In this issue of CHEST, Mackay et al provide us with information about the relationship between airway eosinophilia and neutrophilia, and both quantitative and functional assays of surfactant protein (SP) -D in normal healthy control subjects and in patients with mild asthma and severe asthma. The initial interest in surfactant focused on its ability to reduce surface tension in the lung. This feature of surfactant was historically best demonstrated in preterm neonates. Surfactant is enriched with a relatively unique phospholipid, dipalmitoylphosphatidylcholine, and with four surfactant-associated proteins, SP-A, SP-B, SP-C, and SP-D. SP-B and SP-C, together with dipalmitoylphosphatidylcholine, provide surface tension-lowering properties to the material. The more hydrophilic surfactant-associated proteins, SP-A and SP-D, are involved in innate immunity in lungs. SP-D is synthesized and secreted by alveolar type II cells and club cells, and it binds to surface glycoconjugates and oligosaccharides expressed by a wide variety of microorganisms and associated with the surface of various complex organic antigens.