S1PR5 (sphingosine-1-phosphate receptor 5) is a G protein-coupled receptor that binds the bioactive lipid sphingosine-1-phosphate (S1P) 1. As one of five S1P receptor subtypes, S1PR5 mediates diverse cellular functions including lymphocyte and immune cell trafficking 2. The receptor couples to heteromeric G-proteins and regulates cell migration through S1P gradient sensing, which maintains low S1P in tissues and high levels in blood 1. Mechanistically, S1PR5 plays a critical role in immune cell redistribution. During fasting, S1PR5 is required for natural killer (NK) cell trafficking from peripheral tissues to the bone marrow, enhancing anti-tumor immunity 3. In tuberculosis infection, S1PR5 expression marks exhausted CD4 T cells with vascular localization phenotypes 4. S1PR5 is also expressed on NK cells in transplant rejection scenarios, correlating with active molecular rejection 5. Clinically, S1PR5 represents a therapeutic target for immune-mediated diseases. S1PR5-selective modulators like etrasimod, which also targets S1PR1 and S1PR4, have demonstrated efficacy in ulcerative colitis by preventing lymphocyte egress from lymph nodes 6. S1P modulator therapy shows promise across inflammatory bowel disease, rheumatoid arthritis, and other autoimmune conditions 17. However, safety concerns including leukopenia, cardiovascular events, and infections require careful monitoring during S1PR5-targeted treatment.