Complement mediator systems in Alzheimer’s disease

The brain is “immunologically privileged” in the sense that it cannot generate primary antigen-specific humoral and cellular immune responses. This is because of the integrity of the blood brain barrier, which impedes (but does not entirely prevent) entry of naive T lymphocytes. Furthermore, brain microglia and astrocytes possess only a limited ability to process and present antigens. It is perhaps not surprising, therefore, that the brain is protected from injury due to infection, trauma, or locally damaging processes by the innate or intrinsic immune system, which includes the actions of phagocytic cells, the complement system, cytokines, chemokines, and other cell-derived soluble mediators.