6.3　Natural Killer Cell

Natural killer (NK) cells are the predominant innate lymphocyte subsets that mediate anti-tumor and anti-viral responses.Natural killer cells represent 5%-20% of circulating lymphocytes in humans.They are distinguished by their unique functions and expression of surface antigens.NK cells lack the clonotypic T cell receptor (TCR) of T and NKT cells and its associated signaltransducing adaptor, CD3ε.In humans, subsets of NK cells express the activating Fc receptor,CD16 and most express CD56 [neural cell adhesion molecule (NCAM) or Leu-19].NK cells are most similar to a group of lymphocytes known as innate lymphoid cells (ILCs).ILCs are further categorized into three distinct groups and are present in both humans and mice.NK cells are related to group 1 ILCs as both produce interferon-gamma (IFN-γ) and tumor necrosis factor (TNF)-α upon stimulation.However, unlike Group 1 ILCs, NK cells have cytolytic functions that resemble those of CD8+ cytotoxic T lymphocytes [2].(https://www.daowen.com)