Neonatal lung defense mechanisms: a study of the alveolar macrophage system in neonatal rabbits.

Abnormal function of the alveolar macrophage system may explain the enhanced susceptibility to pulmonary infection in human neonates. This hypothesis was investigated by infecting 1- to 14-day-old rabbits with aerosols of Staphylococcus aureus and then measuring in situ rates of bacterial ingestion, inactivation, and destruction in the lapine lung. The inhaled staphylococci were killed within the lungs of 1-day-old rabbits at a significantly slower rate than that for 7- and 14-day-old rabbits (P less than 0.05). Much of this decrease was due to diminished rates of bacterial ingestion by alveolar macrophages of younger animals. Staphylococci were also killed and destroyed less rapidly within these macrophages, but these differences could not be tested for significance.