Acute apical periodontitis is an inflammatory condition of the periodontal ligament at the tooth apex, often caused by bacterial infection from the pulp. This inflammation can lead to a localized immune response, resulting in regional lymphadenopathy due to the drainage of infectious agents and inflammatory mediators into the regional lymph nodes. The swelling and tenderness of these lymph nodes are signs of the body's attempt to contain the infection. Recognizing regional lymphadenopathy is important for assessing the spread and severity of the infection and guiding appropriate treatment.

Reference: Oral Pathology, Neville et al., 4th Edition.