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The spleen is the largest secondary lymphoid organ in humans. Beyond its classical role in clearance of senescent erythrocytes, it functions as a pivotal node in systemic immune surveillance. Emerging evidence indicates that tumor can remotely remodel splenic niches through a spectrum of soluble mediators, thereby accelerating tumor initiation and progression. Tumor-derived signals divert splenic hematopoietic stem and progenitor cells (HSPCs) toward myeloid- and erythroid-biased extramedullary hematopoiesis (EMH), expanding myeloid-derived suppressor cells (MDSCs) and erythroid progenitor cells (EPCs) that collectively foster immune evasion and metastatic cascades. Consequently, splenic resident immune cells, stromal cells and EMH-related pathways have surfaced as actionable therapeutic targets. In parallel, bidirectional crosstalk between the autonomic nervous system and splenic immunity fine-tunes homeostasis, systemic inflammation and antitumor responses-fueling rising interest in splenic neuromodulation as a therapeutic strategy. In addition, spleen-targeted nanoplatforms are emerging as promising tools to deliver immunomodulatory payloads with improved precision. Nonetheless, inherent structural and functional disparities between human and murine spleens complicate clinical translation of pre-clinical findings. This review provides a concise overview of human lymphoid organs and their functions, with a particular focus on splenic anatomy, cellular composition, and neural regulation. It further delineates tumor-induced splenic rewiring and discusses the prospects of exploiting the spleen as both a biomarker and a therapeutic target in oncology.