Astrocytes are the most generally distributed cell type in mammalian brain.They maintain the microenvironment homeostasis of neurocytes, and regulate their metabolic process. Once the CNS were injured ,astrocytes will soon divide and proliferate to repair it in the form of glial scar. The immune function of astrocytes was gradually recognized except the structure supporting function. Once astrocytes are activated, they will secret series of chemokines, cytokines or inflammatory factors, activate the complement system and induce neuroimmune response, result in various functional disorders of nerve.  The activaton of complement , the expression of regulators and receptors in CNS play important roles in maintaining the microenvironment homeostasis and immune defence in brain. Neurons and gliocytes can synthesize various complement. When they are cultured in vitro,the constitutive expression of complement is low.After the stimulation of cytokines,the expression will upregulate. Complement normally exist in the brain ,they join in defending the invading antigen as an independent immune system. The abnormally activation and disbalanced regulation of complement ,which is the etiological factors of CNS disease,have important relationship with many CNS inflammation and uninflammation disease.Study indicated that over-expression and activation of complement accompany with the whole pathogenesis in Alzheimer's disease, the lesions mediated by complement is one of the important links of action mechanism of Aβtoxicity.  The activation of complement are well ordered cascade reaction, their excessive activation or out of regulation will bring injuries to self tissues. Regulators can interact with diverse complement protein and keep the activation and inhibition of complement in subtle balance. CD46、CD55、CD59 are three important regulators,they function at different stages of complement activation. Membrane cofactor protein,CD46, acts as the