Stimulatory Effects of Immune Reactions upon the Growths of Untransplanted Tumors

The accumulated literature suggests that altering the immune capacities of animals or humans seldom has detectable effects on tumor incidence nor, it seems, does the growth of an untransplanted tumor, either spontaneous or induced, immunize the host against the growth of a subsequent implant of that tumor. These observations suggest that even clearly immunogenic transplanted tumors may not have had their growths modulated by an immune reaction when they were as yet untransplanted in their primary hosts. Also, there are many data that have been interpreted to show that spontaneous tumors of rodents are, even when transplanted, nonimmunogenic. A reconsideration of the available studies, especially those in which either host immune capacity or tumor immunogenicity was titrated, has led me to different conclusions. I believe that the data suggest that probably all tumors, including spontaneous ones, are immunogenic but that the weak immune response in the primary host to the antigens of even the more immunogenic induced tumors usually produces stimulation of tumor growth rather than growth inhibition. The prevalence of immunogenicity suggests that vaccination or other forms of immunotherapy will eventually succeed; however, the analysis also suggests that, in the case of weakly immunogenic tumors, increasing the immune reaction, unless the increase is massive, may have little effect or may actually stimulate rather than inhibit the growths of these tumors in their primary hosts. Immunosnppression might actually be therapeutic in these cases; this may be an unrecognized benefit of the chemotherapy of many human tumors.