of serum with immunosuppressive activity is described. The crude fraction has been shown to be effective in mice only with intravenous injection. When an antigen which is rapidly cleared from the blood is given, a single injection of the protein fraction given 10–20 hours before the exposure to antigen will inhibit the immune response in most animals. Time intervals outside this range are associated with a diminished or absent immunosuppressive effect. The protein fraction does not show activity immediately after preparation, or after reconstitution of freeze-dried material. Within 40 days, however, the activity is restored by storage alone, either frozen or at 4°. The presence of ribonuclease in the fraction is demonstrated, and its possible significance in relation to the immunosuppressive effect is discussed.