Tumor surveillance: how tumors may resist macrophage-mediated host defense.

Both normal human serum and supernatant from explanted malignant tumors contained a heat-stable low-molecular-weight factor that inhibited monocyte activation in vitro. In contrast, serum from individuals with solid tumors enhanced monocyte activation. It is suggested that the systemic activation of monocytes that occurs in malignant disease may be an appropriate host response but that successful tumors may continue to grow because they subvert the normal physiological signal for inhibition of macrophage activation.