If Aspergillus-specific IgE is already positive, why bother testing IgG too?

🧠 ABPA: A quick reminder

ABPA (Allergic Bronchopulmonary Aspergillosis) is an allergic reaction to the fungus Aspergillus fumigatus, most commonly in people with asthma or cystic fibrosis.

Your immune system reacts in two key ways:

1. IgE antibodies – part of an allergic response (like in hay fever or asthma).

2. IgG antibodies – part of a chronic or repeated exposure response, often related to inflammation or tissue damage.

💡 Your main question:

If ABPA-specific IgE is already positive, why bother testing IgG too? Shouldn’t it also be positive?

Not always. Here’s why both are useful and not redundant:

🔬 Why test IgG if IgE is already positive?

✅ 1. They tell us different things:

• IgE (specific to Aspergillus) → Shows allergic sensitisation. This is key for diagnosis.

• IgG (specific to Aspergillus) → Suggests immune system has been exposed repeatedly or persistently to Aspergillus, usually through colonisation in the lungs.

You can have:

• High IgE but normal IgG — early or milder ABPA.

• High IgE and high IgG — more established disease, or significant fungal colonisation.

• High IgG but normal IgE — maybe chronic pulmonary aspergillosis (CPA), not ABPA.

✅ 2. It helps to rule out other conditions

For example:

• CPA is often IgG positive but IgE normal.

• ABPA is usually IgE high, often with IgG also raised, but not always.

So, IgG helps confirm, clarify, or differentiate between Aspergillus-related conditions.

✅ 3. It adds confidence to the diagnosis

If both Aspergillus-specific IgE and IgG are raised, it makes the diagnosis of ABPA stronger — especially when imaging shows mucus plugging, bronchiectasis, or infiltrates.

🧪 Summary:

🧍🏻 Simple analogy:

Think of:

• IgE like the smoke alarm — it reacts to allergens quickly.

• IgG like the carbon monoxide detector — it reacts to long-term exposure that might not cause an immediate “allergic” fire but still shows something’s wrong.