Granzyme B is a serine protease that is produced mainly by cytotoxic T cells (CTL) and natural killer (NK) cells. It plays an important role in cytotoxic lymphocyte-mediated apoptosis, chronic inflammation, and impaired wound healing. Granzyme B is packaged in cytoplasmic granules that are exocytosed towards a bound target cell, which subsequently activates multiple protein substrates that induce apoptosis. Perforin facilitates the transfer of Granzyme B into the target cell. Most circulating CD56+, CD8-, NK cells, and approximately half of circulating CD8+ T cells coexpress both Granzyme A and B. The activation of CD8+ and CD4+ T lymphocytes induce substantial expression of Granzyme B, but not Granzyme A. Recently, Granzyme B has been found to be produced by other cell types such as CD34+ hematopoietic progenitor cells, keratinocytes, basophils, mast cells, plasmacytoid dendritic cells, B cells, and smooth muscle cells. Elevated levels of Granzyme B are also found in some kinds of autoimmune diseases, type 1 diabetes, and cardiovascular diseases.