Inflammation of the dental pulp. Acute pulpitis is characterized by a constant throbbing pain in the affected tooth (often made more severe by reclining), no pain on biting unless the inflammation has spread beyond the confines of the pulp tissue, and the inability to obtain relief from the pain. The pain is associated with the increased pulpal pressure caused by the influx of inflammatory cells, increased vascularity (hyperaemia), and the inability of the pulp chamber to expand. Bacterial invasion may follow the inflammatory response. Chronic pulpitis may be the result of a persistent low-grade irritation or may follow acute pulpitis. It is characterized by a mild intermittent pain of varying intensity over a prolonged period which is difficult for the patient to localize. Pain is usually induced by thermal stimulation or sweet foods. There is a bacterial invasion with an increase in plasma cells and lymphocytes and an influx of macrophages. Chronic pulpitis can resolve itself, progress to acute pulpitis, or result in pulp necrosis following the accumulation of dead bacteria and necrotic debris. In an exposed pulp, the pulpal tissue may extrude through the communication into the oral cavity (hyperplastic pulpitis) to form a pulp polyp; this is more common in primary teeth.