If you’ve ever had the flu and felt brain fog, flat mood, low energy, or disrupted sleep then you have felt your immune system interacting and “talking to” your brain. That dialogue may be subtle, but it’s powerful. Illness (especially complex chronic illness) has a profound impact on the central nervous system.

At the center of this conversation are cytokines. These are small proteins released by immune cells that help coordinate inflammation, antiviral defenses, and tissue repair. During short-lived illness, these signals are critical and help us recover. But when immune activation becomes long-term, as in conditions like Long COVID or chronic Lyme disease, the same cytokines can impact the brain, and affect how neural circuits work.

This crosstalk between the immune and nervous system continues to dynamically evolve in current neuroscience, with important implications for mood, cognition, sleep, and motivation. Additionally, it has encouraged more novel approaches in psychopharmacology for mood disorders such as OCD (obsessive compulsive disorder). Research continues to highlight how inflammation can be a major driver of neurological symptoms and how targeting inflammation can positively impact mood.

Key Inflammation and Neurotransmitter Pathways

Hijacked Tryptophan 

Inflammation can push the amino acid tryptophan (normally a precursor to serotonin) into a different pathway called the kynurenine pathway. This shift influences glutamate signaling. When this happens, people often experience symptoms mirroring low serotonin, like fatigue and mood changes. But the symptoms may not be as simple as low serotonin.

Dopamine and BH₄

Chronic inflammation increases oxidative stress, which can reduce availability of cofactors, like tetrahydrobiopterin (BH₄), which is essential for dopamine synthesis. Dopamine is critical for motivation, energy, and movement control. Reduced dopamine function can show up as fatigue, slow thinking, or difficulty initiating tasks. 

Glutamate and GABA Balance

Inflammatory signals can also tilt the balance between excitatory glutamate and inhibitory GABA. Persistent imbalance here may influence sleep, anxiety, and mood.

Metabolites of Key Neurotransmitters in Inflammatory States

Chronic inflammation exerts significant effects on the monoamine neurotransmitter system, often increasing the turnover of serotonin and dopamine which are critical to mood and behavior. Inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and others can modify monoamine metabolism by influencing synthesis, reuptake, and enzymatic degradation. As a result, the downstream metabolites of these neurotransmitters, 5-hydroxyindoleacetic acid (5-HIAA)for serotonin and 3,4-dihydroxyphenylacetic acid (DOPAC)for dopamine, are frequently found at elevated levels in experimental and clinical settings of chronic inflammation.
Proinflammatory cytokines can enhance serotonin transporter activity and monoamine oxidase (MAO)-mediated metabolism, thus increasing the conversion of serotonin to 5-HIAA (Miller & Raison, 2013). Likewise, inflammation-induced activation of monoamine pathways is thought to push dopamine metabolism toward its catabolic products, including DOPAC, as part of a broader shift in monoaminergic signaling under chronic immune challenge (Souza et al., 2025).

Several clinical studies also support a link between chronic low-grade inflammation and higher monoamine metabolite levels. For example, elevated serum 5-HIAA correlates with high-sensitivity C-reactive protein (hs-CRP), a clinical marker of chronic inflammation, in conditions such as metabolic syndrome (Afarideh et al., 2015). Together, these findings may suggest that chronic inflammation tends to promote monoamine metabolism toward increased production of metabolites like 5-HIAA and DOPAC, reflecting altered neurotransmitter dynamics in the presence of sustained immune activation.

How Do Peripheral Immune Signals Reach the Brain?

The Blood–Brain Barrier Isn’t Impenetrable 

The blood–brain barrier (BBB) is a selective interface. Its job is to protect the brain while letting essential signaling molecules through. But chronic inflammation and immune signals themselves can alter BBB function and allow peripheral cytokines to influence brain cells. 

The Area Postrema (AP): An Interface

One especially important site is the area postrema (AP). This small brainstem structure doesn’t have a typical BBB, making it uniquely sensitive to circulating immune signals. The AP detects blood-borne cytokines and signals to neural networks that regulate autonomic and behavioral responses. This makes it a hub for inflammation-to-brain communication. 

Microglia and Neural Immune Cells

Within the brain, microglia (resident immune cells) react to cytokines. When activated chronically, they impact synapses and influence neurotransmitter metabolism, contributing to symptoms like brain fog and mood fluctuations. 

Additional Pathways

Immune signals also travel along the vagus nerve, engage the HPA axis, and interact with circadian systems, which are channels through which inflammation can influences mood, sleep, energy, and neuroendocrine function.

Lyme Disease and Neuroimmune Interactions 

One key example of neuroimmune impact is Lyme neuroborreliosis. Borrelia burgdorferi can trigger central nervous system inflammation accompanied by robust cytokine and chemokine signaling. Even after antibiotic treatment, a subset of individuals can continue to experience mood or cognitive symptoms that may not stem from ongoing infection alone. Complex immune neural interactions are likely involved, further underscoring the importance of functional testing.

Putting It All Together

Chronic inflammation doesn’t just affect your body, it reaches your brain in multiple ways:

Chronic inflammation can:

• Elevate cytokines that influence neurocircuits
• Shift tryptophan metabolism toward kynurenine
• Reduce cofactors for monoamine production
• Affect BBB interfaces, especially in zones like the area postrema (AP)

The symptoms you feel can include:

• Fatigue and low motivation
• Brain fog and cognitive slowing
• Mood changes or flattened affect
• Sleep disruption

Understanding these biological links can help explain why chronic illness often feels “whole body”. The immune and nervous systems are in constant dialogue and monitoring functional markers during treatment and recovery may provide valuable insight into this complex relationship and guide therapeutic decisions.
iLabs is proud to provide state-of-the-art testing for bacterial, viral, immune, and neurotransmitter status.

Author

Christina Cowger, MA, has been a clinician and educator for over two decades and specializing in the intersection of neurobiology, chronicillness, and mood disorders. Christina has delivered talks and Continuing Education (CE/CME) seminars nationwide, to organizations including California Association of Naturopathic Doctors, Stanford University, Kaiser Permanente, CAMFT, Dominican University, Sonoma State University, Nevada Osteopathic Association and Sutter Health.