Mannose-Binding Lectin (MBL) Deficiency and Aspergillosis

What is MBL?

Mannose-binding lectin (MBL) is a natural protein made by the immune system. Its job is to help the body recognise and fight off germs, including fungi like Aspergillus. It’s part of the “innate” immune system – the first line of defence you’re born with.

How common is MBL deficiency?

MBL deficiency is surprisingly common.

• Around 5–10% of people have very low or absent levels.

• If you include milder reductions, as many as 20–30% of people carry genetic changes that lower MBL activity.

For most, this causes no problems because the immune system has other pathways to fall back on. People often never know they have it.

Why do MBL levels vary?

• Genetics: The MBL2 gene comes in different versions, some producing plenty of MBL and others producing little or none.

• Inheritance: The combination of gene copies from each parent determines your level.

• Normal diversity: Low levels are common and often harmless, showing the immune system has strong backup pathways.

Are some people born more vulnerable to infection?

Yes – but it depends on the situation.

• Children with very low MBL may get more ear, chest, or sinus infections while their immune systems are developing.

• In adults, MBL deficiency usually only matters if there are other risks, such as chronic lung disease, immune suppression, or another immune problem.

• Many people with low MBL live their whole lives without extra infections.

MBL deficiency and aspergillosis

On its own, MBL deficiency rarely causes illness. But in people who already have other risks – such as lung disease (COPD, asthma, bronchiectasis, or TB damage) or a weakened immune system – it may make infections more likely.

Research suggests MBL deficiency can be linked to:

• Chronic pulmonary aspergillosis (CPA)

• Allergic bronchopulmonary aspergillosis (ABPA)

• Invasive aspergillosis in people with suppressed immunity

In these cases, MBL deficiency is not the single cause of aspergillosis, but it may be one of several factors that increase vulnerability.

Can MBL deficiency be treated?

At present, there is no routine treatment to replace MBL itself. Research has explored giving purified MBL, but it hasn’t become a standard therapy – largely because deficiency is so common and most people remain healthy without intervention.

Instead, management focuses on:

• Treating infections promptly with antibiotics or antifungals

• Sometimes using preventive (prophylactic) antibiotics or antifungals in people with frequent or severe infections

• Using immunoglobulin replacement therapy if there are additional immune problems

• Supporting lung health and reducing risks with vaccinations, good self-care, and specialist monitoring

Why measure MBL if it can’t be treated directly?

Even without a direct treatment, measuring MBL can still be useful:

1. Helps explain recurrent infections – finding a low MBL level can give part of the reason why someone is more prone to infections.

2. Part of a bigger immune work-up – it’s often checked alongside other immune tests, and the overall pattern may guide treatment decisions.

3. Risk awareness – knowing about low MBL can make doctors more proactive with antibiotics, antifungals, or vaccinations, and encourage earlier treatment at the first sign of infection.

4. Research value – helps specialists understand why some people develop aspergillosis while others don’t.

Why hasn’t evolution eliminated low MBL?

• Common worldwide: 5–10% of people have very little MBL, and up to 30% have reduced levels. If this were a major disadvantage, numbers would be lower.

• Other immune pathways compensate: The body has strong backup systems, so many people stay healthy even with low MBL.

• Possible advantage: In some infections, high MBL may drive too much inflammation. Lower MBL might have protected against diseases like leprosy or TB.

• Changing disease patterns: In the past, people rarely lived long enough for chronic lung disease to show the effects of low MBL. Today, with longer lives and modern medicine, its role is more visible.

👉 In short: low MBL has not been “selected out” by evolution because it usually isn’t harmful on its own, and in some situations it may even have been protective.

What this means for patients

• Having MBL deficiency is quite common and usually harmless.

• It may become more relevant if you also have underlying lung disease or are on treatments that suppress the immune system.

• If MBL deficiency is suspected, doctors may check for it as part of a wider immune work-up.

• The key point: treatment is aimed at managing infections and lung health, not the MBL level itself.

👉 In short: MBL deficiency is common in the general population. Most people never notice it, but for some with lung disease or weakened immunity, it may add to the risk of aspergillosis. While there’s no direct treatment for the deficiency, testing can help explain recurrent infections, guide wider immune checks, and shape preventive care.