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CD2, also known as the T-cell surface antigen CD2, is a transmembrane glycoprotein primarily expressed on the surface of T cells and natural killer （NK） cells. It belongs to the immunoglobulin superfamily and plays a crucial role in mediating cell adhesion and activation. CD2's principal ligand is CD58 （LFA-3）, which is expressed on antigen-presenting cells and other immune cells. The interaction between CD2 and CD58 facilitates T-cell activation and enhances the immune response. Structurally, CD2 consists of an extracellular domain responsible for ligand binding, a transmembrane domain anchoring it to the cell membrane, and an intracellular domain involved in signal transduction.