Mannose-binding lectin (MBL) deficiency is a condition that affects the immune system. It is a fairly common condition, affecting approximately 5–30 people in every 100. People with this condition have low levels of an immune system protein called mannose-binding lectin in their blood.

People with MBL deficiency may be prone to recurrent infections, including infections of the upper respiratory tract and other body systems. Sometimes those affected may also contract more serious infections, such as pneumonia and meningitis. The exact symptoms caused by infections vary in frequency and severity, depending on the type of infection.

Infants and young children with MBL deficiency seem to be more susceptible to infections, but adults can also develop recurrent infections. In addition, affected individuals undergoing chemotherapy or taking drugs that suppress the immune systems are especially prone to infections.

The treatment of health problems associated with MBL deficiency depends on the severity of the symptoms, but most people do not require any specific treatment. Breakthrough infections are treated with antibiotics as and when they occur. For more severe infections, regular, low-dose prophylactic antibiotics may be prescribed. The quality of life for those affected is good if infections are mild and/or treated promptly.

The symptoms you may recognise and which may have led your doctor to a diagnosis of MBL deficiency are those associated with recurrent respiratory infections. These usually affect the upper airways and include infections of the throat and ear; the latter causing otitis media.

People with MBL deficiency may sometimes also get more serious infections, such as pneumonia and meningitis.

Common causes of infection in MBL deficiency

Common infections in MBL deficiency are due to viruses, e.g. influenza and bacteria, such as Pseudomonas aeruginosa and Staphylococcus aureus.

Children and MBL deficiency

Infants and young children with MBL deficiency seem to be more susceptible to infections. This is because the lectin pathway plays an important role in fighting bacterial infections during the period when the antibody protection from the mother decreases, and the child’s own antibody production is not fully working.

How is MBL deficiency diagnosed?

The diagnosis is made using a sample of the patient’s blood. This is tested to see if the MBL protein is present at normal levels or is reduced. If it is reduced, then the patient’s condition is classified as being a mild, moderate or severe deficiency, with severe being complete deficiency when there is no detectable MBL in the blood.

Genetic changes (known as mutations) in the MBL2 gene can lead to MBL deficiency. This gene provides instructions for making the MBL protein that plays an important role in the body’s immune response. MBL protein attaches to foreign invaders such as bacteria, viruses or yeast and turns on (activates) another important part of the immune system known as the complement system.

Variants in the MBL2 gene reduce the production of the MBL protein, so sometimes MBL deficiency may be referred to as MBL protein deficiency. The reduction of MBL in the blood means that the body does not recognise and fight foreign invaders (e.g. bacteria, viruses or yeast) efficiently. That is why infections can be more common in people with this condition.

Family planning and MBL deficiency

The inheritance pattern of MBL deficiency is not fully understood. People may inherit an increased risk of developing MBL deficiency but not develop symptoms of the condition. Furthermore, not everyone with a change in the MBL2 gene has decreased levels of MBL, and not everyone with decreased protein levels is prone to infection. For these reasons, it is more usual to measure the protein level in the blood than undertake genetic studies. Researchers believe that a number of factors, including other genetic and environmental factors, are involved in the development of MBL deficiency.

Treatment of MBL deficiency depends on the severity of the symptoms. In most cases, no regular treatment of any kind is needed. If infections are mild and infrequent, and the person’s quality of life is not significantly affected, then treatment can be limited to the early use of antibiotics when an infection occurs.

For more information on the use of antibiotics, please read our booklet Use of antibiotics in the treatment of immunodeficiency.

If infections are more severe and/or are occurring frequently and time is being lost from work or school, or in the case of a child if growth and/or development are being affected, then regular, low-dose prophylactic antibiotics may be introduced to prevent the development of infections.

If there are recurrent bacterial infections, appropriate antibiotics should be chosen according to the sensitivities of the organism isolated on culture.

Practising good hygiene plays an important role in keeping people with MBL deficiency well. See our advice page on keeping well and healthy.

Good protection against common bacteria and viruses, in keeping with the recommended vaccine schedule, will help reduce infections in patients with MBL deficiency. Most vaccines are safe to be given to patients with MBL deficiency. The annual flu vaccine is usually advised. 

Immunologists may test vaccine responses to make sure that no other subtle immune defects, such as specific antibody deficiency, are contributing to MBL deficiency to cause frequent infection. 

At present there is no conclusive evidence to support the association of MBL deficiency with the development of particular medical conditions. Doctors will monitor patients’ health by clinical review (check-up) and infrequent blood tests. Most patients will be looked after by their GP in the long term unless a clinical immunologist has identified any complications or additional immune problems. 

You can download our information booklet here