In addition to phospholipids, hydrophobic proteins are necessary for the structure and function of surfactant. These proteins are listed here:  Surfactant protein A (SP-A), the most abundant surfactant protein. SP-A is responsible for surfactant homeostasis (regulating synthesis and secretion of surfactant by type II alveolar cells). It also modulates immune responses to viruses, bacteria, and fungi.
FIGURE 19.15 • Electron micrograph of lung alveoli. This electron micrograph shows two alveolar spaces separated by an alveolar septum containing capillaries, some of which contain red blood cells. Note the areas of thin and thick portions of the alveolar septum. These are shown at a higher magnification in Figure 19.19. 5,800. Inset. Photomicrograph of an alveolus for comparison with the alveolar wall as seen in an electron micrograph. Arrows indicate alveolar capillaries containing red blood cells. 480.
 Surfactant protein B (SP-B), an important protein for the transformation of the lamellar body into the thin surface film of surfactant. SP-B is a critical surfactant-organizing protein responsible for adsorption and spreading of surfactant onto the surface of the alveolar epithelium.
 Surfactant protein C (SP-C), which represents only 1% of the total mass of surfactant protein. Along with SP-B, SP-C aids in orientation of DPPC within the surfactant and maintenance of the thin film layer within the alveoli.
 Surfactant protein D (SP-D), a primary protein involved in host defense. It binds to various microorganisms (e.g., Gram-negative bacteria) and to lymphocytes. SP-D participates in a local inﬂammatory response to acute lung injury and with SP-A modulates an allergic response to various inhaled antigens.
The alveolar septum is the site of the air–blood barrier.
The air–blood barrier refers to the cells and cell products across which gases must diffuse between the alveolar and capillary compartments. The thinnest air–blood barrier consists of a thin layer of surfactant, a type I epithelial cell and its basal lamina, and a capillary endothelial cell and its basal lamina. Often, these two basal laminae are fused (Fig. 19.18). Connective tissue cells and fibers that may be present between the two basal laminae widen the air–blood barrier. These two arrangements produce a thin portion and a thick portion of the barrier (Fig. 19.19). It is thought that most gas exchange occurs across the thin portion of the barrier. The thick portion is thought to be a site in which tissue fluid can accumulate and even cross into the alveolus. Lymphatic vessels in the connective tissue of the terminal bronchioles drain fluid that accumulates in the thick portion of the septum.
Alveolar macrophages remove inhaled particulate matter from the air spaces and red blood cells from the septum.
Alveolar macrophages are unusual in that they function both in the connective tissue of the septum and in the air
FIGURE 19.16 • Electron micrograph of a type II alveolar cell. The type II alveolar cell has a dome-shaped apical surface with a number of short microvilli at its periphery and a relatively smooth-contoured apical center. The lateral cell margins are overlain to a variable degree by the type I alveolar cells that are joined to the type II cell by occluding junctions. Both cell types rest on the basal lamina (BL). The secretory vesicles (G) in this specimen are largely dissolved, but their lamellar character is shown to advantage in
Figure 19.17b. 24,000.