Sialic acid-binding immunoglobulin-like lectin 9 (Siglec-9) is a receptor that expresses on the surface of immune cells. It plays an important role in the body’s immune response. Increased expression of Siglec-9 has been reported in infectious diseases, autoimmune diseases and cancer. Pathogenic microorganism and tumor cells can inhibit the recognition and killing of immune cells by upregulating their own specific sialic acid and binding with Siglec-9 on the surface of host immune cells, and suppress the release of pro-inflammatory cytokines and promote the release of anti-inflammatory cytokines, eventually leading to immunosuppression, tumor immune escape and the like. However, the immunosuppressive function of Siglec-9 may be advantageous for diseases such as neutrophil asthma and autoimmune diseases. Therefore, further research on the mechanism of action of Siglec-9 is of great significance.

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