Upper respiratory tract (nose, oropharynx, larynx) |
Mechanical |
Nasal hairs and sneezing |
Nasal, oropharyngal and sinusal ciliated epithelium |
Saliva, mucus |
Vocal cords |
Innate immunity |
Complement |
Proteases |
Lactoferrin |
Acquired humoral immunity |
Secretory immunoglobulin (Ig)A and IgM |
Lower respiratory tract (tracheobronchial tree) |
Mechanical |
Mucociliary clearance |
Cough and impaction on bronchial branching |
Acquired cellular immunity |
Bronchial-associated lymphoid tissue (BALT) |
Humoral immunity |
Secretory IgA and IgM |
Lung parenchyma (alveoli and lung interstitium) |
Surfactant products (SP-A, SP-B, SP-D) |
Phagocytic cellular mechanisms |
Resident alveolar macrophages |
Phagocytosis |
Oxygen and nitrogen metabolites |
Lysozyme, acid hydrolases |
Recruited polymorphonuclear neutrophils (from pulmonary microvessels) |
Phagocytosis |
Oxygen and nitrogen metabolites |
Lactoferrin, defensins (human neutrophil peptides 1–4) |
Bacterial/permeability increasing protein |
Cationic antimicrobial protein (CAP/azurocidin) |