Periodontal disease infection with oral biofilm microorganisms initiates host immune response and signs of periodontitis, including bone resorption. This review delineates some mechanisms underlying the host immune response in periodontal infection and alveolar bone resorption. Activated T lymphocytes have been historically implicated in experimental periodontal bone resorption. An experimental rat adoptive transfer/gingival challenge periodontal disease model has been demonstrated to require antigen-specific T lymphocytes and gingival instillation of antigen and LPS for bone resorption. Interference with costimulatory interactions between T cells and antigen-presenting cells abrogated bone resorption, further emphasizing the significance of immune response in periodontal disease. Receptor activator of nuclear factor kappaB ligand (RANKL), a critical osteoclast differentiation factor, is expressed on T lymphocytes in human periodontal disease as determined by immunohistochemical and confocal microscopic analyses. Interference with RANKL by systemic administration of osteoprotegerin (OPG), the decoy receptor for (and inhibitor of) RANKL, resulted in abrogation of periodontal bone resorption in the rat model. This finding indicated that T cell-mediated bone resorption is RANKL-dependent. In additional experiments, treatment of T cell-transferred rats with kaliotoxin (a scorpion venom potassium channel inhibitor) resulted in decreases in T-cell RANKL expression, diminished induction of RANKL-dependent osteoclastogenesis, and abrogation of bone resorption, implicating an important role of immune response/RANKL expression in osteoclastogenesis/bone resorption. In other experiments, adoptive transfer of antigen-specific, RANKL-expressing B cells, and infection with the antigen-bearing Actinobaccillus actinomycetemcomitans gave rise to periodontal bone resorption, indicating that B cells also have the capacity to mediate bone resorption, probably via RANKL expression. In humans, prominent T lymphocytes have been identified in periodontal disease, and diseased tissues showed elevated RANKL mRNA expression, as well as decreased OPG mRNA expression. Mononuclear cells from periodontal lesions involving T cells and B cells of patients induced osteoclastogenesis in vitro. In summary, a biofilm interface initiates immune cell infiltration, stimulating osteoclastogenesis/bone resorption in periodontal disease. This resorption can be ameliorated by inhibition of RANKL activity or by diminishing immune cell stimulation. These two procedures, if localized, have the potential to lead to the prevention or therapeutic management of periodontal disease and therefore require further study.