Immune-mediated processes in neurodegeneration: where do we stand?

Neurodegeneration is a pathological condition that predominantly affects neurons. It represents a large spectrum of disorders with heterogeneous symptoms and distinct clinical features. In addition to the devastating effects it can have on the affected individual, it constitutes a heavy burden to the society in terms of health care costs. Although the exact cause of neurodegeneration is not known, there are plenty of evidences supporting the notion that the immune system is strongly associated with various forms of neurodegenerative diseases. Given the numerous functions of immune cells, a change in their expression can either be beneficial or deleterious to the host. A better understanding of the molecular and cellular processes in neurodegeneration is therefore needed. This could facilitate the development of new therapeutic targets and provide effective means to dampen the progression of neurodegenerative disorders. The overarching aim of this paper is to provide an overview of the roles that the innate and adaptive immune systems play in the central nervous system, and to discuss their beneficial or detrimental effects during neurodegeneration. This paper also critically examines the contribution of immune and inflammatory-mediated responses in the development of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), and amyloid lateral sclerosis by illustrating key findings from animal and human studies.