Immune System & the CNS
The immune system is comprised of cellular and humoral components. Collectively, the immune system participates in tissue maintenance, restoration, and defense against infectious agents such as bacteria and viruses. The immune system has two important arms, the innate arm mediated by macrophages and natural killer cells, and the adaptive arm mediated by T-cells and B-cells. Innate immunity is generally non-specific, providing the initial response to tissue damage or infection. Adaptive immunity develops in response to specific antigens resulting in both humoral immunity (mediated by B cells) and cell-mediated immunity (mediated by T-cells). B cells recognize antigens and develop into antibody-secreting cells. In cell mediated immunity, antigen presenting cells (APC) help sensitize T helper cells, which then secrete cytokines that regulate the activity of other immune cells. Cytolytic T lymphocytes recognize antigens on target-cells and destroy these cells.

The Immune System in the CNS - Immune Privilege
The CNS of vertebrate animals is uniquely protected from toxins, invading pathogens, inflammatory cells and macromolecules through a protective mechanism called the "blood-brain barrier". It is composed of tight junctions at capillaries within the CNS and provides a protective physical barricade. Existence of the blood-brain barrier has in part precluded the need for an active immune system within the CNS. Thus, the immune system operates differently in the CNS than in the rest of the body. This has made possible for humans and other mammals to develop "Immune-Privilege", a mechanism that preserves the intricate and precise circuits of the CNS against inadvertent damage by immune cells. In the CNS, macrophages - are scarce and inactive relative to their counterparts in the rest of the body. Microglial cells, which may be regarded as CNS-resident macrophages, also function at a relatively slow pace. Consequently, rejection of grafts implanted in CNS tissue is very slow or absent, and CNS response to trauma and degeneration is atypical. While once thought to be the result of "immune ignorance", it later became apparent that Immune Privilege is achieved by suppression of the immune system via a biochemical mechanism (the immune-brain barrier) that is distinct from the blood-brain barrier.