Granzyme B, also known as GZMB, is the most prominent member of the granzyme family of cell death-inducing serine proteases expressed in the granules of cytotoxic T lymphocytes (CTLs) and NK cells. Granzyme B enters the target cells depending on another membrane-binding granule protein, perforin, results in the activation of effector caspases and mitochondrial depolarization through caspase-dependent and -independent pathways, and consequently induces rapid cell apoptosis. Over 3 substrates of GZMB have been identified including the key substrate caspase-3, ICAD, and Bid. GZMB is suggested to protect the host by lysing cells bearing on their surface 'nonself' antigens such as bacterial and viral infected-cells and tumor cells and accordingly plays an essential role in immunosurveillance.

Granzymes are key components of the immune response that play important roles in eliminating host cells infected by intracellular pathogens. Several granzymes are potent inducers of cell death. A total of eight granzymes (A-G and M) have been identified in the mouse, but only five are known in humans (A, B, H, M, and granzyme 3), and granzyme H appears to be specifically human. Human granzyme H is a neutral serine protease that is expressed predominantly in the lymphokine-activated killer (LAK)/natural?killer (NK) compartment of the immune system. In adenovirus-infected cells in which granzyme B (gzmB) and downstream apoptosis pathways are inhibited, granzyme H directly cleaves the adenovirus DNA-binding protein (DBP), a viral component required for viral DNA replication. This virus demonstrated that gzmH directly induces an important decay in viral DNA replication. Interestingly, gzmH also cleaves the adenovirus 1K assembly protein, a major inhibitor of gzmB, and relieves gzmB inhibition. Granzyme H has a very high amino acid identity (>9%) with many portions of the granzyme B sequence, particularly near the amino terminus of the molecule despite performing a distinct enzymic function.