Canine Annexin V (also known as ANXA5, Annexin A5, placental anticoagulant protein I (PAP-I), lipocortin V, or anchorin CII) is a calcium-dependent phospholipid-binding protein belonging to the annexin family, which plays important roles in membrane organization, cellular signaling, inflammation, and regulation of apoptosis. In dogs (Canis lupus familiaris), Annexin V binds with high affinity to phosphatidylserine exposed on the outer leaflet of the plasma membrane during early stages of apoptosis or cellular damage in the presence of calcium ions. Because of this property, FITC-labeled canine Annexin V (Annexin V–FITC) is widely used in flow cytometry and fluorescence microscopy assays to detect and quantify apoptotic cells. When conjugated with fluorescein isothiocyanate (FITC), Annexin V allows fluorescent labeling of cells displaying externalized phosphatidylserine, enabling sensitive detection of early apoptosis, often combined with viability dyes such as propidium iodide (PI) to differentiate early apoptotic cells from late apoptotic or necrotic cells. In canine health and research, Annexin V–FITC assays are commonly used to study cell death mechanisms in infectious diseases, immune-mediated disorders, inflammatory conditions, and cancer, including lymphoma, osteosarcoma, and mammary tumors. It is also utilized to evaluate immune cell viability, drug-induced cytotoxicity, and reproductive cell health. As a biomarker of apoptosis and membrane remodeling, canine Annexin V supports veterinary and translational research into host–pathogen interactions, tumor biology, immune regulation, and therapeutic responses.