Sheep erythrocytes ingested by guinea pig peritoneal macrophages in vitro, and permitted to undergo digestion for various periods, were found after some hours to lose the capacity to induce antibodies while gaining the ability to invoke delayed hypersensitivity. These observations may be related to the known predilection of small molecular immunogens to act as good inducers of delayed reactivity and poor stimulators of antibody. They may be related also to the activity of mycobacterial adjuvant as a vehicle for the induction of delayed hypersensitivity on the basis that this melange activates macrophages to phagocytose and enzymatically degrade macromolecular antigens rapidly. The thesis that small fragments of antigenic molecules may preferentially invoke hypersensitivity can be interpreted on the basis of current concepts of multicellular involvements in immune responses.

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