Defensive Cells

Alveolar macrophages are migrating mononuclear cells present in the interstitium and lumenal surface of the alveoli [32]. These cells phagocytize (envelop and, when possible, enzymatically degrade) foreign substances, particles, or microorganisms in the alveoli, after which they remain in the alveolus or migrate to the mucociliary escalator or into lymph tissue. Upon activation, macrophages release a variety of enzymes and biologically active mediators [33,34] that may influence airway function. The synthesis and release of matrix metalloproteinases (MMPs) by activated alveolar macrophages can contribute to lung tissue remodeling [35,36].

Mast cells are located in the walls of the central and peripheral airways and may be found free the lumen of the airways [37]. Activation by antigen cross-bridging of surface antibodies elicits cellular degranulation of the mast cell and the release of biologically active preformed and newly generated mediators. Also released are proteases, including chymase and tryptase, which can modify airway function by degrading biologically active proteins and peptides [38]. In addition, tryptase activates protease-activated receptors (PARs), leading a variety of unanticipated biological actions, such as induction of airway smooth muscle proliferation [39,40]. Mast cells serve an important role in the response of the airways to challenge by antigens (or allergens).

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