The brain was long understood to be immunologically privileged — a compartment so sensitive and precisely regulated that it was largely sealed off from the body's immune system. The blood-brain barrier kept pathogens out. Immune cells were mostly excluded. The central nervous system operated, in this view, like a clean room: tightly controlled, minimally disturbed.

This picture was never quite right, and the last two decades of neuroscience have revised it substantially.

The brain has its own immune system. It is not the same as the body's peripheral immune system, and it doesn't work in exactly the same way — but it is real, it is active, and its dysregulation is increasingly understood to be at the center of conditions ranging from postoperative cognitive dysfunction and chemobrain to Alzheimer's disease, Parkinson's disease, and Long COVID neurological symptoms.

The key cell in this system is called the microglial cell — and understanding what microglia do, and what happens when they don't stop doing it, is one of the more important advances in neuroscience of the past twenty years.

What microglia are

Microglia are the brain's resident immune cells. They make up roughly 10–15% of all cells in the central nervous system and are distributed throughout the brain and spinal cord. Under a microscope, a resting microglial cell looks like a small body with long, branching extensions reaching out in all directions — a shape that reflects their primary function in the resting state: surveillance.

Microglia are constantly scanning their local environment. Those branches are sampling the surrounding tissue, detecting molecular signals that indicate damage, pathogens, debris, or dysfunction. When everything is normal, they are relatively quiet. When something is wrong, they respond.

Microglia are not the same cells as the macrophages that carry out immune surveillance in the rest of the body, though they are related. They arise from a different lineage — they colonize the developing brain in early embryogenesis, from the yolk sac, before the blood-brain barrier is established. From that point on, they largely self-renew in place. They are, in a meaningful sense, not just cells in the brain — they are brain cells.

What microglia do when they're working correctly

When microglia detect a problem — an invading pathogen, a dead or dying neuron, an abnormal protein accumulation — they activate. Activation is not a single state but a spectrum. Activated microglia can:

  • Phagocytose (engulf and destroy) pathogens, cellular debris, and abnormal proteins
  • Release cytokines — signaling molecules that recruit other immune cells and coordinate the inflammatory response
  • Prune synapses — a process that is essential in normal brain development and appears to continue in a modified form in the adult brain, shaping neural circuit function
  • Provide trophic support — under some conditions, microglia release factors that support neuronal survival and repair

None of this is inherently pathological. Microglial activation is essential for brain health. Infections, injuries, and accumulated debris all require a functioning microglial response. The problem is not activation — it is chronic, unresolved activation.

When microglia become the problem

Microglial activation is designed to be temporary. The threat is neutralized, the debris is cleared, the cytokine signals decline, and microglia return to their resting surveillance state.

Sometimes this resolution doesn't happen.

Chronically activated microglia shift into a state that researchers describe as neuroinflammatory. In this state, they are no longer primarily protective — they are actively damaging. Chronically activated microglia:

  • Sustain high levels of pro-inflammatory cytokines (including TNF-α, IL-1β, and IL-6)
  • Prune synapses excessively, disrupting neural circuits
  • Release reactive oxygen species that damage nearby neurons
  • Impair the function of astrocytes — another key support cell for neurons
  • Create an inflammatory environment that further activates other microglia, in a self-amplifying feedback loop

The downstream result is neuronal dysfunction and, over time, neuronal death. The cognitive consequences — impaired memory, reduced attention, slower processing, executive dysfunction — follow directly from disrupted neural circuit function.

The trigger problem: how peripheral inflammation reaches the brain

If microglia are the effectors of neuroinflammation, what sets them off in the first place? In the context of conditions like postoperative cognitive dysfunction (POCD) and chemotherapy-induced cognitive impairment (CICI), the trigger is outside the brain.

Surgery is a controlled physiological injury. Chemotherapy is a systemic toxic challenge. Both trigger substantial peripheral inflammatory responses — surges of cytokines that circulate through the bloodstream coordinating the body's response to the insult. Under normal circumstances, the blood-brain barrier filters most of this out.

But the blood-brain barrier is not perfectly impermeable, and it becomes more permeable under inflammatory conditions. Some cytokines — particularly TNF-α — can cross or signal through the barrier. Others trigger changes in the barrier's permeability itself. And in elderly patients, whose blood-brain barriers are less robust than those of younger people, this penetration is more significant.

Once peripheral cytokines enter the brain, they encounter microglia. Microglia respond to cytokine signals the same way they respond to any perceived threat: they activate. And in the context of an already-inflamed peripheral environment — surgery, chemotherapy, any sustained systemic insult — they can stay activated long after the peripheral trigger has resolved.

This is the mechanism behind POCD and CICI: not direct damage from anesthesia or from chemotherapy drugs crossing the blood-brain barrier, but a relay. Peripheral inflammation signals to brain-resident immune cells, which then generate a secondary neuroinflammatory response. That response disrupts neural function. The patient experiences it as cognitive impairment.

Why this matters for Alzheimer's and other neurodegenerative conditions

Microglial dysfunction is not unique to the post-surgical or post-chemotherapy context. It is increasingly recognized as a central feature of neurodegenerative disease.

In Alzheimer's disease, microglia are activated by the accumulation of amyloid-beta plaques — abnormal protein deposits that aggregate between neurons. Initially, this activation may be protective: microglia attempt to clear the plaques. Over time, as plaque burden increases and clearance fails, microglial activation becomes sustained and damaging. Genome-wide association studies have identified multiple Alzheimer's risk genes that are expressed specifically in microglia — a finding that has substantially shifted the field toward viewing Alzheimer's partly as an immune disorder of the brain, not just a protein aggregation disease.

In Parkinson's disease, activated microglia are found in abundance in the substantia nigra — the brain region where dopamine-producing neurons are lost — and their presence correlates with disease severity and progression.

The picture that emerges across these conditions is of microglia as a shared effector mechanism: when the brain encounters a chronic threat — whether it's a peripheral inflammatory insult, an abnormal protein accumulation, or a metabolic challenge — microglia activate, and if that activation is not resolved, it drives damage. The initial trigger differs. The downstream biology overlaps substantially.

This overlap has important implications for drug development. A therapeutic approach that can modulate microglial activation without suppressing it entirely — interrupting the damaging chronic state while preserving the protective acute response — would be relevant not just to POCD and CICI but to the full spectrum of neuroinflammatory and neurodegenerative conditions.

The cytokine at the center: TNF-α

Among the cytokines involved in microglial activation, one has received the most attention as a therapeutic target: TNF-α, or tumor necrosis factor alpha.

TNF-α is a pleiotropic cytokine — it plays multiple roles in immune function, cell survival, and cellular death pathways. In the context of neuroinflammation, it is both a product of activated microglia and a driver of further microglial activation, making it a central node in the self-amplifying inflammatory feedback loop.

TNF-α is also, crucially, a validated therapeutic target with a long clinical history. The class of anti-TNF therapies — used to treat rheumatoid arthritis, Crohn's disease, and other autoimmune conditions — is one of the most commercially successful in pharmaceutical history. The challenge in the neuroinflammation context is that most existing TNF-α inhibitors are large biological molecules (antibodies) that do not cross the blood-brain barrier. They can suppress TNF-α in the periphery but don't reach the site of neuroinflammation itself.

This is what makes brain-penetrant TNF-α inhibitors — small molecules that can cross into the central nervous system — a significant area of interest. If the damage is happening in the brain, the treatment needs to reach the brain.

The emerging field

Microglial biology has become one of the most active areas in neuroscience, and the therapeutic implications are beginning to reach clinical development. There is now a growing recognition that conditions previously understood as separate — surgical cognitive impairment, chemotherapy cognitive impairment, Alzheimer's, Long COVID neurological symptoms, traumatic brain injury, major depression with inflammatory features — may share a common neuroinflammatory mechanism at the level of microglial activation.

This does not mean they are the same disease or that a single treatment will address all of them. But it does mean that advances in understanding and treating neuroinflammation in one context may have broader relevance than the initial indication.

At Nulyn Science, we are pursuing NS-001 — a brain-penetrant inhibitor of the cytokines that drive microglial activation — initially in POCD and CICI. Our preclinical data demonstrates that NS-001 reduces microglial activation in animal models of both conditions. The Phase 2 trial in POCD planned for the second half of 2026 will be the first clinical test of whether this translates.

The biology, however, points toward a broader story. The same microglia that are disrupted by the inflammatory insult of surgery or chemotherapy are the ones implicated in the slow-burning inflammation of Alzheimer's disease. The mechanism is more specific than "brain inflammation." But it is also more generalizable than any single condition.

We are, in neuroscience terms, early. The field is moving fast.