Immunoglobulin D (IgD)

Immunoglobulin D (IgD) is a type of antibody that protects your body from germs in your mucous membranes. Mucous membranes (mucosa) line the parts of your body that connect the inside to the outside. IgD is especially important in defending your mouth, nose and respiratory tract.

We hear a lot about antibodies. You might know that they’re an important part of your immune system. But what’s so special about them? Antibodies are proteins that help your body to recognize bacteria, viruses, toxins and other potentially harmful substances.

Each antibody has a unique shape that fits a specific antigen. Antigens are molecules that attach to a cell or an organism and mark it as either part of your own body (“self”) or foreign and potentially dangerous (“non-self”). When an antibody encounters an antigen that it fits — like on the surface of a virus or bacterium — it tells your immune system that it’s found a potential problem. Your immune system can then take action to eliminate the problem.

Where are IgD antibodies in your body?

IgD is found in your:

• Blood
• Respiratory tract — like your sinuses, airways and lungs
• Nose and mouth
• Tonsils
• Tears

Most of these immunoglobulins are found on the surface of B cells or other immune cells. They help these immune cells recognize germs (pathogens) and other harmful substances.

What do IgD antibodies look like?

Like all antibodies, IgD is made up of proteins and shaped like a “Y,” with a straight end and a forked end. While IgD can float freely, the straight end (the Fc region) is usually attached to a B cell or another type of immune cell. This end is the same on all IgD antibodies.

The forked end (receptor or Fab region) is unique for each antibody — its shape fits a single antigen like a lock and key. This end looks for antigens that fit that shape and activates your immune system when it finds one.

What does this immunoglobulin do?

IgD’s main function is as a B-cell receptor. This means it acts like an antenna, looking for anything in your body that could harm you. If it finds an antigen that fits its specific shape, it attaches (binds) to it, which:

• Alerts other immune cells that it’s found something harmful
• Blocks the pathogen from entering your cells
• Activates the B cell

The activated B cell will make copies of itself with the same IgD proteins. These proteins can find other pathogens of the same shape and bind to them.

Experts know that other antibodies (IgA, IgE, IgG and IgM) all play specific roles in your immune system, but IgD’s specific roles aren’t as clear. There’s evidence that it plays a part in:

• Mucosal immunity: IgD recognizes germs, toxins and other harmful substances in your mucous membranes and binds to them. This is important for protecting your body in places where germs often enter it (like through your nose or mouth).
• Maintaining homeostasis: It maintains the right amount of healthy bacteria while protecting you from harmful bacteria.
• Controlling allergies: IgD may be able to block allergens from causing inflammation in your respiratory tract. But there’s also some evidence that it plays a role in developing allergies.
• Taking over for other antibodies: IgD might be able to function like IgM antibodies if your IgM antibodies aren’t working properly.

What tests are there for IgD?

Your provider can order immunoglobulin tests to check your levels of IgD and other antibodies. This is a blood test that can help diagnose immune system diseases and infections.

What causes low IgD?

Diseases that affect your immune system’s ability to make antibodies and other immune cells can cause low levels of IgD. These include:

• Selective IgD deficiency: This is a condition where you have low levels of IgD but normal levels of other antibodies.
• Hypogammaglobulinemia: This is a group of conditions that cause low levels of all types of antibodies.

What are the symptoms of low IgD?

Unlike other immunoglobulin deficiencies, people with low levels of IgD don’t seem to get more infections or have any specific symptoms.

How do you treat immunoglobulin D deficiency?

Low IgD levels don’t usually cause symptoms or need treatment.

What does high IgD mean?

Causes of high IgD include:

• Chronic infections
• Autoimmune diseases
• Cigarette smoking
• Lymphoma
• Multiple myeloma

These can all cause your body to make too many immune cells and lead to inflammation. Treatment depends on the underlying condition.