Sialic acid-binding immunoglobulin-like lectin H (Siglec-H) is a type I transmembrane receptor belonging to the Siglec family, characterized by its ability to recognize sialylated glycans. In mice, Siglec-H is selectively expressed on plasmacytoid dendritic cells (pDCs) and is widely used as a defining surface marker for this cell population. Unlike many inhibitory Siglecs, Siglec-H lacks classical ITIM motifs and instead associates with the adaptor protein DAP12 (TYROBP), enabling it to modulate intracellular signaling. Functionally, Siglec-H plays a role in regulating type I interferon production, endosomal Toll-like receptor (TLR7/9) responses, and antiviral immunity. It can act as a negative regulator of excessive interferon responses, thereby maintaining immune homeostasis. In mouse models, Siglec-H is implicated in viral infection, autoimmune diseases (e.g., lupus-like conditions), and immune tolerance. Due to its restricted expression and functional relevance, Siglec-H is a key target in pDC biology, innate immunity, and immunoregulation research.