Members of the bactericidal/permeability-increasing protein family have antimicrobial properties and bind lipophilic substances, therefore targeting gram-negative bacteria. The bactericidal permeability increasing protein (BPI) is an antibacterial and endotoxin-neutralizing molecule that is abundant in the granules of polymorphonuclear leukocytes (neutrophil granules). Sharing structural and sequence homologies with BPI, BPIL1 (bactericidal/permeability-increasing protein-like 1) is a 458 amino acid secreted protein that contains the family’s common conserved feature of two cysteine residues that are critical for protein function. While BPIL1 is primarily expressed at low levels in tonsil tissue, it has been found to be upregulated in hypertrophic tonsils, suggesting that it may play a role in the pathogenesis of inflamed disease tissue.