Inflammation in the nervous system is usually caused by white blood cells (WBCs), called lymphocytes (mostly CD4+ cells); monocytes (macrophages) from the blood stream usually cause inflammation in the nervous system. Cells and fluids in the blood are normally restricted from entering the nervous system by the blood-brain barrier. This is formed by endothelial cells lining the venules with “tight junctions” uniquely occurring in the brain and spinal cord. A second layer of “foot processes” from astrocytes (star-like cells) but-tresses this barrier.
In the process of inflammation, these WBCs eat holes through the lining of the smallest blood vessels (venules) and enter the nervous sys-tem. Lymphocytes and macrophages are not normally present in the nervous system. However, in some patients, a different type of immune reaction occurs where antibody is important in damaging myelin. In some, antibody may be the sole cause of myelin dam-age.
In another group of patients with a different variation of MS, antibody damage may lead to additional damage that another kind of lymphocyte (CD8+ cells) causes. In ordinary cases of MS, it is not yet clear how important CD8-mediated damage is.