Immunostimulation of cancer versus immunosurveillance.

A moderate antitumor immune reaction is optimal for tumor growth and most (perhaps all) untransplanted tumors, rather than being inhibited, are probably dependent, at least early in their progression, upon the immune reaction. Some tumors, as for example most human skin tumors, have a higher incidence in immunodepressed patients than they do in the general population. This could mean that the normal immune reaction usually inhibits the growth of these tumors. More probably, the increased incidence in immuno-depressed heart and kidney transplant patients is caused by a lowering of a tumor-stimulatory immune reaction to a level that is even more stimulatory. Other tumors, such as human mammary tumors, have, in contrast to the skin tumors, a lower than expected incidence in immunodepressed patients. Mammary tumors are postulated to possess, on average, a low immunogenicity that arouses an immune reaction that is usually equal to or less than that required for optimal tumor growth; a further lowering of the reaction, as occurs in heart or kidney transplant patients, thus results in a lowered tumor incidence. In such patients, an immunostimulating adjuvant or vaccine might, we predict, sometimes accelerate rather than inhibit tumor growth.