Stress, Natural Killer Cells, and Cancer
RONALD B. HERBERMAN
It is now fairly well established that the way individuals cope with stress may affect the development and progression of infections and diseases, including cancer. Of particular interest to researchers attempting to better understand the potential immunological mechanisms that may mediate the influences of the central nervous system on host defenses against cancer and other diseases are natural killer (NK) cells.
NK cells are a third subtype of lymphocytes, besides B and T cells, that mediate immune responses against a variety of intracellular pathogens, including tumor cells and a wide variety of infectious microbes (Whiteside & Herberman, 1994). They are called “natural” killers because, unlike cytotoxic T cells, they do not need to recognize a specific antigen before swinging into action. Rather, NK cells have innate cytolytic activity and are stimulated to broader and higher levels of activity by cytokines,1 which are low-molecular-weight proteins that are released by other immune cells (as well as other cells in the body) in response to infectious agents, such as bacteria, viruses, fungi, and parasites (Rabin, 1999, p. 64). When stimulated by cytokines, NK cells have an enhanced ability to selectively seek out and kill, 2 target cells that fail to express “self” MHC class I molecules.
This important “immunosurveillance” property, combined with the relative ease with which NK cell activity can be measured (see chapter 11), have made them a major focus of research on the effects of stress on susceptibility to a number of viral illnesses (e.g., AIDs, hepatitis) and cancers.