The growths of many and perhaps all tumors may be stimulated rather than inhibited by a quantitatively low level of immunity. The reason tumors have antigens may be that tumors do not develop in vivo in the absence of at least a minimal immune reaction; in this sense, cancer may be considered an autoimmune disease. This review, based largely on the work of our own laboratory, outlines the data showing that the titration of anti-tumor immunity exhibits the phenomenon of hormesis, i.e. the dose-response curve is non-linear such that low levels of immunity are generally stimulatory but larger quantities of the same immune reactants may inhibit tumor growth. Evidence is also reviewed that suggests that the immune response may vary qualitatively and quantitatively during progression, such that there seems to be, during oncogenesis, a very low level of immune reaction that aids initial tumor growth, followed by a larger reaction that may cause remission of early neoplasms, followed, if the neoplasm survives, by a relative immunologic tolerance to the tumor that may be dependent, at least in part, on suppressor cells. This knowledge may help to explain some clinical observations concerning the relationships among tumor types and the organ distribution of metastases.

Tumor implants grew better in radiated or in newborn nu/nu mice than in adult nu/nu controls when, and only when, the tumors were demonstrably immunogenic in normal mice. This result suggests primary immunity. No evidence of immunological memory was found by immunization-challenge type experiments in the nude mice.

Ten mouse strains in which aryl hydrocarbon hydroxylase can be induced, or F1 hybrids of these strains, were ranked according to their sarcoma susceptibility when exposed to a high concentration (5 percent) of the chemical carcinogen 3-methylcholanthrene. This rank order was reversed when the concentration of 3-methylcholanthrene was reduced to 0.05 percent.

A highly immunogenic mouse tumor was studied with the use of microcytotoxicity test, performed at lymphoid cell:target tumor cell ratios ranging from 1:1 to 1,000:1 at various times during tumour growth and after tumor excision. Although the tumor was highly immunogenic, in vitro lymphoid cell-mediated cytotoxicity was shown only with lymphoid cells from hyperimmunized mice, i.e., mice that had "seen" the tumor more than once. Instead, the lymphoid cells harvested 2 and 3 weeks after tumor inoculation, as well as after excision of the tumor, stimulated the target cells. At 4 weeks after inoculation of the tumor, there appeared to be neither stimulation nor inhibition of the target tumor cells, though at this point the growth of this highly immunogenic tumor had been arrested in vivo.

The therapeutic efficacy of intralesional BCG (Bacillus Calmette‐Guerin; one immunizing dose every 2 weeks for a minimum of five treatments) was studied in 19 melanoma patients. Of 15 patients evaluable for response, five experienced significant objective improvement (two complete and three partial remissions). Objective improvement was limited to those patients with dermal metastatic disease. In vitro cytotoxicity in the presence of patient's serum bore, on average, a relationship to the clinical disease. In certain individual cases, serum blocking and/or lymphocyte stimulation may have had prognostic significance.