Purity ~95% (SDS-PAGE). Bactericidal/permeability increasing protein (BPI) is a 456 residue protein which is part of the innate immune system. BPI was initially identified in neutrophils, but is found in other tissues including the epithelial lining of mucus membranes. It is an endogenous antibiotic protein with potent killing activity against Gram-negative bacteria. It binds to compounds called lipopolysaccharides produced by Gram-negative bacteria. Lipolysaccharides are potent activators of the immune system; however BPI at certain concentrations can prevent this activation. Bacterial/Permeability-Increasing Protein (BPI) is present in the azurophilic granules of polymorphonuclear leukocytes (PMN). BPI is toxic only toward Gram-negative bacteria. This specificity is attributable to the strong attraction of BPI for the lipopolysaccharides (LPS) in the bacterial envelope. BPI is also an important antigen for anti-neutrophil cytoplasmic autoantibodies (ANCA) in vasculitis. Source: Human