BPI is part of a functionally related protein family that includes lipopolysaccharide binding protein, phospholipid transfer protein, and cholesteryl ester transfer protein, highlighting BPI′s importance in the broader context of immune defense mechanisms. BPI plays a crucial role in the immune response by acting as an antibacterial and endotoxin-neutralizing agent, primarily found in the granules of polymorphonuclear leukocytes, such as neutrophils. The human BPI gene, which spans 31.5 kb and is located on chromosome 20q11.23-q12, consists of 15 exons and encodes a 456 amino acid protein. BPI is strategically localized to the cell surface of epithelial cells, which serve as the first line of defense against bacterial invasion. Such localization enables BPI to block endotoxin-mediated signaling, thereby safeguarding mucosal surfaces from the harmful effects of gram-negative bacteria and their endotoxins.