Natural killer cells

Definition

Natural killer (NK) cells are a type of lymphocyte in the innate immune system that play a crucial role in the body's defense against tumors and virally infected cells. These cells can recognize and eliminate compromised cells without prior sensitization, making them vital for early immune responses and bridging the innate and adaptive immune systems.

5 Must Know Facts For Your Next Test

1. Natural killer cells account for about 5-10% of all circulating lymphocytes in the blood and are primarily found in peripheral blood, spleen, and liver.
2. These cells are capable of recognizing stressed cells in the absence of antibodies and MHC, allowing them to act faster than T cell-mediated responses.
3. NK cells release cytotoxic granules containing perforin and granzymes to induce apoptosis in targeted cells.
4. Natural killer cells also secrete various cytokines, such as interferon-gamma (IFN-γ), which enhances the immune response by activating other immune cells.
5. Dysfunction or reduced activity of NK cells has been linked to increased susceptibility to infections and the development of certain cancers.

Review Questions

• How do natural killer cells differentiate between healthy cells and those that are infected or cancerous?
  • Natural killer cells use a combination of activating and inhibitory receptors to assess the health of target cells. Healthy cells typically express major histocompatibility complex (MHC) molecules that bind inhibitory receptors on NK cells, preventing attack. However, infected or cancerous cells often downregulate MHC expression or present stress signals, leading to a lack of inhibitory signaling and subsequent activation of NK cell cytotoxicity.
• Discuss the role of natural killer cells in the context of bridging innate and adaptive immunity.
  • Natural killer cells play a pivotal role in connecting innate and adaptive immunity through their rapid response to infected or tumorigenic cells. They produce cytokines like interferon-gamma (IFN-γ) that activate other immune components, including macrophages and T cells. This not only enhances the immediate immune response but also helps to shape adaptive immunity by facilitating antigen presentation and promoting memory cell formation.
• Evaluate the implications of natural killer cell dysfunction in disease states such as cancer and chronic infections.
  • Natural killer cell dysfunction can lead to severe consequences in disease progression. In cancer, reduced NK cell activity allows tumors to evade immune detection and grow unchecked. Similarly, chronic infections can result from an inadequate NK cell response, prolonging the infection period and potentially leading to more severe health issues. Understanding these implications highlights the importance of NK cell functionality for effective immune surveillance and potential therapeutic targets for immunotherapy.

Related terms

Cytotoxic T cells: A subtype of T lymphocytes that directly kill cancerous or virus-infected cells after being activated by specific antigens.

Innate immunity:

The first line of defense against pathogens, consisting of physical barriers, immune cells, and biochemical responses that act quickly and non-specifically.

Immunotherapy: A treatment that uses the body's own immune system to fight diseases like cancer, which can involve enhancing the activity of natural killer cells.