Afferent and efferent immunological pathways of the brain. Anatomy, function and failure.

Immunological privilege appears to be a product of unique lymphatic drainage systems for the brain and receptor-mediated entry of inflammatory cells through the blood-brain barrier. Most organs of the body have well-defined lymphatic vessels that carry extracellular fluid, antigen presenting cells, lymphocytes, neoplastic cells and even bacteria to regional lymph nodes. The brain has no such conventional lymphatics, but has perivascular pathways that drain interstitial fluid (ISF) from brain parenchyma and cerebrospinal fluid (CSF) from the subarachnoid space to cervical lymph nodes. ISF and solutes drain along narrow, ∼100 nm-thick basement membranes within the walls of cerebral capillaries and arteries to cervical lymph nodes; this pathway does not allow traffic of lymphocytes or antigen presenting cells from brain to lymph nodes. Although CSF drains into blood through arachnoid villi, CSF also drains from the subarachnoid space through channels in the cribriform plate of the ethmoid bone into nasal lymphatics and thence to cervical lymph nodes. This pathway does allow the traffic of lymphocytes and antigen presenting cells from CSF to cervical lymph nodes. Efferent pathways by which lymphocytes enter the brain are regulated by selected integrins on lymphocytes and selective receptors on vascular endothelial cells. Here we review: (1) the structure and function of afferent lymphatic drainage of ISF and CSF, (2) mechanisms involved in the efferent pathways by which lymphocytes enter the brain and (3) the failure of lymphatic drainage of the brain parenchyma with age and the role of such failure in the pathogenesis of Alzheimer's disease.