Neutrophil serine protease inhibitor that plays an essential role in the regulation of the innate immune response, inflammation and cellular homeostasis (PubMed:17664292, PubMed:21683252, PubMed:21248149, PubMed:30692621). Acts primarily to protect the cell from proteases released in the cytoplasm during stress or infection (PubMed:17664292). These proteases are important in killing microbes but when released from granules, these potent enzymes also destroy host proteins and contribute to mortality. Regulates the activity of the neutrophil proteases elastase, cathepsin G, proteinase-3, chymase, chymotrypsin, and kallikrein-3. Acts also as a potent intracellular inhibitor of granzyme H (PubMed:12189154). During inflammation, limits the activity of inflammatory caspases CASP1 and CASP4 by suppressing their caspase-recruitment domain (CARD) oligomerization and enzymatic activation (PubMed:30692621). In addition, promotes the proliferation of beta-cells when secreted (PubMed:26701651). Monomer. Interacts (via C-terminus) with CASP1 and CASP4 (via CARD domain); these interactions regulate the activity of inflammatory caspases. Ubiquitous with higher expression in pancreas, spleen and bone marrow. Mutant mice do not differ from wild-type in growth, litter size or life span when maintained in a specific pathogen-free environement (PubMed:17664292). However, they have increased mortality in association with late-onset failed bacterial clearance, and specifically increased neutrophil death (PubMed:17664292, PubMed:30692621). Mutant mice also show a severe defect in the bone marrow reserve of mature neutrophils demonstrating a key role for in cellular homeostasis (PubMed:21683252, PubMed:21248149). In addition, Serpinb1a-deficiency leads to maladaptive beta-cell proliferation in insulin-resistant states (PubMed:26701651). Belongs to the serpin family. Ov-serpin subfamily.