Mechanisms of disease: The blood-brain barrier

OBJECTIVE: The blood-brain barrier (BBB) is often perceived as a passive membrane. However, evidence has demonstrated that the BBB plays an active role in normal homeostasis and in certain disease processes. METHODS: Approximately 300 peer-reviewed publications that discussed normal or abnormal BBB function were reviewed. RESULTS: The role of the BBB and how it contributes to disorders of the central nervous system vary, depending on the specific disease process. CONCLUSION: In health and disease and extending to old age, endothelial cells, neurons, and glia constitute a neurovascular unit that regulates the BBB. Advances toward penetrating the BBB must account for both normal and abnormal functions of the neurovascular unit.