The CNS is immunologically unique, with low or undetectable expression of HLA molecules. With a defective BBB, white blood cells are able to leak into CNS tissue. Such permeability of the BBB in MS is not understood. A macroscopic view of the demyelination process is identified in the MS plaque. This acute lesion consists of macrophages, lymphocytes, plasma cells, and demyelinated axons.
Macrophages are a key component in the physical demyelination process.
Through an unusual form of phagocytosis, the macrophage strips away the myelin lamellae. Macrophages also secrete proteasomes and complement that may also be involved. Although systemic immune attack was previously considered to be a primary cause of MS, the cells may also be involved. Specific genomic regions may harbor MS susceptibility genes. Linkage studies show that these candidate genes can produce significant genetic effects.