Abstract: The invention involves cellular and biological responses to relatively low concentrations of thionin (such as Pyrularia thionin), which responses are distinct from the direct cytotoxic effect of high concentrations of thionins on cells. These two separate biological responses of immune cells to thionins are related to the presence of two separate and distinct binding sites on the immune cell, a high affinity binding site that reacts with thionins at low concentrations to stimulate the immune cell to provide antitumor protection, and a low affinity site that is responsible for the direct toxicity of thionins to cells. This invention provides, in certain embodiments, methods for using a thionin, either alone or in combination with an interleukin, for stimulation of the immune system.