Macrophages are large complex single cells capable of moving about the lung and performing a multitude of important functions. In their defensive function, they kill and digest bacteria, degrade antigen, synthesize immunoregulatory substances such as interferon, chemotactic factors, and tumor-inhibiting factors. In their non-defense function they synthesize arachidonic acid metabolites, platelet and fibroblast activating factors, enzyme inhibitors and binding proteins.