Pulmonary alveolar cells and tissues owe their defensive capacity partly to properties shared with components of other exposed surfaces and partly to properties developed locally to a unique degree. Among the various surfaces of the body the epithelium differs greatly, but the connective tissue remains much the same. It is the principal region where cellular and immunological defense measures of the body are enacted. Alveolar connective tissue forms only a very thin layer beneath an equally thin epithelium, and, being free from chronic inflammatory cells, normally contributes little to alveolar defense. This normally is entrusted to alveolar macrophages and to epithelial lining cells. Certain properties of these cells fit them for a defensive role. These are considered in the final pages of this survey.