Diagrammatic representation of the defense strategies of the lung. (a) The surface of the alveolar epithelium is “patrolled” by PMNs and macrophages, which phagocytose incoming planktonic bacteria quickly and easily. (b) The alveolar phagocytes are unable to engulf bacteria in matrix-enclosed biofilm fragments, even when these invaders are reacted with specific antibodies. (c) Biofilm fragments grow and burgeon in the colonized lung, and release occasional planktonic cells that react with antibodies and are phagocytosed. (d) The mature biofilm reaches a “standoff” with the immune system, and parts of the microbial community become calcified to form a long-term pulmonary nidus.