The primary immune recognition is based on structures common among invading pathogens. Bacterial surface molecules, such as lipopolysaccharide (LPS) and peptidoglycan (PGN), are known to elicit immune reactions ranging from cytokine release to fever. Recently, a family of proteins called peptidoglycan recognition protein (PGRP) has been identified in mouse and human that binds to peptidoglycans expressed on Gram-positive bacteria. Peptidoglycan (PGN) is an essential cell wall component of virtually all bacteria and, thus, it is an excellent target for recognition by the eukaryotic innate immune system. The PGRPs (PGRP-L, PGRP-S, PGRP-Ia, and PGRP-Ib) define a new family of human pattern recognition molecules. PGRP-L is primarily expressed in the liver. Although liver is not considered a primary immune organ, liver participates in host defenses by producing acute phase proteins (by hepatocytes) in response to infections and by clearing microorganisms from blood. PGRP-S is present in neutrophils and inhibits growth of Gram-positive bacteria and, therefore, may function as a neutrophil antibacterial protein. However, PGRP-S may have another, as yet unidentified function because in humans it is expressed in the bone marrow 50-100 times higher than in neutrophils.