Trypan Blue is commonly used in cell biology as a vital stain to assess cell viability. It is excluded by live cells with intact cell membranes but penetrates dead or damaged cells, staining them blue. This property makes it a standard tool in cell counting protocols, especially when using a hemocytometer to determine the number of viable versus non-viable cells in a sample. It is widely used in laboratory research, particularly in tissue culture, immunology, and vaccine production, where accurate cell counts and viability measurements are critical. Trypan Blue is also used in ophthalmic surgery, especially during cataract procedures, to stain the anterior capsule of the lens, improving visibility and facilitating safer surgical maneuvers in cases with poor red reflex. Additionally, it serves as a marker for blood–retinal barrier disruption in eye research and has been used in studies involving tumor visualization and surgical guidance due to its selective accumulation in necrotic tissues.