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Figure 4. Pathogenesis of acute (a,b), chronic (c), and cystic (d,e) apical periodontitis (AP) lesions. The acute lesion may be primary (a) or secondary (b) and is characterized by the presence of a focus of neutrophils (PMNs). The major components of chronic lesions (c) are lymphocytes (Ly), plasma cells (Pc), and macrophages (Ma). Periapical cysts can be differentiated into true cysts (d), with completely enclosed lumina, and pocket cysts (e), with cavities open to the root canal. Arrows indicate the direction in which the lesions can change. (From Nair, 1998b.)
