Numerous modalities have been used in the treatment of cancer and for some time now host factors, including immunity in a classical sense, have been thought to be of importance.
"Host factors" may be involved in the malignant process but the evidence accumulated so far is indirect and the mechanisms poorly understood. The existence of resistance factors has been suggested because (1) occasional spontaneous regression of established tumor does occur, (2) widespread metastases may appear many years after the apparent successful treatment of a primary lesion, (3) some patients may survive for long periods of time even after incomplete removal of their cancer, (4) occasional regression of metastatic disease may occur after the primary tumor has been treated, and (5) evidence exists that large numbers of tumor cells may be extruded into the lymphatic and vascular systems without the development of metastases.12
Most of the initial animal experiments performed