The importance of host resistance and immunological factors in influencing the growth of tumors has been suspected for over 50 years.1 A great deal of experimental evidence has accumulated in the past two decades to support this thesis, and further studies in the immunologic field are now warranted on the basis of this evidence.
The mechanisms by which host cells inhibit cancer growth or destroy cancer cells is not completely understood. Much of the early experimental work was concerned with attempts to demonstrate circulating antibodies against malignant tissues, identify specific tumor antigens, and develop effective cancer vaccines. In 1953, Foley2 demonstrated specific antigenicity in a class of experimentally induced tumors induced by methylcholanthrene in syngeneic mice. He demonstrated that the removal of a growing transplant of an induced tumor was followed by resistance to subsequent challenge with the same tumor. These experiments have been confirmed many times and