What distinguishes natural killer cells from other lymphocytes?

Natural killer (NK) cells are a unique type of lymphocyte in the immune system, distinguished primarily by their ability to induce cell death in infected or cancerous cells without needing prior sensitization to an antigen. This means they can respond rapidly to a wide variety of threats, as they do not require the specific recognition of a pathogen that other lymphocytes, like T and B cells, do. This feature allows NK cells to act as a crucial first line of defense in the innate immune response.

In contrast, other lymphocytes, such as T cells, require antigen stimulation for activation; this means they need to recognize a specific pathogen's antigen to begin their immune response. The assertion that natural killer cells specifically target bacteria is incorrect; NK cells primarily recognize and eliminate virus-infected cells and tumor cells, not just bacteria. Additionally, while some lymphocytes are involved in adaptive immunity, NK cells function mainly in the innate immune system, which is responsible for the immediate response to pathogens. Hence, it is their ability to kill without prior sensitization that sets them apart from other lymphocytes.