Yin and Yang: complement activation and regulation in Alzheimer's disease.

The spectrum of inflammatory diseases is nowadays considered to include diverse diseases of the central nervous system (CNS). Current evidence suggests that syndromes such as Alzheimer's disease (AD) have important inflammatory and immune components and may be amenable to treatment by anti-inflammatory and immunotherapeutic approaches. Compelling evidence has been reported that complement activation occurs in the brain with Alzheimer's disease, and that this contributes to the development of a local inflammatory state that is correlated with cognitive dysfunction. The complement system is a critical element of the innate immune system recognizing and killing, or targeting for destruction, otherwise pathogenic organisms. In addition to triggering the generation of a membranolytic complex, complement proteins interact with cell surface receptors to promote a local inflammatory response that contributes to the protection and healing of the host. Complement activation causes inflammation and cell damage, yet it is an essential component in trying to eliminate cell debris and potentially toxic protein aggregates. It is the balance of these seemingly competing events--the "Yin" and the "Yang"--that influences the ultimate state of neuronal function. Knowledge of the unique molecular interactions that occur in the development of Alzheimer's disease, the functional consequences of those interactions, and the proportional contribution of each element to this disorder, should facilitate the design of effective therapeutic strategies for this disease.