Innate and adaptive immunity in neurodegenerative disease

Neurodegenerative diseases (NDs) are a group of neurological disorders characterized by the progressive loss of selected neurons. Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common NDs. Pathologically, NDs are characterized by progressive failure of neural interactions and aberrant protein fibril aggregation and deposition, which lead to neuron loss and cognitive and behavioral impairments. Great efforts have been made to delineate the underlying mechanism of NDs. However, the very first trigger of these disorders and the state of the illness are still vague. Existing therapeutic strategies can relieve symptoms but cannot cure these diseases. The human immune system is a complex and intricate network comprising various components that work together to protect the body against pathogens and maintain overall health. They can be broadly divided into two main types: innate immunity, the first line of defense against pathogens, which acts nonspecifically, and adaptive immunity, which follows a defense process that acts more specifically and is targeted. The significance of brain immunity in maintaining the homeostatic environment of the brain, and its direct implications in NDs, has increasingly come into focus. Some components of the immune system have beneficial regulatory effects, whereas others may have detrimental effects on neurons. The intricate interplay and underlying mechanisms remain an area of active research. This review focuses on the effects of both innate and adaptive immunity on AD and PD, offering a comprehensive understanding of the initiation and regulation of brain immunity, as well as the interplay between innate and adaptive immunity in influencing the progression of NDs.