Neutrophils from schizophrenic patients are initiating far more C4A production than healthy controls.
For generations, schizophrenia has resisted explanation, its origins scattered across genetics, environment, and the brain's own architecture. Now, Stanford researchers have found an unexpected messenger in the story: neutrophils, the immune system's most common foot soldiers, appear to produce and consume a protein called C4A at rates far exceeding those seen in healthy individuals — linking the body's inflammatory machinery to the synaptic losses long observed in schizophrenic brains. Published in May 2026, the finding does not yet complete the picture, but it places a new and testable piece at the center of one of medicine's most enduring puzzles.
- Schizophrenia affects one in a hundred people worldwide, yet its biological mechanisms have remained elusive enough that current treatments can only manage symptoms, not address causes.
- The discovery that neutrophils — ordinary immune cells, not brain cells — produce the C4A protein upends assumptions about where schizophrenia's pathology originates and who is responsible for it.
- Patients' neutrophils churn out far more C4A than healthy controls, yet paradoxically retain less of it, suggesting the protein is being rapidly activated and consumed somewhere in the body in a process not yet understood.
- The finding creates a potential shortcut around one of psychiatry's greatest obstacles: drugs targeting neutrophils could intervene in the disease without needing to cross the blood-brain barrier.
- A measurable blood marker for C4A activity could transform schizophrenia diagnosis from a clinical judgment call into an early, objective test — potentially catching risk before symptoms ever appear.
Neutrophils are the immune system's most abundant and expendable defenders — short-lived white blood cells that rush to sites of infection and often perish there. What Stanford Medicine researchers discovered this spring is that these cells are also, unexpectedly, factories for a protein called C4A. The finding, published in the Proceedings of the National Academy of Sciences, opens a new corridor into understanding schizophrenia, a disorder affecting roughly one in a hundred people globally and causing hallucinations, delusions, and deep cognitive impairment.
The connection had been hiding across years of separate observations. Genetic studies had already identified C4A as the strongest common genetic risk factor for schizophrenia. Patients tend to have elevated neutrophil counts. And clozapine — the most effective antipsychotic available — has a curious side effect: it depletes circulating neutrophils. Lead author Agnes Kalinowski, a clinical psychiatry professor at Stanford, recognized these scattered pieces and began asking whether they belonged to the same picture.
C4A is part of the complement system, an ancient immune toolkit. In the brain, it participates in synaptic pruning — the developmental process of eliminating excess neural connections. In schizophrenia, this pruning goes too far: the cerebral cortex ends up with roughly 30 percent fewer synapses than in healthy brains. The more copies of the C4A gene a person carries, the greater their risk — and symptom severity tracks with C4A levels in cerebrospinal fluid.
When Kalinowski's team examined blood samples, they confirmed that neutrophils do produce C4A — and that patients' neutrophils produce far more of it than those of healthy controls. Yet those same patient cells retain less of the protein, because it is being rapidly activated and consumed. A telltale activation fragment appeared at elevated levels in patients' blood plasma, pointing to an accelerated cycle of production and depletion. 'Something, somewhere, is using it up,' Kalinowski noted. 'We don't know exactly where that's happening, but we're hoping to find out.'
The implications are significant. Drugs targeting neutrophil activity could potentially interrupt schizophrenia's pathology without crossing the blood-brain barrier — a longstanding obstacle for psychiatric medicine. Blood tests measuring C4A activity might one day enable early diagnosis, before symptoms emerge. Kalinowski describes the work as assembling a jigsaw puzzle, piece by careful piece — and the neutrophil-C4A connection, she suggests, is one that helps the rest snap into place more quickly.
Neutrophils are the foot soldiers of your immune system—short-lived white blood cells that arrive first at sites of infection, engulf pathogens, and often die in the process. They make up roughly half of all circulating white blood cells in a healthy person. What Stanford Medicine researchers discovered this spring is that these cellular warriors are also factories for a protein called C4A, something nobody suspected they could produce. The finding, published May 11 in the Proceedings of the National Academy of Sciences, opens a new door into understanding schizophrenia—a disorder that affects roughly one in every hundred people worldwide, causing hallucinations, delusions, and profound cognitive impairment.
The connection between neutrophils and schizophrenia had been hiding in plain sight, scattered across years of separate observations. Large genetic studies had identified C4A as a major risk factor for the disease. Patients with schizophrenia tend to have elevated neutrophil counts in their blood. And clozapine, the most effective antipsychotic medication available, has a curious side effect: it depletes circulating neutrophils. Agnes Kalinowski, a clinical assistant professor of psychiatry at Stanford and the study's lead author, saw these puzzle pieces and began asking whether they might fit together. "Connecting such seemingly disparate relationships could give us a better understanding of schizophrenia and, eventually, better treatments and diagnostics for it," she said.
C4A belongs to the complement system, an ancient evolutionary toolkit of roughly fifty proteins that work in sequence to recognize and respond to inflammatory threats. When activated, complement proteins punch holes in bacterial membranes and trigger a cascade of immune responses. In the brain, however, C4A plays a different role entirely. It participates in synaptic pruning—the brain's process of eliminating excess connections between nerve cells. During development, the human brain initially creates an enormous number of synapses, potentially hundreds of trillions. Pruning ordinarily refines this excess into a more efficient network. But in schizophrenia, something goes wrong. The cerebral cortex, crucial for higher thinking, contains roughly 30 percent fewer synapses than in healthy brains and is measurably thinner. The number of C4A gene copies a person inherits—some carry two copies, others several—is the strongest common genetic risk factor for schizophrenia yet identified. Symptom severity correlates with C4A levels in patients' cerebrospinal fluid.
When Kalinowski and her colleagues examined gene-expression databases and tested blood samples from volunteers, they found that neutrophils do indeed manufacture C4A. More striking: neutrophils from schizophrenia patients produced far more of the protein than neutrophils from healthy controls. Yet paradoxically, those same patient neutrophils retained less of the protein itself. What was happening was that the C4A was being made in abundance but then consumed—used up somewhere in the body. The telltale marker of C4A activation, a small fragment called C4-ana, appeared at higher levels in schizophrenia patients' blood plasma, suggesting the protein was being activated and depleted at an accelerated rate. "Schizophrenia patients' neutrophils appear to be exhausting their initially plentiful C4A supplies," Kalinowski observed. "Something, somewhere, is using it up. We don't know exactly where that's happening, but we're hoping to find out."
The implications ripple outward. If neutrophils do play a direct role in driving schizophrenia's pathology, then drugs designed to block neutrophil activation might interrupt the disease process without needing to cross the blood-brain barrier—a major obstacle for most psychiatric medications. A simple blood test measuring neutrophil activity or C4A levels could potentially flag risk before symptoms emerge, transforming diagnosis from a clinical guessing game into something measurable and concrete. Current treatments for schizophrenia remain palliatives. Clozapine can be life-changing, reducing hallucinations and delusions, but it carries serious side effects including weight gain and increased heart attack risk. The heritability of schizophrenia is remarkably high—about 80 percent—yet environmental factors like early childhood stress, high fever, and adolescent marijuana use also contribute. Kalinowski frames the work as assembling a jigsaw puzzle. "We don't have a full explanation of schizophrenia yet, but we're putting together the jigsaw puzzle," she said. "Figuring out where each piece goes helps you snap the rest of them into place more quickly." The neutrophil-C4A connection is one piece. Where it leads—to new drugs, new diagnostics, new understanding of how the immune system shapes the mind—remains to be discovered.
Notable Quotes
Neutrophils are able to be little factories for C4A, and neutrophils from schizophrenic patients are initiating far more C4A production than healthy controls' neutrophils are.— Agnes Kalinowski, lead author, Stanford Medicine
Drugs that block neutrophil activation wouldn't need to penetrate the blood-brain barrier, because they'd be working in our peripheral bloodstream.— Agnes Kalinowski
The Hearth Conversation Another angle on the story
So neutrophils are making this protein C4A. But C4A was already known to be made in the liver. Why does it matter that neutrophils make it too?
Because neutrophils are everywhere in the bloodstream, and they're making far more of it in schizophrenia patients than in healthy people. The liver makes C4A at a baseline level. But if neutrophils are ramping up production in patients, that's a new source of the protein—and it's one that's somehow getting consumed or activated at an accelerated rate.
Consumed by what? Where is it going?
That's the mystery they're trying to solve. The C4A is being made in abundance but then used up. The evidence is in the activation markers—the fragments that appear when C4A gets activated. Those are elevated in schizophrenia patients' blood. So the protein is being made, activated, and depleted. But the exact location and mechanism? Still unknown.
And in the brain, C4A does something completely different—it prunes synapses. So are we saying immune activation in the blood is causing over-pruning in the brain?
That's the hypothesis they're exploring. C4A in the brain is linked to synaptic pruning, and schizophrenia brains have 30 percent fewer synapses than normal. If neutrophils are producing excess C4A that somehow reaches the brain, it could be driving excessive pruning. But they haven't proven that pathway yet.
What about clozapine? That depletes neutrophils and it's the best drug we have. Doesn't that suggest neutrophils are the problem?
Exactly. It's one of those hints that seemed disconnected until now. Clozapine works—it reduces hallucinations and delusions. One of its effects is lowering neutrophil counts. If the new research is right, maybe it works partly because it's dampening neutrophil activity, not just because of its direct effects on dopamine receptors in the brain.
So the next step is drugs that target neutrophils directly?
That's one possibility. And the beauty of it is those drugs wouldn't need to cross the blood-brain barrier. They'd work in the peripheral bloodstream. That's a huge advantage over current psychiatric medications, which have to penetrate the brain and often cause side effects.