Allograft tolerance: presumptive evidence that serum factors from tolerant animals that block lymphocyte-mediated immunity in vitro are soluble antigen-antibody complexes.

Serum factors present in rats rendered operationally tolerant to skin allografts by inoculation of allogeneic bone-marrow cells as newborns inhibit or "block" the cytotoxic effect of immune lymphocytes in vitro. These blocking factors were specifically removed from tolerant serum by absorption with allogeneic cells and later eluted from the absorbing cells in glycine buffer (pH 3.1). Blocking activity of the eluted material was resolved into fractions of low and higher molecular weight, which may be soluble histocompatibility antigen and specific alloantibody, respectively. Both antigen and antigen-antibody complexes may block in vitro, depending upon the assay used.