In tumor immunology, it is more important that tumor cells can escape from the immune system of the host than being able to provoke only a weak reaction from the host. With regard to antigenicity, tumor cells can express only oncodevelopmental antigens which were expressed in the developmental process of normal cells. Tumor cells are essentially self, although their antigenic heterogeneity is evident. Under this premise, immunity against tumor is essentially autoimmunity. Actually, most so-called 'tumor specific antigens' have been demonstrated to be self antigens. Immunotherapy against tumor cells may be based on the overly optimistic premise that autoimmunity is not harmful to the host. However, immunotherapy must face the essential duplicity between autoimmunity and tumor rejection. It is, therefore, hypothesized that autoimmunity is prepared and developed as an inhibition mechanism against retro-differentiation of normal cells but not against tumors. Autoimmunity is a check valve mechanism which inhibits reversion of the developmental process of normal cells as shown in hemolymph protein of larvae.

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