Acid-stable proteinase inhibitor with strong affinities for trypsin, chymotrypsin, elastase, and cathepsin G (PubMed:3533531, PubMed:3462719, PubMed:2039600, PubMed:2110563, PubMed:10702419, PubMed:24121345). Modulates the inflammatory and immune responses after bacterial infection, and after infection by the intracellular parasite L.major. Down-regulates responses to bacterial lipopolysaccharide (LPS) (By similarity). Plays a role in regulating the activation of NF-kappa-B and inflammatory responses (PubMed:10702419, PubMed:24352879). Has antimicrobial activity against mycobacteria, but not against salmonella. Contributes to normal resistance against infection by M.tuberculosis. Required for normal resistance to infection by L.major. Required for normal wound healing, probably by preventing tissue damage by limiting protease activity (By similarity). Together with ELANE, required for normal differentiation and proliferation of bone marrow myeloid cells (PubMed:24352879). Interacts with GRN; interaction protects progranulin from proteolysis. Detected in blood plasma (PubMed:24352879). Detected in bone marrow myeloid cells (PubMed:24352879). Detected in airway sputum (PubMed:2039600). Detected in parotid gland secretions (PubMed:3462719). Detected in seminal plasma (at protein level) (PubMed:3485543). Detected in uterus cervix (PubMed:3533531). Down-regulated in response to low ELANE activity. Up-regulated by ELANE treatment in bone marrow cells. The pathologies of several chronic and acute diseases of the respiratory tract involve an imbalance between the proteases of cells involved in inflammatory responses and the inhibitors of these proteases. The inflammation-mediated release of neutrophil elastase in the lungs of patients whose levels of active alpha-1-antiprotease are compromised by genetic background, cigarette smoking, air pollutants, or a combination of all three can result in severe lung damage and a decreased lifespan. The relatively small size of this protein, its lack of glycosylation and its stability make this protein a candidate for use as a therapeutic agent in diseases mediated by leukocyte elastase-antielastase imbalances.