The Phosphatidyl Serine detection kit provides a rapid and reliable method for the detection of apoptosis by flow cytometry. The method enables detection at the single-cell level (3) and also allows the distinction between apoptosis and necrosis. During the early stages of apoptosis, phosphatidyl serine (PS) becomes exposed on the outside of the cell membrane. In vivo, this is a signal to phagocytes to engulf the dying cell before it looses its plasma membrane integrity and releases inflammatory mediators into the surroundings. This early stage of apoptosis can be specifically detected by PS binding proteins, such as Annexin V. During the early stage of apoptosis, the cell membrane is intact and the cells exclude propidium iodide (PI). Later, during in the apoptotic process in vitro, the membrane becomes porous and PI becomes associated with DNA in the nucleus. The uptake of PI is an indication of necrosis. In a single cell suspension, these events can be analysed using the Phosphatidyl Serine detection kit, which contains the PS binding protein, Annexin V FITC or PE, plus calcium buffer and PI. (See table 1 below). The binding of Annexin V to phosphatidyl serine during early apoptosis is illustrated in Figure 1.

Figure 1. Flow cytometry analysis of peripheral blood lymphocytes labelled with Annexin V FITC and PI after overnight incubation with dexamethasone. Apoptotic cells exclude PI and express phosphatidyl serine which is detected by Annexin V FITC. Necrotic or dead cells are permeable for PI which associates with nuclear DNA and is visible as red fluorescence. The dot plot shows three populations: viable cells (lower left), apoptotic cells (lower right) and apoptotic cells which have lost plasma membrane integrity (upper right).