Natural killer cells, also known as NK cells or large granular lymphocytes (LGL), are a type of cytotoxic lymphocyte essential to the innate immune system. They are a member of the rapidly increasing family of innate lymphoid cells (ILC) and account for 5–20% of all circulating lymphocytes in humans.

NK cells function similarly to cytotoxic T cells in the adaptive immune response of vertebrates. NK cells respond rapidly to virus-infected cells and other intracellular pathogens acting approximately 3 days after infection, as well as tumour growth.

Immune cells often identify the major histocompatibility complex (MHC) displayed on infected cell surfaces, initiating the release of cytokines that result in the lysis or apoptosis of the infected cell.

However, NK cells are exceptional in that they can recognise and kill stressed cells in the absence of antibodies and MHC, enabling a much speedier immune response.

They were dubbed "natural killers" due to the belief that they do not require activation to kill cells lacking "self" MHC class 1 signals. This function is particularly crucial since dangerous cells without MHC I signals cannot be identified and eliminated by other immune cells, such as T lymphocyte cells.

CD56+, CD3- cells are NK cells. NK cells are one of the three types of cells that differentiate from the common lymphoid progenitor; the other two are B and T lymphocytes.

It is known that NK cells differentiate and mature in the bone marrow, lymph nodes, spleen, tonsils, and thymus before entering the bloodstream. NK cells and natural killer T cells (NKTs) differ in phenotype, origin, and effector roles; NKT cell activity frequently stimulates NK cell activity by secreting interferon gamma.

Unlike NKT cells, NK cells typically display the surface markers CD16 (FcRIII) and CD57 in humans, NK1.1 or NK1.2 in C57BL/6 mice.

The NKp46 cell surface marker is currently a preferred NK cell surface marker expressed in humans, multiple mouse strains (including BALB/c mice), and three common monkey species.

In addition to their role as effectors of innate immunity, both activating and inhibitory NK cell receptors have essential functional roles, including self tolerance and the maintenance of NK cell activity.

Numerous tests have showed that NK cells can easily adapt to their immediate surroundings and acquire antigen-specific immunological memory, which is crucial for responding to subsequent infections with the same antigen.

The significance of NK cells in both the innate and adaptive immune responses is growing in cancer studies using NK cell activity as a potential treatment.