The CD66 antigens are a group of glycoproteins belonging to the carcinoembryonic antigen (CEA) family. Several members (e.g., CD66a, CD66b, CD66c, CD66d) are predominantly and abundantly expressed on the surface of granulocytes (especially neutrophils) and their precursors. These molecules play roles in cell adhesion, activation, and intracellular signaling. During an inflammatory response, activated neutrophils rapidly migrate to the infection site, carrying their surface CD66 antigens with them. This makes CD66 an excellent target for imaging agents that can non-invasively localize hidden infections based on neutrophil accumulation.