Perforin
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Perforin is a cytolytic protein found in the granules of CD8 T-cells and NK cells. Upon degranulation, perforin inserts itself into the target cell's plasma membrane, forming a pore.
Although purified perforin is sufficient to lyse cells at high doses, the biology of perforin itself does not explain the ability of CD8 T-cells and NK cells to induce apoptosis in target cells. This induction of apoptosis may require at least one other granule protein, granzyme B.
There have been mice generated that lack perforin. Studies of these mice led to the discovery of additional mechanisms by which immune cells kill their target - through Fas-FAS ligand interactions. This led to the discovery of additional Fas-like molecules. However, perforin continues to play an important part in CD8 T-cell and NK cell function.