Things to Remember
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Your body fights parasites with special weapons that can hurt you too: When you have a parasitic infection, your immune system sends out special white blood cells (eosinophils) that release toxic chemicals to kill the parasite. The problem? These same chemicals can damage your own tissues and cause inflammation throughout your body. This immune response - not the parasite itself - is often what makes you feel sick.
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Parasite infections can look like sudden allergies or asthma: Parasites trigger high levels of IgE antibodies (the same ones involved in allergies), which can cause mysterious rashes, hives, itching, or even new-onset asthma in adults. If you develop unexplained allergic symptoms or breathing problems, especially with no family history, it's worth investigating whether a parasitic infection might be the hidden cause.
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Your immune system might stay "stuck on" even after treatment: Even after successfully treating a parasite, your immune system can remain in overdrive for weeks or months. Your blood work might still show elevated immune markers, and you might still have symptoms while your body slowly winds down from months of fighting.
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"Leaky gut" is real and causes problems beyond your intestines: Parasites damage your intestinal lining, which can allow food particles and bacterial toxins to leak into your bloodstream. This triggers body-wide inflammation that can cause seemingly unrelated symptoms like joint pain, brain fog, fatigue, and even depression - because inflammatory chemicals can affect your brain chemistry.
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Mysterious symptoms might all connect back to your gut: If you're being treated for depression, joint pain, and digestive issues separately, consider whether they might all stem from gut damage. Doctors often treat each symptom in isolation without looking at what's happening in your intestines, which could be the root cause of multiple problems.
This article explains how your immune system detects and fights parasitic infections, and why those responses sometimes cause the symptoms that make you feel sick.
The kitchen table had a jar of honey on it. Local honey, the handwritten label said - from someone's backyard hives. I was there visiting a woman who'd been exhausted for months, and she'd mentioned buying it because she'd read somewhere that local honey helps with allergies. It's one of those folk remedies that has a grain of physiological truth buried under layers of wishful thinking.
Key Immune Cells in Parasitic Infections: What Each Does
| Immune Component | Normal Function | Response to Parasites | Signs of Activation | What It Means Clinically |
|---|---|---|---|---|
| Eosinophils | 1-3% of white blood cells; general immune surveillance | Increase dramatically (often >1500 cells/μL); release toxic proteins to damage parasite membranes | Elevated eosinophil count on blood work; may persist weeks after treatment | Eosinophilia indicates active or recent parasitic infection; also causes inflammation and tissue damage |
| IgE Antibodies | Low levels; primarily anti-parasite immunity | Spike significantly; bind to parasite antigens to tag for destruction | Allergy-like symptoms: hives, rashes, persistent itching, sometimes asthma | High total IgE suggests chronic parasitic infection; can take 6-12 months to normalize after treatment |
| Mast Cells | Release histamine during allergic reactions | Activated by IgE binding; degranulate and release inflammatory chemicals | Itching, flushing, hives, gastrointestinal cramping | IgE-mediated degranulation causes "allergic" symptoms that are actually anti-parasitic responses |
| Th2 Helper Cells | Coordinate immune response to large pathogens | Drive eosinophil production and IgE synthesis; promote mucus production | Increased mucus, cough, airway reactivity, sometimes asthma-like symptoms | Sustained Th2 activation explains why parasitic infections mimic or trigger allergic conditions |
But it made me think about how our immune systems actually work. Because when we talk about parasitic infections, most people picture the worm itself - the thing you might see in stool, the organism causing the problem. But what actually makes you sick isn't usually the parasite directly. It's your body's response to it.
And that response is extraordinary. Terrifying, sometimes. But extraordinary.
The Eosinophil Story
Your body has a specific battalion reserved for parasites: eosinophils - white blood cells that specialise in attacking large pathogens like worms. They're named for the way they look under a microscope when stained with eosin dye, this reddish-pink colour that makes them stand out from other immune cells. Under normal circumstances, they make up only 1-3% of your white blood cell count. But when parasites arrive, that percentage can skyrocket.
Eosinophils release toxic granules - little packets of proteins and enzymes designed to damage parasite cell membranes. It's chemical warfare at the cellular level. Major basic protein, eosinophil peroxidase, eosinophil cationic protein - these are the weapons. They work beautifully against helminths (parasitic worms - the multicellular organisms that live in intestines, blood, or tissues). The problem is they also cause collateral damage.
When eosinophil counts rise dramatically - a condition called eosinophilia - you get inflammation. Lots of it. Tissue damage. Organ dysfunction if severe. I've seen eosinophil counts above 1500 cells per microlitre (normal is under 500) in people with chronic Strongyloides infections. Their bodies were waging war, and the battlefield was their own intestines.
Here's the strange part: sometimes the eosinophilia persists even after the parasite is gone. The immune system, revved up for months or years, doesn't always calm down immediately. People treat the infection successfully, but their blood work still shows elevated eosinophils weeks later. It's like the body's alarm system got stuck in the "on" position.
The IgE Connection
Then there's IgE - Immunoglobulin E, an antibody class primarily involved in allergic responses and anti-parasitic immunity. IgE binds to parasite antigens (foreign proteins that trigger immune responses) and tags them for destruction. When IgE levels are high, your body is either fighting a parasite or thinks it is.
This is why people with chronic parasitic infections often develop what looks like allergies. Rashes that won't resolve. Hives that come and go without apparent trigger. Itching - persistent, maddening itching - that's actually the result of IgE-mediated mast cell degranulation (release of histamine and inflammatory chemicals from immune cells). The body mistakes its own tissues for invaders sometimes.
I once saw someone who'd developed sudden-onset asthma at age forty-three. No family history, no obvious triggers. Pulmonary function tests showed classic reactive airway disease - bronchospasm (narrowing of airways in response to irritants) that responded to inhalers. But her total IgE was profoundly elevated. Turned out she had a Toxocara infection - roundworms from dogs that had been migrating through her lungs for months, triggering massive immune responses every time they passed through pulmonary tissue.
The asthma resolved after treatment. But it took nearly a year for her IgE levels to normalise. Her immune system had been so persistently activated that it took time to wind back down. The body remembers threats, even after they're gone.
The Gut Barrier Breaks Down
Most intestinal parasites don't just sit passively in your bowel. They attach to the intestinal wall, sometimes burrowing into the mucosa (the innermost lining of the intestine - the protective barrier between gut contents and bloodstream). This mechanical damage disrupts the epithelial barrier - the single layer of cells that separates your gut contents from your bloodstream.
When that barrier fails, you get increased intestinal permeability. People call it "leaky gut," which sounds vague and alternative-medicine-ish, but it's a real, measurable phenomenon. Tight junctions (protein structures that seal the gaps between intestinal cells) loosen. Larger molecules that should stay in the gut start crossing into the bloodstream. Food antigens. Bacterial endotoxins. Inflammatory mediators.
The immune system, already on high alert because of the parasite, now encounters a flood of foreign material it wouldn't normally see. This triggers systemic inflammation - low-grade but persistent. You get symptoms that seem unrelated to the gut: joint pain, brain fog, chronic fatigue, even mood changes. Depression rates are higher in people with chronic parasitic infections, and while part of that is probably the psychological burden of being unwell, part of it is likely inflammatory cytokines (signalling proteins released by immune cells) crossing the blood-brain barrier and affecting neurotransmitter metabolism.
I don't think we fully appreciate how much gut health affects everything else. Or maybe we do, conceptually, but we don't act like it clinically. We treat symptoms in isolation - prescribe an antidepressant for the mood, a nonsteroidal for the joint pain, maybe a proton pump inhibitor if there's reflux - without ever asking what's happening at the level of the intestinal barrier.
The Microbiome Gets Wrecked
Here's something most people don't know: parasites alter your gut microbiome - the community of trillions of bacteria, fungi, and other microorganisms living in your intestines. They change which bacterial species dominate, often favouring pathogenic strains over beneficial ones. Giardia, for instance, can significantly reduce populations of Lactobacillus and Bifidobacterium - two genera associated with gut health and immune regulation.
When the microbiome shifts, you lose some of its protective functions. Microbial diversity decreases. Short-chain fatty acid production (like butyrate - a compound produced by gut bacteria that nourishes intestinal cells and reduces inflammation) drops. The gut becomes more permeable, more inflamed, less able to regulate itself.
And here's the cruel part: even after the parasite is eradicated, the microbiome doesn't always recover on its own. I've seen people clear Blastocystis or Giardia with appropriate antiparasitic therapy, only to develop post-infectious irritable bowel syndrome - persistent abdominal pain, bloating, altered bowel habits that last months or years. The infection is gone, but the damage to the microbiome remains.
Probiotics help sometimes. Sometimes they don't. There's emerging evidence that fecal microbiota transplantation (transferring stool from a healthy donor to restore gut bacteria) might help in severe cases, but it's not standard practice yet. We're still learning how to rebuild what gets destroyed.
The Immune System Never Quite Forgets
One of the most unsettling aspects of parasitic infections is immune memory. Your body produces specific antibodies against parasites you've encountered, and those antibodies can persist for years - sometimes decades - after the infection is resolved. This creates diagnostic confusion.
I've had patients test positive for Toxoplasma IgG antibodies (indicating past infection with Toxoplasma gondii - a protozoan that causes toxoplasmosis) and panic, thinking they have an active infection. They don't. The antibodies just mean they were exposed at some point - maybe decades ago. But distinguishing past infection from active infection requires looking at IgM antibodies (which indicate recent infection) and sometimes PCR testing (polymerase chain reaction - a test that detects parasite DNA directly).
The immune system's memory also means you can have exaggerated responses to re-infection. If you've had hookworm before and encounter it again, your immune system might respond so aggressively that the secondary infection causes more symptoms than the first one did. It's protective, but it's also exhausting for your body.
I'm not sure we've figured out how to balance immune memory with immune tolerance. The body needs to remember threats. But it also needs to know when to stand down.
What This Means Practically
When someone presents with chronic fatigue, unexplained eosinophilia, persistent GI symptoms, or vague systemic complaints, parasites should be on the differential. Not at the top, maybe, but somewhere in the middle. And testing should be comprehensive - not just a single stool O&P, but serological testing (blood tests for antibodies) for common organisms, complete blood count with differential to check eosinophil levels, total IgE if there's any suggestion of allergic-type symptoms.
But even that won't catch everything. Some parasites - like Strongyloides - can persist for decades without ever showing up on routine testing. Diagnosis sometimes requires duodenal aspirate or even empiric treatment trials, which feels unsatisfying but is occasionally necessary.
Treatment isn't just about killing the parasite. It's about supporting the immune system afterward, helping the gut heal, restoring the microbiome if possible. That might mean probiotics, anti-inflammatory strategies, time. There's no quick fix.
The honey jar was still on the table when I left. I didn't have the heart to tell her that local honey probably wasn't going to solve her fatigue. But I did order a parasite panel. Sometimes the simplest explanations - infection, inflammation, immune dysregulation - are the ones we overlook.
