Granzymes are a family of serine proteases expressed by cytotoxic T lymphocytes and natural killer (NK) cells and are key components of immune responses to pathogens and transformed cells. Granzymes are synthesized as zymogens and are processed into mature enzymes by cleavage of a leader sequence. They are released by exocytosis in lysosome-like granules containing perforin，a membrane pore-forming protein. Granzyme A is the most abundant serine protease in cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. Granzyme A has a specific function in CTL and NK cells. It induces caspase-independent cell death when introduced into target cells by perforin. Granzyme B has the strongest apoptotic activity of all the granzymes as a result of its caspase-like ability to cleave substrates at aspartic acid residues thereby activating procaspases directly and cleaving downstream caspase substrates
