HTR7 encodes a G-protein coupled receptor that binds serotonin and is coupled to G(s) alpha proteins to activate adenylate cyclase. The receptor mediates serotonergic signaling in the central and peripheral nervous systems and in immune cells, with roles spanning neurotransmission, circadian rhythm regulation, and immune homeostasis. Beyond its classical neurological functions, HTR7 regulates immune cell behavior. Enteric serotonergic neurons control intestinal plasmacytoid dendritic cell function through HTR7 signaling to enhance IgA-mediated mucosal immunity 1. In macrophages, serotonin modulates polarization toward an anti-inflammatory M2 state partly through HTR7 2. HTR7 expression is also elevated on megakaryocytes in immune thrombocytopenic purpura, where pharmacological inhibition with SB269970A restored platelet levels in a mouse model 3. Clinically, HTR7 variants influence antidepressant response: the rs7905446 G allele correlates with better response to selective serotonin reuptake inhibitors (SSRIs) including paroxetine and fluoxetine in bipolar and major depressive disorder 4. In Alzheimer disease, HTR7 expression is reduced in prefrontal cortex and associates with psychotic symptoms including hallucinations 5. Recent evidence indicates that serotonin-activated macrophages promote chemoresistance in ovarian cancer through HTR7-dependent metabolic signaling, and SSRI treatment may sensitize tumors to cisplatin 6.