Defective opsonization of Haemophilus influenzae by sera of elderly patients

The elderly are prone to respiratory infection but the role of disordered immunity is uncertain. Opsonization of bacteria is an important early host defence mechanism required for optimal phagocytosis. There are conflicting reports in the literature as to the ability of the elderly to perform this function and one reason may be the use of in vitro tests in which the 'target' microorganism is clinically unrealistic. We have tested the hypothesis that use of a more clinically relevant microorganism Haemophilus influenzae, which commonly infects the respiratory tract of such persons, may detect abnormalities of opsonization in the elderly. Using an in vitro luminol-enhanced chemiluminescence (CL) assay we have compared sera from 45 elderly subjects (age greater than 70 years without known immune deficiency or current infection) with sera from 12 young, healthy subjects, for their capacity to opsonize Staphylococcus aureus (Oxford strain) and H. influenzae (non-capsulated) expressed as percentage CL obtained using pooled normal human serum (PNHS). There was no significant difference (median) between the elderly (95%) and the young (108%) with regard to S. aureus but opsonization of H. influenzae was poor in the elderly (63%) compared with that in the younger group (87%; P = 0.002); the sera of 11 elderly subjects having an opsonic capacity less than 30% that of PNHS. We conclude that use of the more clinically relevant microorganism, H. influenzae, appears to differentiate a subgroup of the elderly with reduced serum opsonic capacity. It remains to be determined prospectively whether these individuals are in fact more prone to infection.