Is cancer dangerous to the immune system?

The hypothesis of immunologic surveillance of neoplasia is predicated on the theory that the immune system is capable of discriminating self from foreign antigens, and that tumor-specific antigens are regarded by the immune system as nonself. We propose here an alternate view, that the immune system has evolved to detect danger by employing 'professional' antigen-presenting cells as sentinels of tissue distress. In this model, cancers do not appear dangerous to the immune system, so that the default response of T cells to tumors is to be turned off. We discuss the implications for immunotherapy of malignancy.