Natural killer (NK) cells are lymphocytes but are not B cells. Unlike B cells, which are adaptive immune cells that produce antibodies and have antigen-specific receptors, NK cells belong to the innate immune system and kill virus-infected or tumor cells directly using cytotoxic granules and activating/inhibitory receptors. They do not produce antibodies or form memory in the same way B cells do.

An example of a natural killer (NK) cell is the circulating CD56dimCD16+ NK lymphocyte. These CD3−, CD56+ cells make up the majority of peripheral blood NK cells, are highly cytotoxic, and rapidly destroy virus‑infected or tumor cells without prior sensitization. Another example is the tissue‑resident CD56brightCD16− NK subset, which produces cytokines and helps regulate immune responses.

Natural killer (NK) cells are named because they are innate lymphocytes that can recognize and kill infected or tumor cells without prior sensitization ("natural"), and they directly induce target-cell death ("killer") using mechanisms such as perforin/granzyme release, death-receptor signals, and cytokine secretion. Their activity is regulated by a balance of activating and inhibitory receptors sensing stress, missing self, or abnormal ligands.

B cells are killed or depleted by targeted therapies (anti‑CD20 antibodies like rituximab, anti‑CD19 CAR‑T causing B‑cell aplasia), cytotoxic chemotherapy and radiotherapy, high‑dose steroids and other immunosuppressants, severe infections or sepsis, bone marrow failure and congenital immunodeficiencies, and age‑related apoptosis. Some viral infections can infect or dysregulate B cells, leading to loss of function or cell death.

Natural killer (NK) cells circulate in the blood and also reside in primary and secondary lymphoid organs (bone marrow, spleen, lymph nodes) and in tissues such as liver, lungs, tonsils, gut mucosa and the uterine decidua during pregnancy. They develop in bone marrow, patrol peripheral tissues for infected or tumor cells, and accumulate in inflamed sites to provide rapid innate immune responses.

Lacking natural killer (NK) cells weakens innate immune defense against virus‑infected and transformed cells, causing recurrent or severe viral infections (especially herpesviruses), reduced tumor surveillance, and higher cancer risk. People may have frequent, prolonged infections, impaired clearance of infected cells, and abnormal immune regulation. Severe congenital NK cell deficiency can cause life‑threatening infections in infancy; partial deficits increase susceptibility and worsen outcomes.