A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Microglia, resident immune cells of the brain, respond to damage by morphing from resting (ramified) to activated (amoeboid). Activated microglia produce cytokines (TNFα, IL-6) and reactive oxygen species that can be neuroprotective or neurotoxic. Chronic neuroinflammation is implicated in neurodegeneration.
Image microglial morphology during activation. Measure cytokine production using multiplex assays.
Microglia are immune cells invading the brain—they're resident. Inflammation is always bad—appropriate inflammation is needed for repair.
From your study of glial cells, you know that the brain contains far more than just neurons — glial cells provide structural support, insulate axons, regulate the extracellular environment, and maintain the blood-brain barrier. Among these, microglia stand apart: they are the brain's resident immune cells, derived not from neural tissue but from yolk sac macrophage precursors that colonize the brain early in development and remain there for life. In the healthy brain, microglia exist in a "surveilling" state, extending and retracting long, branching processes that continuously sample the local environment for signs of damage, infection, or abnormal cellular debris.
When microglia detect a threat — a pathogen breaching the blood-brain barrier, a dying neuron, or protein aggregates associated with neurodegeneration — they undergo a dramatic transformation called activation. Their morphology shifts from highly branched (ramified) to compact and rounded (amoeboid), resembling the macrophages of the peripheral immune system you may have encountered in studying the innate immune response. Activated microglia migrate toward the injury site, phagocytose (engulf) debris and pathogens, and release a cocktail of signaling molecules including cytokines (TNF-alpha, interleukin-1 beta, interleukin-6), chemokines that recruit additional immune cells, and reactive oxygen species (ROS) that kill pathogens. This acute inflammatory response is genuinely protective: it clears damage, walls off infection, and initiates tissue repair.
The problem arises when inflammation fails to resolve. Chronic neuroinflammation — sustained microglial activation lasting weeks, months, or years — shifts the balance from protective to destructive. The same cytokines and ROS that kill pathogens in the short term damage healthy neurons and oligodendrocytes when produced continuously. TNF-alpha at chronically elevated levels promotes excitotoxicity by increasing glutamate release and impairing glutamate uptake by astrocytes. IL-1 beta disrupts long-term potentiation, impairing synaptic plasticity and memory. Reactive oxygen species damage DNA, proteins, and lipid membranes in surrounding neurons. This self-perpetuating cycle — neuronal damage triggers more microglial activation, which causes more damage — is now recognized as a central feature of neurodegenerative diseases including Alzheimer's, Parkinson's, and ALS.
Astrocytes, the other major glial population, participate in neuroinflammation as well. Activated microglia release signals that push astrocytes into a reactive state (sometimes called reactive astrogliosis), in which they can lose their normal supportive functions — glutamate buffering, potassium homeostasis, blood-brain barrier maintenance — and instead secrete additional inflammatory mediators. The interaction between microglia and astrocytes creates a feedforward loop that amplifies and sustains inflammation. Understanding neuroinflammation therefore requires seeing it not as a simple immune response but as a dialogue between cell types, where the outcome — repair or degeneration — depends on the intensity, duration, and molecular specificity of the inflammatory signals involved.