This chapter discusses developmental immunology, with an emphasis on the pulmonary system in the primates. It is well established that newborn infants have a limited ability to mount an effective immune response against microbes. As there are limited numbers of precursor or 'memory' lymphocytes within neonatal airways, it is likely that the infant is most dependent upon immune cell expansion in regional lymph nodes to respond to environmental exposure to antigens and infectious agents. The predominance of macrophages within airways of infants is likely to be representative of the first level of innate immunity established for the 'naive' lung. As the infant matures and comes in contact with environmental antigens, cells from the adaptive arm of the immune system contribute to the resident leukocyte population of the airways. Dendritic cells are the primary antigen-presenting cells of the lung and are critical for the immunosurveillance of inhaled antigens. By the 13th week of gestation, HLA-DR positive dendritic-like cells are observed in the human fetal lung. HLA-DR positive cells within the fetal lung are dispersed as single cells within the interstitium, do not stain for markers of macrophage/monocyte lineage, and increase in frequency with gestational age.
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