Natural Killer (NK) Cells- Definition, Structure, Immunity, Functions


Natural killer (NK) cells are a type of white blood cells that belong to the innate immune system. They are called natural killers because they can kill infected or abnormal cells without prior activation or recognition. They are essential for fighting against viral infections, cancer and other diseases.

NK cells are part of the lymphocyte family, which also includes B cells and T cells. However, unlike B cells and T cells, which have specific receptors for recognizing antigens, NK cells have a variety of receptors that can sense stress signals, such as the absence of self molecules or the presence of foreign molecules on the surface of target cells.

NK cells can be activated by cytokines, such as interferon-alpha and interleukin-12, which are produced by other immune cells in response to infection or inflammation. Once activated, NK cells can perform two main functions: cytotoxicity and cytokine production.

Cytotoxicity is the ability of NK cells to kill target cells by releasing granules that contain perforin and granzymes. Perforin forms pores on the membrane of the target cell, allowing granzymes to enter and trigger apoptosis (programmed cell death). NK cells can also induce cell death by expressing death ligands, such as Fas ligand and TRAIL, that bind to death receptors on the target cell.

Cytokine production is the ability of NK cells to secrete various molecules that modulate the immune response. For example, NK cells can produce interferon-gamma, which enhances the activity of macrophages and cytotoxic T cells; tumor necrosis factor-alpha, which induces inflammation and cell death; and interleukins, which regulate the differentiation and proliferation of other immune cells.

NK cells are found in various tissues and organs of the body, such as the blood, spleen, lymph nodes, liver, lung and uterus. They can also migrate to sites of infection or injury. NK cells are heterogeneous in their phenotype and function, depending on their developmental stage, activation state and microenvironment.

NK cells play a crucial role in immunity by eliminating infected or abnormal cells that escape from other immune mechanisms. They also interact with other immune cells to coordinate and regulate the immune response. NK cells are involved in various diseases, such as viral infections, cancer, autoimmune disorders and pregnancy complications.

In this article, we will discuss the definition, structure, immunity and functions of NK cells in more detail. We will also explore some of the current research and applications of NK cells in immunology and medicine.