Protein D (PD) is a highly conserved surface lipoprotein(M, 42 000) found in all Hae mophilus influenzae (Hi), including nontypeable Haemophilus influenzae (NTHi).PD is a virulence factor with glycerophosphodiesterase activity, which can lead to the release of phosphorylcholine from host epithelial cells, and plays an important role in the pathogenesis of respiratory tract infection caused by Hi.Pneumococcal polysaccharide conjugate vaccine using NTHi PD as a carrier protein can protect against acute otitis media caused by both pneumococci and NTHi.PD is not only a potential carrier protein candidate for polysaccharide conjugate vaccines but also the first NTHi antigen which can induce protective response in human.