What neuroinflammation is
Neuroinflammation is inflammation that takes place within the central nervous system. The classical signs of inflammation in peripheral tissue — redness, swelling, heat, pain — do not translate directly to the brain, which is protected by the blood-brain barrier and has its own resident immune cells. In the central nervous system, inflammation is defined by cellular and molecular changes: activation of microglia and astrocytes, production of cytokines and chemokines, and infiltration of peripheral immune cells when barriers are disrupted.
A 2014 review in Nature Neuroscience by Heneka and colleagues described neuroinflammation as a normal protective response to acute injury that can become pathological when sustained. The same biology that helps clear pathogens or damaged cells can, when chronic, contribute to neuronal dysfunction. This dual nature is the central theme of two decades of neuroinflammation research.
The field has expanded dramatically with the development of imaging techniques, biofluid biomarkers, and single-cell sequencing methods that allow researchers to characterize neuroinflammatory states in living patients rather than only at autopsy.
Microglia — the brain's resident immune cells
Microglia are the resident macrophage-lineage cells of the central nervous system. They originate from yolk-sac precursors during embryonic development and self-renew within the brain throughout life. In a resting state they extend dynamic processes that survey the brain microenvironment continuously.
When microglia encounter pathogens, damaged cells, or other signals, they shift to an activated state. Researchers historically described "M1" (pro-inflammatory) and "M2" (anti-inflammatory) microglia, though single-cell sequencing studies including a 2019 paper by Hammond and colleagues in Immunity showed that microglial states exist on a spectrum with many distinct subpopulations rather than a clean binary.
Sustained microglial activation has been documented in post-mortem brain tissue from patients with various neuroinflammatory and neurodegenerative conditions. A 2022 paper in Brain by Yang and colleagues, examining brain tissue from patients who died with severe COVID-19, described changes in microglia and choroid plexus that were not present in matched controls.
Cytokine signaling in nervous tissue
Cytokines are small signaling proteins. In the brain, key inflammatory cytokines studied in neuroinflammation research include interleukin-1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), and interleukin-6 (IL-6). Chemokines such as CCL2 recruit peripheral immune cells when barrier function is compromised.
Cytokine signaling in the brain is not identical to cytokine signaling elsewhere. The blood-brain barrier limits which molecules can cross. Some peripheral inflammation signals reach the brain through indirect routes — the vagus nerve, circumventricular organs, or compromised barrier regions. A 2017 paper in Nature Reviews Neuroscience by Dantzer summarized how peripheral inflammation produces "sickness behavior" in animal models through these signaling routes.
This is part of why post-viral or post-infectious cognitive and fatigue symptoms have a biologically plausible substrate even when the original pathogen is no longer detectable: persistent peripheral inflammation, or compromised barrier function, can sustain central effects.
How PET imaging tracks it
PET (positron emission tomography) with specialized tracers allows researchers to visualize activated microglia in living brains. The most common class of tracers binds the 18-kDa translocator protein (TSPO), which is upregulated on activated microglia and astrocytes. First-generation TSPO tracers (such as [11C]PK11195) have largely been replaced by second-generation tracers ([11C]PBR28, [18F]DPA-714) with better sensitivity.
A 2014 paper in Brain by Albrecht and colleagues used TSPO-PET to demonstrate elevated microglial activation in patients with chronic low back pain. A 2019 paper in Brain by the same group documented similar findings in fibromyalgia. A 2023 paper in JAMA Network Open by VanElzakker and colleagues used TSPO-PET to examine neuroinflammation in long-COVID patients with cognitive symptoms.
Important caveats: TSPO-PET findings depend on genetic polymorphisms in the TSPO receptor (high vs low binders), require careful kinetic modeling, and remain primarily research methods. Routine TSPO-PET is not part of standard clinical workup for cognitive symptoms.
CSF and blood-based markers
Cerebrospinal fluid analysis allows direct sampling of the central nervous system milieu. Key markers researchers use include:
- Cytokines (IL-6, IL-8, TNF-α, IFN-γ) measured in CSF can be elevated in neuroinflammatory conditions.
- Neurofilament light chain (NfL) is released when neurons are damaged. Now measurable in blood with sensitive assays. Used in MS, ALS, and emerging in long-COVID research.
- Glial fibrillary acidic protein (GFAP) reflects astrocyte activity. Elevated in many neuroinflammatory states.
- Soluble TREM2 is a marker of microglial activity studied in Alzheimer's research.
Blood-based biomarkers for brain inflammation and injury have advanced rapidly. NfL, GFAP, and tau measured with ultrasensitive platforms (such as Simoa) can detect signals previously requiring CSF sampling. A 2023 paper in Nature Medicine by Wang and colleagues documented elevated NfL in subsets of long-COVID patients with cognitive symptoms.
Conditions where neuroinflammation is studied
Neuroinflammation has been studied across a wide range of neurological conditions. A non-exhaustive list from the research literature:
- Multiple sclerosis — the prototype neuroinflammatory disease, with decades of imaging, CSF, and immune research.
- Post-viral fatigue and long COVID — an active area since 2020.
- Alzheimer's disease — microglial dysfunction is now considered a core feature in research models, not just a downstream effect.
- Parkinson's disease — activated microglia documented in substantia nigra in research studies.
- Major depression — subsets of patients show elevated inflammatory markers; "inflammation depression" is an active research topic.
- Chronic pain conditions — TSPO-PET studies show microglial activation in fibromyalgia and chronic low back pain research populations.
- Encephalitis (various causes) — the acute neuroinflammatory disorder by definition.
Linking neuroinflammation across these conditions does not mean they share a single mechanism. Different conditions have different cellular, molecular, and clinical pictures. The shared theme is that the central nervous system can mount inflammatory responses that affect function.
Neuroinflammation in long-COVID research
Yang et al., Nature
Single-cell sequencing of post-mortem brain tissue from COVID-19 patients revealed dysregulation of choroid plexus cells and microglia. The findings supported a neuroinflammatory contribution to neurologic symptoms in severe COVID-19. PubMed ↗
Greene et al., Cell
Dynamic contrast-enhanced MRI documented blood-brain-barrier disruption in long-COVID patients with cognitive symptoms but not in matched controls. The study provided objective imaging evidence consistent with a neuroinflammatory substrate. PubMed ↗
Douaud et al., Nature
Pre- and post-infection MRI scans from UK Biobank participants documented measurable changes in brain structure after even mild COVID-19, including in olfactory- and memory-associated regions. The changes were small but statistically significant. PubMed ↗
VanElzakker et al., research preprint and publication
Pilot TSPO-PET imaging in long-COVID patients with cognitive symptoms found patterns suggestive of microglial activation in subsets of patients. Sample sizes were small; replication and larger cohorts are needed. PubMed ↗
Wang et al., Nature Medicine
Blood-based neurofilament light chain was elevated in subsets of long-COVID patients with cognitive symptoms. Findings were consistent with low-level neuronal injury or stress in a subpopulation. PubMed ↗
What this does not mean
- This is not evidence that everyone with cognitive symptoms has neuroinflammation. Many causes exist.
- This is not a recommendation for any "anti-neuroinflammatory" supplement. No supplement has been shown in robust trials to reduce neuroinflammation in patients with post-viral conditions.
- This is not a substitute for clinical evaluation by a licensed clinician.
- This is not evidence of permanent brain damage. Most patients improve over time.
General practices the research community discusses
When researchers discuss general lifestyle factors relevant to brain health and inflammation, several themes appear consistently. These are framed as general wellness, not treatment:
- Sleep adequacy. Sleep is when the glymphatic system appears to be most active in research models. See our article on sleep architecture and cellular repair.
- Regular physical activity within tolerance. Moderate exercise is associated with lower neuroinflammatory markers in animal research and observational human studies.
- Mediterranean-style diet, omega-3 intake. Associated with lower systemic inflammation in observational research. See our diet overview.
- Stress management. Chronic stress is associated with elevated peripheral and central inflammatory markers in animal models and human research.
- Avoiding tobacco, excess alcohol. Both associated with elevated inflammatory markers across populations.
- Gut health. See microbiome and immune recovery for the research-community framing.
None of these is presented as a treatment for any specific condition. Decisions about evaluation, testing, or interventions belong with your licensed healthcare provider.
Authoritative sources to read directly
- NIH RECOVER Initiative
- NIH NINDS — neurological disorders
- CDC Long COVID
- Mayo Clinic: Long-term effects
- Cleveland Clinic: Long COVID
- American Academy of Neurology