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PubMed This is a summary of 22 peer-reviewed journal articles Updated
Pulmonology

Biology & Differential Diagnosis: Eosinophils, IL-5, and The EGPA Pitfall

At a Glance

Severe eosinophilic asthma is driven by inflammatory proteins like IL-5 that recruit too many eosinophils to the lungs. It is vital to rule out EGPA, a rare blood vessel inflammation that mimics asthma but can cause severe damage to nerves, skin, and organs if left untreated.

Understanding why your lungs are so inflamed requires looking at the microscopic “messengers” in your immune system. While most asthma is managed with a standard inhaler, severe eosinophilic asthma is driven by specific proteins called cytokines that act like a recruitment siren for inflammatory cells. [1][2]

The Recruitment Team: IL-5, IL-4, and IL-13

In a healthy body, your immune system uses these cytokines to fight off infections. In eosinophilic asthma, they are overproduced, creating a cycle of chronic inflammation known as Type 2 inflammation. [1] This is a specific type of immune response that your doctor will look for when deciding on treatments.

  • IL-5 (Interleukin-5): This is the “primary recruiter.” Its main job is to tell your bone marrow to produce more eosinophils (a type of white blood cell) and then guide them into your lungs. [3][4] Once there, IL-5 keeps them alive much longer than they should be, leading to a massive buildup that damages the airway lining. [5]
  • IL-4 and IL-13: These two work together as the “construction crew.” They are responsible for airway remodeling—the process where the lungs produce too much mucus and the airway walls become thick and scarred. [6][1] They also tell the body to produce IgE, an antibody that triggers allergic reactions. [7]

Modern “biologic” medications work by essentially “mopping up” these cytokines or blocking their receptors, preventing the recruitment siren from ever going off. [5][8]

The “EGPA Pitfall”: When It’s Not Just Asthma

There is a rare but serious condition called Eosinophilic Granulomatosis with Polyangiitis (EGPA)—formerly known as Churg-Strauss Syndrome—that can look exactly like severe asthma in its early stages. [9][10]

EGPA is a form of vasculitis, which means inflammation of the blood vessels. [11] Because blood vessels go everywhere in your body, EGPA can damage the heart, kidneys, skin, and nerves, whereas asthma is mostly limited to the lungs. [12][13]

Why It Gets Missed

Many people with EGPA have a history of severe asthma and nasal polyps for years before the “vasculitic” stage begins. [14] Sometimes, the high doses of steroids used to treat asthma actually “hide” the EGPA symptoms. When the steroid dose is lowered, the underlying vasculitis flares up. [15]

Red Flags for EGPA

If you have been diagnosed with severe eosinophilic asthma, you should be aware of “red flag” symptoms that suggest the inflammation might be systemic (affecting your whole body) rather than just in your lungs.

System Red Flag Symptoms
Nerves Mononeuritis multiplex: Sudden numbness, “pins and needles,” or weakness in a hand or foot (e.g., “foot drop”). [16][17]
Skin Purpura: Small, purple or red spots on the skin that don’t fade when pressed, or painful skin lumps. [11]
Sinuses Severe, chronic sinus pain or nasal polyps that recur even after surgery. [18]
Heart/Kidneys Unexplained chest pain, shortness of breath that feels different from asthma, or blood in the urine. [19][20]
Bloodwork An exceptionally high eosinophil count (usually over 1,000 cells/μL or 1.0 x 10⁹/L). [18][15]

If you experience these symptoms, it is vital to tell your doctor. EGPA requires a different, more intensive monitoring and treatment plan than isolated asthma to prevent long-term organ damage. [21][22]

Common questions in this guide

What is IL-5 and how does it affect my asthma?
IL-5, or Interleukin-5, is an inflammatory protein that acts like a recruitment siren for your immune system. In severe eosinophilic asthma, your body overproduces IL-5, which signals your bone marrow to make excess eosinophils and guides them into your lungs, causing chronic inflammation.
What is EGPA and how is it different from asthma?
EGPA is a rare form of vasculitis, or blood vessel inflammation, that can affect organs throughout the body. While it often starts with symptoms that look exactly like severe asthma, EGPA can eventually cause serious damage to the heart, kidneys, skin, and nerves if it isn't properly monitored and treated.
Why does EGPA get misdiagnosed as severe asthma?
Many patients with EGPA experience severe asthma and recurrent nasal polyps for years before the broader blood vessel inflammation begins. Additionally, the high doses of steroids commonly used to treat severe asthma can mask the symptoms of EGPA, making it harder to detect until steroid doses are lowered.
What are the red flag symptoms of EGPA?
Warning signs of EGPA include unexplained nerve pain or numbness, sudden weakness like foot drop, painful skin lumps, or purple spots that do not fade when pressed. Severe chronic sinus pain and an exceptionally high blood eosinophil count (over 1,000 cells/μL) are also major red flags that require immediate medical attention.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What was my highest ever recorded blood eosinophil count, and is it high enough to warrant investigation for EGPA?
  2. 2.Should I be tested for ANCA (Anti-neutrophil cytoplasmic antibodies) to help clarify my diagnosis?
  3. 3.Given my history of asthma, are there signs of systemic involvement in my heart, kidneys, or nerves that we should be screening for?
  4. 4.How do the biological treatments we are considering (like mepolizumab or benralizumab) differ in their ability to treat both severe asthma and potential underlying vasculitis?
  5. 5.If my symptoms continue to resist high-dose steroids, at what point do we consider a biopsy or more advanced imaging like a Cardiac MRI?

Questions For You

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References

References (22)
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This page provides educational information about eosinophilic asthma biology and EGPA. It is for informational purposes only and does not replace professional medical advice. Always consult your healthcare provider if you experience new symptoms or suspect EGPA.

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