Nk refers to natural killer cells, which are important immune cells in the body and participate in hypersensitivity and autoimmune diseases in some cases. Because naturalkillercell is slow to type in online chat, it is directly replaced by nk or NK cells on the network.

The exact source of NK cells is still unclear. They come directly from bone marrow, and their development and maturity depend on the microenvironment of bone marrow.