Apical periodontitis can be caused by both exogenous and endogenous factors.  Exogenous factors include microbes and their toxins and noxious metabolic by-products, chemical agents, mechanical irritation, foreign bodies, and trauma.  Endogenous factors include the host’s metabolic products, such as urate and cholesterol crystals, as well as cytokines or other inflammatory mediators that activate osteoclasts. Occasionally, foamy macrophages and giant cells are seen, especially associated with cholesterol crystal deposits, which are products of disintegrated cell membranes, disintegrating erythrocytes of stagnant blood vessels within the lesion, lymphocytes, plasma cells, macrophages that die and disintegrate in chronic apical lesion.