The immune system and bone.

T cells and B cells produce large amounts of cytokines which regulate bone resorption and bone formation. These factors play a critical role in the regulation of bone turnover in health and disease. In addition, immune cells of the bone marrow regulate bone homeostasis by cross-talking with bone marrow stromal cells and osteoblastic cells via cell surface molecules. These regulatory mechanisms are particularly relevant for postmenopausal osteoporosis and hyperparathyroidism, two common forms of bone loss caused primarily by an expansion of the osteoclastic pool only partially compensated by a stimulation of bone formation. This article describes the cytokines and immune factors that regulate bone cells, the immune cells relevant to bone, examines the connection between T cells and bone in health and disease, and reviews the evidence in favor of a link between T cells and the mechanism of action of estrogen and PTH in bone.