At day 3, we found infants born at a low gestational age are more likely to have greater numbers of CD14 + mononuclear phagocytes in the airway but fewer of these cells are functionally polarized, suggesting increased recruitment of monocytes or a failure to mature these cells in the lung. These findings suggest airway macrophage phenotype may be an important determinant of clinical outcome in neonatal inflammatory lung disease. In Vinpocetine humans, macrophages are thought mostly to differentiate post-natally with very few macrophages populating the sterile environment of the normal human lung at birth. In vivo studies show circulating monocytes appear from 20 weeks gestation and macrophages are recoverable from the lung before birth, having matured under the influence of local factors including alveolar type 1 cells, bronchial epithelium, cytokines and surfactant. Acquisition of secondary lysosomes is dependent on breathing air, suggesting other maturation processes do not occur until birth. In rabbits, numbers of macrophages increase shortly before birth, continuing throughout the first month of life. Work using a murine lung ZINC00881524 explant model demonstrated not only that macrophages were present in the fetal lung but also that they were critical to lung immune responses. Macrophages containing apoptotic neutrophils have been visualized in BAL from preterm infants, supporting a functional role for these cells in the preterm infant lung. Our current data show that both preterm and term infants have populations of macrophages in the lung at birth, as determined both by light microscopy and by co-expression of CD14 with CD36 or HLADR, presentational molecules with known immunoregulatory functions. HLA-DR is an antigen presentation receptor expressed on mature airway macrophages. CD36 is a scavenger receptor expressed by both monocytes and macrophages, appearing at the peak of macrophage differentiation. As well as facilitating bacterial phagocytosis, CD36 expression is associated with a macrophage phenotype that is more likely to efferocytose, and thus confers both a functional and antiinflammatory phenotype upon the macrophage, and has been shown to play protective roles in inflammation.