The monoclonal antibody 31 recognizes secretory leukocyte proteinase inhibitor (SLPI).SLPI was identified as an alarm reactant and expression is induced by inflammatory factorslike LPS, IL1β, TNFα and neutrophils elastase. SLPI is a relatively small basic antiproteaseof 11.7 kDa and is a cationic non-glycosylated protein consisting of 107amino acids. SLPIhas a high affinity for the neutrophil serine proteinases, elastase and cathepsin G.Orthologues of SLPI have been found in mice, rate, pigs and sheep. It consists of two highlysimilar WAP (‘whey acid protein’) /four-disulphide core domains. SLPI contain 16 cysteineresidues which assemble into eight disulphide bridges (four in each WAP domain). SLPI isconstitutively expressed at many mucosal surfaces and is produced by a variety of epithelialcells, including respiratory, intestinal and amniotic epithelia. Expression is also detected inmast cells, neutrophils and macrophages. Expression of SLPI gene is significantlyincreased by progesterone and by the pro-inflammatory cytokines TNF-α and IL1-β.Although SLPI has been shown to inhibit a spectrum of proteases (including HNE,cathepsin G, trypsin, chymotrypsin and chymase), its main action in this regard is likely tobe the inhibition of elastase, as indicated by its low dissociation constant and favourablekinetics of inhibition for this enzyme. SLPI has been described in several body fluids likeseminal fluid, bronchial fluids, cervical fluids and saliva. It has been found to beantibacterial, antifungal, anti-retroviral, and to have an important role in mucosal defence.SLPI might also facilitate tumor spread, contributing to wound healing, is elevated in sepsisand levels seem to correlate with oral candidiasis in HIV-1 positive patients. The reactivity ofthe antibody 31 with isolated domains of SLPI was evaluated using domains obtained bycleavage using partial acidic hydrolysis. Therefore, monoclonal antibody 31 recognizes alsoother SLPI cleavage products.