Natural killer cells are a part of lymphocytes called "null cells" These are large granular lymphocytes and the 1st line of defence against viruses and bacteria
They belong to null cells as they do not rely on specific match or memory.
They control both tumor and microbial spread in the body.
Unlike T cells ( which mature in thymus), natural killer cells develop in bone marrow. They identify the viruses and other harmful cells by lack of major histocompatibility complexes (MHC)