CD84 (SLAMF5) is a homophilic adhesion molecule and self-ligand receptor of the SLAM family broadly expressed on immune cells 1. Functionally, CD84 regulates diverse immunological processes: it enhances T cell proliferation and IL-21 secretion for B cell help 2, promotes NK cell cytotoxicity 1, and maintains B cell tolerance in germinal centers to prevent autoimmunity 1. In myeloid cells, CD84 serves as a marker for myeloid-derived suppressor cells (MDSCs) in breast cancer and cancer patients 34, and enhances macrophage LPS-induced inflammatory responses 1. CD84 signaling is regulated by adapter proteins SH2D1A/SAP and SH2D1B/EAT-2 1. Clinically, CD84 dysregulation associates with multiple pathologies: in acute myeloid leukemia, CD84 functions as a critical survival factor regulating mitochondrial metabolism and antioxidant defense via NRF2 stabilization 5; in prostate cancer, CD84-expressing PMN-MDSCs promote resistance to therapy through factor X signaling 6; in lung cancer, CD84 expression in tumor-associated macrophages predicts better prognosis in adenocarcinoma 7. Notably, CD84 genetic variants predict response to etanercept in rheumatoid arthritis patients, with higher expression correlating with better therapeutic outcomes 8. These findings establish CD84 as both an emerging biomarker and therapeutic target across cancer and autoimmune disorders.