The immune system provides protection against the development of cancer. Malignant cells are considered foreign (nonself) because they are no longer completely normal. These cells often express cell surface antigens that are different from normal cells, allowing recognition by macrophages, helper T lymphocytes, and natural killer cells. Once malignant cells have been recognized as foreign, defensive and offensive actions are initiated by the immune system to eliminate or destroy them. This continuing protection, or immunosurveillance, is crucial in suppressing cancer development.