Final answer:
Natural Killer (NK) cells are the innate viral and tumor fighters of the human immune system, capable of destroying cells with foreign or abnormal markers.
Step-by-step explanation:
The innate viral and tumor fighters in the human immune system are Natural Killer (NK) cells. NK cells are lymphocytes that provide nonspecific immunity by recognizing and destroying cells that do not express "self" proteins on their plasma membrane, or that contain foreign or abnormal markers, such as cancer cells or cells infected with a virus. In concert with cytotoxic T cells, which are part of the adaptive immune response, NK cells work against virally infected cells as a first line of defense. NK cells are also known to kill foreign cells and are not primarily involved in signaling functions like macrophages, which can phagocytize pathogens and produce chemicals that promote inflammation.