Multiple sclerosis (MS) is a chronic neurologic disease characterized in early phases by a cellular immune response and later by multiple areas of demyelination or plaques in the central nervous system (CNS) white matter. The clinical manifestations of the disease are highly variable, but probably are related to the extent of breakdown of the blood-brain barrier associated with inflammation in the acute phase, and with the extent of demyelination in the chronic phase. The initiating events of MS are not known, but current hypotheses include immune responses to an initiating viral infection and autoimmune responses to CNS myelin antigens. Both inflammatory and CNS resident cells contribute to the immunopathology of the disease. Chronic lesions are characterized by glial scarring and depletion of both oligodendrocytes and axons.