Human Osteopontin (OPN) (also known as SPP1, Secreted Phosphoprotein 1, bone sialoprotein I, ETA-1, or early T-lymphocyte activation-1) is a multifunctional phosphorylated glycoprotein involved in immune regulation, cell adhesion, and tissue remodeling. Osteopontin interacts with integrins (such as αvβ3, αvβ1, and α4β1) and CD44 receptors, activating signaling pathways that regulate cell migration, inflammatory responses, and extracellular matrix remodeling. In humans, OPN is produced by osteoblasts, macrophages, dendritic cells, epithelial cells, and activated T lymphocytes, and plays key roles in bone remodeling, immune activation, and tissue repair. OPN is also an important mediator of inflammatory and immune-mediated diseases, including rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, and atherosclerosis, where it contributes to leukocyte recruitment and cytokine signaling. In addition, osteopontin is strongly implicated in fibrosis and cancer, influencing tumor progression, angiogenesis, and immune cell infiltration in cancers such as breast, lung, and colorectal cancer. Because of its involvement in immune regulation, inflammation, and extracellular matrix remodeling, human osteopontin is widely studied as both a biomarker and therapeutic target in inflammatory diseases, fibrotic disorders, and oncology.