Phagocytes
Alveolar macrophages remove foreign material that escapes the mucociliary defense mechanism. Nonpathogenic
particles and microbes are handled by simple phagocytic activity and are removed in the mucus flow or by the lymphatic system. Pathogenic microorganisms are neutralized with the aid of secre-tions such as lysozyme, interferons, proteolytic enzymes and enzyme inhibitors, opsonins, lactoferrins, complement factors, oxygen radicals and free radical scavengers,
and specific immunoglobulins in the mucus. Neutrophils, with phagocytic activity and a potent array of pathogen-killing enzymes, are recruited from the blood into the alveolus and assist the macrophages in the phagocytic activity. In healthy pigs, the normal ratio between the cellular elements in the bronchoalveolar mucus is 70–90% alveolar macrophages, 5–18% lymphocytes, 4–12% neutrophils, and up to 5% eosinophilic granulocytes. The phagocytic cell system also comprises intravascular macrophages, which in the pig are particularly numerous in the pig lungs .