Immune signaling and function in neurodegeneration.

Neurodegenerative diseases arise from interactions among pathogenic proteins, immune responses, and diverse environmental or age-related stressors that disrupt CNS homeostasis. CNS resident microglia detect self-derived danger signals through pattern recognition receptors, and their activation can promote clearance of aberrant proteins, including amyloid-β, tau, α-synuclein, and TAR DNA-binding protein 43. However, microglial activation may also drive maladaptive states that amplify neuroinflammation. Microglial transitions are further shaped by receptor-mediated signaling and antigen presentation pathways that integrate environmental cues with functional responses. Adaptive immune cells contribute additional layers of regulation, with CD8+ and CD4+ T cells exerting neuroprotective or neurotoxic effects depending on disease context, activation state, and antigen specificity. The identification of granzyme K-expressing CD8+ T cells in several neurodegenerative conditions highlights the growing recognition that distinct T cell subsets may have specialized roles in disease. Aging, repetitive head injury, and viral infection further alter microglial phenotypes, weaken barrier integrity, promote T cell recruitment, and prime the CNS for chronic inflammation. In this review, we synthesize current knowledge of innate and adaptive immune mechanisms in neurodegeneration, examine how external factors influence these responses, and consider how these insights may guide future therapeutic strategies.