Background:  Immune privilege of the central nervous system (CNS) is thought to be maintained by the tight endothelial junctions of the blood-brain-barrier (BBB) and the presence of an immunosuppressive microenvironment. In fact, the stablization of CNS internal environment depends on the long-term surveillance and protection of the innate immune system. Among them, microglia cells and complement system are two important defensive powers in CNS whose disordered metabolism will lead to microenvironment disbalance, inflammation outbreak,lots of inflammatory factors releasing and“bystander effecs”of normal tissue.  Alzheimer’s disease (AD) is a kind of neural degeneration disease with progressive memory decrease and irreversible cognitive damage. Its characteristical pathogenesis is the deposition of multiple extracellularβ-amyloid(Aβ), microglia cells and complement activation,cytokine releasing which lead to neurofibrillary tangles(NFTs) and neuron loss.This self attack by host defense on neurons is autotoxic and not autoimmune in nature. Inflammation plays an important role in it.  It is widely considered that microglia is the most important immune cell in CNS. It can not only show phagocytosis as mononuclear phagocytes but also can be involved in inflammatory response. It is confirmed that at the early stage of AD,activated microglia can phagocytize Aβ,but with the inflammation develops,microglia can be overactivated by plenty of Aβwith phagecytosis of Aβdecreasing, quantities of proinflammatory cytokines and oxyradical secretion, varied immunological molecular expression,which impares neural tissue and aggravates neural inflammation.  Complement system is one of the innate immune system. It can be activated to form C5b-9_((n)) membrane attack complex(MAC) by microorganism or inflammatory media. C5b-9_((n)) , as an multieffect immune factor, can insert into cell membrane making cell lysis. Recently, the new researches demonstrates that non-lethal dose MAC can deposit on many