Tubular myelin in host defense. Surfactant phospholipids that are secreted into the alveolar space form lattice-like arrays of intersecting membranes called tubular myelin (a). Tubular myelin formation is dependent on SP-A, which is visible as electron-dense structures at the corners of the lattice (30). (b)The images suggest a model in which the C-type lectin domains of SP-A are buried at the membrane intersections and the N-terminal domains extend toward the centers of individual squares. The association of SP-A with tubular myelin places the collectin in the first line of defense against alveolar pathogens, in a configuration that does not disrupt surface activity.