D-Neopterin is widely used as a biomarker for immune system activation, particularly in clinical diagnostics. It is produced by monocytes and macrophages when the immune system is stimulated by inflammatory or infectious conditions. Elevated levels are commonly observed in patients with viral infections, autoimmune diseases, certain cancers, and neurodegenerative disorders such as Parkinson's disease. It is also used to monitor disease progression and response to therapy in conditions like HIV, hepatitis, and tuberculosis. Due to its stability in serum and urine, D-Neopterin is a practical and non-invasive tool for assessing chronic immune activation. Additionally, it has been studied in prenatal screening and transplant rejection monitoring, where immune response plays a critical role.