Equine 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 key roles in membrane organization, cellular signaling, inflammation, and regulation of apoptosis. In horses (Equus caballus), Annexin V binds with high affinity to phosphatidylserine that becomes exposed on the outer leaflet of the plasma membrane during early apoptosis or cellular stress in the presence of calcium ions. Because of this property, FITC-labeled equine Annexin V (Annexin V–FITC) is widely used in flow cytometry and fluorescence microscopy assays to detect and quantify apoptotic cells. Conjugation with fluorescein isothiocyanate (FITC) enables fluorescent labeling of cells displaying phosphatidylserine, allowing sensitive identification of early apoptotic cells, often used alongside viability dyes such as propidium iodide (PI) to distinguish early apoptosis from late apoptosis or necrosis. In equine health and research, Annexin V–FITC assays are used to investigate cell death mechanisms in inflammatory conditions, infectious diseases, and cancer, as well as to evaluate immune cell viability and drug-induced cytotoxicity. It is also applied in equine reproductive biology to assess sperm membrane integrity and early embryonic cell viability in assisted reproductive technologies. As a biomarker of apoptosis and membrane remodeling, equine Annexin V supports studies of host–pathogen interactions, tissue injury, immune regulation, and reproductive cell health in veterinary and comparative biomedical research.