Immunoglobulin G (IgG)

IgG (immunoglobulin G) is a type of antibody that your body makes to help you fight off infections. Your immune system makes these immunoglobulins to defend itself from harmful invaders, like viruses and bacteria, that have infected you in the past. This is called the secondary immune response.

Antibodies are proteins that each have a unique shape that fits into an antigen like a key into a lock. Antigens are proteins or other markers that identify a cell or organism as either part of your own body (“self”) or foreign (“non-self”) and potentially harmful. When an antibody fits an antigen — like on the surface of a virus or bacterium — it alerts the rest of your immune system that it’s found something harmful. This allows your immune system to identify and target the harmful organisms (pathogen). Other immune system cells damage, destroy or try to flush it out of your system.

IgG is the most common immunoglobin in your blood, accounting for up to 75% of blood antibodies.

IgG subclasses

There are four IgG subclasses. While they all play a role in your immune system, they have slightly different specialties:

• IgG1: IgG1 is the most common subclass. It plays an important role in fighting viral infections.
• IgG2: IgG2 is best at fighting off certain bacterial infections.
• IgG3: IgG3 most often fights off viral infections.
• IgG4: IgG4’s role isn’t as clear as the other subclasses, but experts think it’s important for fighting off parasitic infections.

What do IgG antibodies do?

IgG responds to pathogens by:

• Alerting other immune cells to destroy a pathogen: IgG complexes can attach to pathogens (germs and other foreign substances) and recruit other immune cells to destroy them. They can do this by coating them to attract other immune cells (opsonization) or activating a series of proteins (the complement system) that react with each other to damage or destroy the pathogen.
• Neutralizing them: If an IgG antibody attaches (binds) to a pathogen, it can prevent it from attaching to and entering your body’s cells.

When does your body make this immunoglobulin?

When your immune system encounters a potentially harmful antigen, it causes either a primary or secondary immune response, depending on whether your body remembers seeing the pathogen before. During the primary response, your body makes IgM antibodies. These help fight off the harmful intruder until your body switches to making IgG antibodies, which can spread more widely throughout your body and have better tools for fighting infections.

After this first encounter, your immune system creates memory cells — like taking a photo for a “wanted” poster. When it encounters the same pathogen again, your body can switch more quickly to creating IgG antibodies, which recognize the suspect and can mobilize quickly to detain it.

Vaccines are designed to get your body to generate this memory without getting sick first. When your immune system sees a virus or bacterium you’re vaccinated against, it’ll already have a wanted poster and have IgG antibodies prepared to fight the invader.

Where are IgG antibodies in the body?

IgG is the most common antibody in your blood and in the fluid that surrounds cells. It’s small and can move easily into tissues in other parts of your body. IgG is the only antibody that can cross the placenta, which provides babies some immune protection for about six months after they’re born.

What does IgG look like?

Like all immunoglobulins, these are made up of chains of proteins that form a “Y” shape. The forked end is different for each antibody. This is called the receptor, or the Fab region. The receptor recognizes the unique shape of antigens on the surface of pathogens (or any “non-self” substance). It binds to these antigens to alert your immune system that it should destroy the substance.

The other end of the “Y” shape is the same on all antibodies of the same type (IgA, IgD, IgE, IgG and IgM). This is called the Fc or constant region. The Fc region fits onto immune cells. If the receptor of IgG is bound to a pathogen when an immune cell connects to the Fc region, the cell knows to get rid of the pathogen.

What tests are there for IgG?

Your provider can order immunoglobulin tests to check your levels of IgG and other antibodies. These are blood tests that can help diagnose immune system diseases and infections.

A provider can also test your levels of IgG to see if you have immunity to specific infectious diseases. They might want to check these levels (called titers) to see if you developed immunity through a vaccine or natural immunity (getting an infection and having your immune system develop immunoglobulins against it). You might have this test done for:

• Measles
• Mumps
• Rubella
• Chickenpox

What is a normal level of IgG?

For adults, the range of normal IgG levels is between 600 and 1600 mg/dL (milligrams per deciliter). These numbers can vary by lab and are lower in kids and babies.

What causes high IgG?

Causes of high IgG antibodies include:

• Infections
• Liver cirrhosis
• Autoimmune diseases, which include lupus (systemic lupus erythematosus/SLE), scleroderma, rheumatoid arthritis and Sjögren’s syndrome
• Monoclonal gammopathy of undetermined significance (MGUS), which can cause immunoglobulins
• Multiple myeloma, which can produce increased amounts of abnormal IgG antibodies

In newborns, high IgG levels could be a sign of a congenital (present at birth) infection. Toxoplasmosis, rubella, syphilis and cytomegalovirus (CMV) can all pass from a pregnant female to the fetus through the placenta.

What causes low IgG?

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

• Hypogammaglobulinemia: This is a group of conditions that prevent you from making enough B cells, or prevent B cells from maturing and making antibodies. They include common variable immunodeficiency (CVID) and X-linked agammaglobulinemia (XLA).
• Primary IgG deficiencies: Primary IgG deficiencies can make you have low levels of IgG without another underlying cause. You usually have normal levels of other antibodies, like IgM and IgA.
• Medications or illnesses that weaken your immune system: Immunosuppressive medications, cancer, HIV and other conditions can reduce your body’s production of antibodies.

What are the symptoms of low IgG?

If you have low immunoglobulin levels, you can get frequent infections, including:

• Sinusitis
• Pneumonia
• Bronchitis
• Ear infections

High levels can also cause frequent infections if you’re making abnormal IgG antibodies (that don’t work properly).

How do you increase levels of IgG?

People with weakened immune systems sometimes need to increase their levels of IgG. Immunoglobulin replacement therapy, or intravenous immunoglobulin (IVIG) treatments, can give you additional antibodies to help you fight infections.

IgG is also often used in monoclonal antibody treatments, which can help people with certain infections fight off the illness. They’re used in some cancer treatments, too.

Does low IgG mean you have an autoimmune disease?

No. In fact, many people with autoimmune conditions have increased levels of IgG.

Do high IgG levels mean you have allergies?

No. The connection between IgG antibodies and allergies is unclear. Healthcare providers use your symptoms, IgE antibody levels and other allergy tests to diagnose allergies. There’s no evidence that IgG levels can help diagnose allergies.