Reversible binding of a T cell factor on IgG-coated sepharose beads

Supernatants of alloantigen-activated T cells contain a number of factors, including an immunoglobulin-binding factor (IBF) which inhibits complement-induced hemolysis of sheep erythrocytes coated with anti-Forssman IgG antibodies and a factor which suppresses IgM antibody synthesis in vitro. These two factors may be identical, since they are simultaneously retained on Sepharose beads to which IgG has been coupled and can be recovered by elution at pH 2.8. They do not bind to Sepharose beads to which IgM of F(ab')2 fragment of IgG has been coupled, demonstrating that they have a selective affinity for the Fc region of IgG. In addition, the fixation of IBF on the Fc portion of IgG reversibly inhibits subsequent binding of the first component of complement (C1), thus indicating that IBF does not irreversibly alter the C1 binding site(s) of IgG.