In order for MS to develop, there have to be antigens in the brain that elicit a T-cell mediated reaction. These antigens are unknown. They may be components of bacteria and viruses or myelin proteins. Antigens are presented to T4 cells by MHC class II molecules on macrophages and astrocytes. The perivascular lymphocytes in acute MS plaques are T-cells.
Cytokines damage the blood-brain barrier causing efflux of fluid, humoral factors, and cells.
Antibody-complement action can also cause demyelination. Macrophages ingest myelin debris. Demyelination causes loss of saltatory conduction. Linear conduction along demyelinated axons is slow because the internodal axon membrane has few ion channels. In addition, lack of insulation of axons allows impulses to disperse laterally to adjacent demyelinated axons.