Granzyme B is a serine protease most commonly found in the granules of cytotoxic lymphocytes (CTLs), natural killer cells (NK cells) and cytotoxic T cells. It is secreted by these cells along with the pore forming protein perforin to mediate apoptosis in target cells.

Granzyme B has also more recently found to be produced by a wide range of non-cytotoxic cells ranging from basophils and mast cells to smooth muscle cells. The secondary functions of granzyme B are also numerous. Granzyme B has shown to be involved in inducing inflammation by stimulating cytokine release and is also involved in extracellular matrix remodelling.

Elevated levels of granzyme B are also implicated in a number of autoimmune diseases, several skin diseases, and type 1 diabetes.

In humans, granzyme B is encoded by GZMB on chromosome 14q.11.2, which is 3.2kb long and consists of 5 exons. It is one of the most abundant granzymes of which there are 5 in humans and 10 in mice. Granzyme B is thought to have evolved from a granzyme H related precursor and is more effective at lower concentrations than the other granzymes.

The enzyme is initially in an inactive precursor zymogen form, with an additional amino terminal peptide sequence. This sequence can be cleaved by cathepsin C, removing 2 amino acids.Cathepsin H has also been reported to activate granzyme B.

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