Histology, Natural Killer Cells


Natural killer or NK cells belong to the granular lymphocyte of the innate immune arm. In particular, natural killer cells are designed to exert cytotoxicity against virus-infected cells and immunosurveillance of cancer cells. Various immunoreceptors on the NK cell surface are configured to sense any alteration of self cells caused by malignant transformation or viral infection, which are potentially nonspecific. In contrast, lymphocytes of the adaptive immune arm, B and T lymphocytes express one specific recognition receptor. NK cell receptors are grouped as inhibitory and activating receptors that are polymorphic and germline-encoded.

Inhibitory receptors override activating receptors when encountering normal cells. However, surface molecular changes by a viral infection or tumor formation stimulate the activating receptors. Upon activation, natural killer cell release granzyme, perforin, effector molecules of the TNF family, and Fas-ligand to induce apoptosis of the target cell. Besides, NK cells synthesize and release other cytokines and chemokines such as Il-10, gamma-interferon, GM-CSF to recruit other immune effector cells to the activated site. NK cells also exhibit antibody-dependent cell cytotoxicity when antibodies tag the target antigen or cell. Nowadays, interventions targetting the inhibiting-activating state of NK cells has been using in clinical therapies.[1]

Issues of Concern

The term natural killer derives from the fact that these cells, in vitro, can kill target cells without the need for activation. NK cells represent the third lymphocyte population (together with type T and B cells). They play important roles in the innate immune system and the management of the immunological response.

These cells interact with several systems, including the nervous system; there is evidence linking their actions and the development of autism spectrum disorder. They can be assessed (the level of infiltration in solid tumors) to understand the prognosis in solid tumors; if the NK cells are low, the prognosis is generally positive. An alteration of their response (in excess) stimulates the senescence of the tissues, laying the foundations for aging. Natural killer cells are important for the body's salutogenic response in dealing with various physiological and non-physiological stimuli throughout life.


Morphologically, NK cells are large, granular, and bone marrow-derived lymphocytes. Moreover, phenotypically, they are defined as CD56 and CD16 in humans; thus, they lack the common T cell marker, CD3, and T cell receptor(TCR). They constitute 10% of the total peripheral blood mononuclear cell population of circulating lymphocytes and the third-largest population of lymphocytes after B and T cells.  NK cells, dendritic cells, B and T lymphocytes are derived from the single source, lymphoid progenitor cells. NK cell differentiation and maturation occur in bone marrow, spleen, lymph nodes, and thymus before entering into the peripheral circulation.[2]

A variety of activating or inhibitory has been demonstrated on the surface of NK cells as their immunoreceptor. Inhibitory receptors have cytoplasmic tyrosine-based inhibition motifs and recognize MHC class I molecules expressed by most of the nucleated cells. Activating receptors recognize the ligands exhibited by pathological cells and interact with the cell signaling pathway for effector activities. CD16, NKp30, NKp44, NKp46, NKG2C/CD94, and NKG2D/CD94 are examples of activating receptors. NK cells have their characteristic cytoplasmic granules that structurally and functionally similar to lysosomes, hence called “secretory lysosomes.” These granules have a bilayered membrane that separates the lytic content from the cytoplasm. They contain both varieties of lytic enzymes, unique to lytic granules and typical for lysosomes.[3]


Natural killer cells' effector functions are regulated by the balance of inhibitory and activating receptors and depend on and the intracellular signaling by these receptors. In physiological conditions, most of the circulating NK cells are in the resting state. However, upon activation by various cytokines, they infiltrate into virus-infected or tumor tissues. Almost every healthy cells express MHC I molecules, a potential ligand for inhibitory NK cell receptors, and exhibit self-tolerance. However, MHC I molecules are downregulated upon viral infection and tumor transformation, exerting subthreshold inhibitory signaling to the NK cell, leading to its activated state. The upregulation of natural killer cells activating receptors occurs in cellular stresses resulting from infection or tumor growth.[4][5][6]

NK cells can kill tumor cells without previous sensitization. They directly kill tumor cells by releasing lytic granules that contain granzyme and perforin. NK cells, together with cytotoxic CD8+ T lymphocytes, generate immunity against virus-infected and tumor cells. In most circumstances, tumor cells downregulate their MHC I to escape from cytotoxic T cells. However, the lower expression of MHC I make tumor cells susceptible to lysis by NK cell.[7]

Histochemistry and Cytochemistry

NK cells are considered as the innate equivalent of CD8 positive T lymphocytes. Various cell markers are used for immunocytochemistry. Natural killer cells are usually classified as CD3 negative and CD56 positive cells and subdivided into CD56dim and CD56bright variety. CD56dim and CD56bright classification are made according to their homing properties.

About 90% of circulating and spleen natural killer cells are CD56dim, CD16 positive, and express perforin. In comparison, NK cells in lymph nodes(LNs) and peripheral lymphoid organs(e.g., tonsils) are CD56bright and CD16 negative, which lacks the 'perforin.' These cells express various cytokines, including IFN-γ(gamma-interferon) upon stimulation.[7][8][9]

CD56 dim cells mainly perform a cytotoxic action, while CD56 bright cells are mainly cytokine secreting cells. CD16 mediates antibody-dependent cell-mediated cytotoxicity (ADCC).

Microscopy Light

Natural killer cells are distinct groups of lymphocytes that constitute five to ten percentage of peripheral circulating lymphocytes. On Wright-Giemsa-staining, they are visible as large granular lymphocyte morphology. They possess large nuclei containing coarse chromatin and prominent nuclei. Their abundant basophilic cytoplasm contains plenty of azurophilic granules that are negative for peroxidase staining.[10]

Microscopy Electron

NK cells are large and exhibit various shapes and express microvilli more prominently in the area of effector-to-target contact. At the same time, microvilli are scant cell surface in the absence of target cells. Membranes are not extensively fused in the region of target and effector cell contact.

NK cells have structurally distinct granules that consist of two different compartments. The outer compartment includes the lysosome-associated phosphatase acid enzymes and trimetaphosphatase. No enzymatic activities associated with the inner compartment have been identified. At the time of degranulation, a limited area of the cytoplasm exhibits vacuole-like areas possessing granules and apparent granular debris. Degranulation appears to be implicated in NK cells' cytotoxic activity. Convoluted nuclei with distinctive polarity against the dense granules and pseudopodia are typical. In the cytoplasm, both Mitochondria and polysomes are observed.[10][11]


In the presence of intracerebral hemorrhage and stimulation of alarmins (IL) and chemokines, the NKs are stimulated in a short time, which in turn stimulates the production of cytokines and chemokines. With the latter (and myeloid cells), the NKs manage the extent of the immune response. In this context, NK stimulates the presence of perihematomal edema through cytotoxic activity and an increase in the inflammatory response. The end result is further neurological deterioration of the patient.

Clinical Significance

Natural killer cells are essential components of innate immunity. Although their function may be compromised in many primary immune deficiency disorders, rare isolated Natural Killer cell deficiency may also occur. Patients with NK cell deficiency (NKD) are typically affected by herpesviral or papillomavirus infections. There are two potential varieties of NK cell deficiency: (1) the classical natural killer cell deficiency (CNKD) and (2) functional natural killer cell deficiency(FNKD).[12][13] 

CNKD reflects the significant absence of NK cells among peripheral blood mononuclear cells. Fewer than 1% of NK cells in peripheral blood should be present to consider this condition. Besides, this outcome should be stable over time, evident by a minimum of three independent tests separated by one month each. Unlike CNKD, patients with FNKD have NK cells found in their peripheral blood within the spectrum of age-specific average values; however, permanently deficient in their functions. It should be regarded only in the absence of other identified primary or secondary immune defects that may impact on NK cell's function. Both CNKD and FNKD patients are highly susceptible to human papillomavirus (HPV), herpes simplex virus (HSV), cytomegalovirus (CMV), Epstein-Barr virus (EBV), varicella-zoster virus (VZV) infection.[14][15][16][17]

Article Details

Article Author

Masum Rahman

Article Editor:

Bruno Bordoni


2/7/2021 4:57:55 AM



Mandal A,Viswanathan C, Natural killer cells: In health and disease. Hematology/oncology and stem cell therapy. 2015 Jun;     [PubMed PMID: 25571788]


Lanier LL, NK cell recognition. Annual review of immunology. 2005;     [PubMed PMID: 15771571]


Krzewski K,Coligan JE, Human NK cell lytic granules and regulation of their exocytosis. Frontiers in immunology. 2012     [PubMed PMID: 23162553]


Abel AM,Yang C,Thakar MS,Malarkannan S, Natural Killer Cells: Development, Maturation, and Clinical Utilization. Frontiers in immunology. 2018;     [PubMed PMID: 30150991]


Zhang Y,Huang B, The Development and Diversity of ILCs, NK Cells and Their Relevance in Health and Diseases. Advances in experimental medicine and biology. 2017;     [PubMed PMID: 28921473]


Stabile H,Fionda C,Gismondi A,Santoni A, Role of Distinct Natural Killer Cell Subsets in Anticancer Response. Frontiers in immunology. 2017;     [PubMed PMID: 28360915]


Sojka DK,Yang L,Yokoyama WM, Uterine Natural Killer Cells. Frontiers in immunology. 2019     [PubMed PMID: 31118936]


Fuchs A, ILC1s in Tissue Inflammation and Infection. Frontiers in immunology. 2016;     [PubMed PMID: 27047491]


Poli A,Michel T,Thérésine M,Andrès E,Hentges F,Zimmer J, CD56bright natural killer (NK) cells: an important NK cell subset. Immunology. 2009 Apr;     [PubMed PMID: 19278419]


Tsuchiyama J,Yoshino T,Mori M,Kondoh E,Oka T,Akagi T,Hiraki A,Nakayama H,Shibuya A,Ma Y,Kawabata T,Okada S,Harada M, Characterization of a novel human natural killer-cell line (NK-YS) established from natural killer cell lymphoma/leukemia associated with Epstein-Barr virus infection. Blood. 1998 Aug 15;     [PubMed PMID: 9694726]


Frey T,Petty HR,McConnell HM, Electron microscopic study of natural killer cell-tumor cell conjugates. Proceedings of the National Academy of Sciences of the United States of America. 1982 Sep;     [PubMed PMID: 6291036]


Orange JS, Natural killer cell deficiency. The Journal of allergy and clinical immunology. 2013 Sep;     [PubMed PMID: 23993353]


Orange JS, How I Manage Natural Killer Cell Deficiency. Journal of clinical immunology. 2020 Jan;     [PubMed PMID: 31754930]


Biron CA,Byron KS,Sullivan JL, Severe herpesvirus infections in an adolescent without natural killer cells. The New England journal of medicine. 1989 Jun 29;     [PubMed PMID: 2543925]


Etzioni A,Eidenschenk C,Katz R,Beck R,Casanova JL,Pollack S, Fatal varicella associated with selective natural killer cell deficiency. The Journal of pediatrics. 2005 Mar;     [PubMed PMID: 15756234]


Ornstein BW,Hill EB,Geurs TL,French AR, Natural killer cell functional defects in pediatric patients with severe and recurrent herpesvirus infections. The Journal of infectious diseases. 2013 Feb 1;     [PubMed PMID: 23175766]


Lopez C,Kirkpatrick D,Read SE,Fitzgerald PA,Pitt J,Pahwa S,Ching CY,Smithwick EM, Correlation between low natural killing of fibroblasts infected with herpes simplex virus type 1 and susceptibility to herpesvirus infections. The Journal of infectious diseases. 1983 Jun;     [PubMed PMID: 6304202]


Jan CI,Huang SW,Canoll P,Bruce JN,Lin YC,Pan CM,Lu HM,Chiu SC,Cho DY, Targeting human leukocyte antigen G with chimeric antigen receptors of natural killer cells convert immunosuppression to ablate solid tumors. Journal for immunotherapy of cancer. 2021 Oct;     [PubMed PMID: 34663641]