Natural Killer (NK) Cells and SARS-COV-2
Children have high number of NK cells
Lymphocytes are also higher in children due to frequent infections.
Children’s immune system also offers cross-reactivity to other viruses (for example measles vaccine offers immune training to have cross-reactivity to other pathogens.)
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Cellular Immune Responses to Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) Infection in Senescent BALB/c Mice: CD4+ T Cells Are Important in Control of SARS-CoV Infection

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Excerpt
Hence, in viral infections, NK cells are thought to play a central role primarily in the early stages of the response, in which they function to restrain viral replication until the adaptive immune response is sufficiently developed to control the infection.
Studies of mice with a mutation in the perforin gene demonstrated that perforin-dependent cytotoxicity plays an important role in controlling MCMV replication in the spleen but little or no role in the liver (172,173).
Important: NK cells are useful in the early stage of the infection by reducing the replication and giving a chance to the adaptive arm to become ready.
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Functional exhaustion of antiviral lymphocytes in COVID-19 patients
We showed that the total number of NK and CD8+ T cells was decreased markedly in patients with SARS-CoV-2 infection. The function of NK and CD8+ T cells was exhausted with the increased expression of NKG2A in COVID-19 patients. Importantly, in patients convalescing after therapy, the number of NK and CD8+ T cells was restored with reduced expression of NKG2A. These results suggest that the functional exhaustion of cytotoxic lymphocytes is associated with SRAS-CoV-2 infection. Hence, SARS-CoV-2 infection may break down antiviral immunity at an early stage.
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Mechanism of NK cells exhaustion
multiple mechanisms might contribute to the exhaustion of NK cells, such as dysregulated NK cell receptors signaling, as well as suppressive effects by regulatory cells or soluble factors within the microenvironment.
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CD107+ NK Cells
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NKG2A/CD94 receptors complex
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Natural Killer Cells: Development, Maturation, and Clinical Utilization
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IFN gamma is produced by NK Cells, CD4H1, and CD8 Cytotoxic
