What Are the 5 Types of Antibodies?

Antibodies are specialized Y-shaped proteins made by the immune system. They help the body fight disease by "recognizing" viruses, bacteria, and other pathogens (disease-causing microorganisms) by the molecules on their surface called antigens. Each antibody produced by the immune system is matched to a specific antigen—like a key to a lock—which it then binds to either to kill it or tag it for neutralization by other immune cells.

There are five broad categories (isotypes) of antibodies that the immune system produces, each of which has distinct functions and ways of defending the body against disease and infection.

Samples for medical screening

Immunoglobulin Isotypes

Antibodies are also known as immunoglobulins (immuno- meaning immune and -globulin meaning protein), the isotypes of which are designated by the letters "Ig." There are five different isotypes produce by the human body:

Immunoglobulin G (IgG)

Immunoglobulin G (IgG) accounts for around 75% of all antibodies in the human body. They are produced by white blood cells called B cells that originate in the bone marrow. Depending on the antigen it is matched to, IgG can either tag a pathogen for neutralization or bind to a toxin (poison) to directly neutralize it.

IgG can sometimes trigger an undesirable response in people with autoimmune diseases (in which the immune system inadvertently attacks its own cells and tissues).

Immunoglobulin A (IgA)

Immunoglobulin A (IgA) is the isotype primarily found in mucosal tissues (such as those in the mouth, vagina, and intestines) as well as saliva, tears, and breast milk. It accounts for 15% of all antibodies in the human body and is produced by B cells secreted from a thin layer within mucosal tissues called the lamina propria.

IgA is one of the body's first-line defenses against infection. It not only binds to pathogens to tag them for destruction but also prevents them from sticking the lining of tissues, called the epithelium.

IgA is also associated with hypersensitive reactions in people with celiac disease and several other autoimmune disorders.

Immunoglobulin A (IgM)

Immunoglobulin M (IgM) is also one of the first antibodies recruited by the immune system to fight infection. IgM populations rise very quickly when the body is first confronted with disease and then plummet just as quickly when there are enough IgG to take over. IgM is also produced by B cells and, when bound to a pathogen, will spur other antibodies and immune cells into action.

In addition to activating the immune response, a subset of IgM helps B cells "remember" a pathogen after it has been neutralized. If the pathogen were to return, the immune system can respond more robustly due to the presence of these memory B cells.

Immunoglobulin E (IgE)

Immunoglobulin E (IgE) is the antibody responsible for the allergic response that is mostly found in the lungs, skin, and mucosal membranes. IgE is produced by B cells secreted by lymph nodes or other lymphoid tissues situated near the site of the antigen (known in this case as an allergen).

When IgE binds to an allergen, it triggers a cascade of events in which white blood cells called basophils and mast cells degranulate (break open) and release an inflammatory compound called histamine into the bloodstream. It is histamine that is responsible for the symptoms of allergy.

IgE also helps to protect the body from parasitic infections, including helminths (parasitic worms).

Immunoglobulin D (IgD)

Immunoglobulin D (IgD) is important in the early stages of the immune response. Unlike other antibodies, it does not actively circulate but instead binds to B cells to instigate the immune response. As a signaling antibody, IgD help incites the release of front-line IgM to fight disease and infection.

IgD accounts for only around 0.25% of antibodies in the human body. Despite its vital role in "kick-starting" the immune response, IgD is arguably the least understood antibody with little known about how it might participate in other parts of the immune system.

Antibo Testing

Because immunoglobulins are matched to a specific pathogen, they can be used to diagnose diseases based on their unique structure. These tests, commonly known as antibody tests, are used to detect disease-specific antibodies in a sample of blood.

Antibody tests are available to diagnose (or help diagnose) a wide variety of infectious and autoimmune diseases, including:

Antibody tests do not detect the actual pathogens but rather the antibodies produced in response to the infection. Depending on the disease, it may take time for enough antibodies to be produced to reach detectable levels. If tested too early, during the so-called "window period," the test may deliver a false-positive result.

An antibody test can confirm that an infection has taken place, as with COVID-19 or HIV, although it cannot tell you when.

Immunoglobulin can also be used to characterize the stage of infection. Because IgM levels increase before the IgG response kicks in, a disease-specific IgM and IgG test can tell if a certain infection occurred more recently or less recently. Herpes simplex is one of the diseases in which IgM and IgG tests are commonly performed for this purpose.

In people with allergies, IgE tests can be used to confirm that an allergic response has occurred or for diagnostic purposes to see if IgE levels increase when you are intentionally exposed to an allergen.

A Word From Verywell

When taking an antibody test, it is important to remember that antibodies are the consequences of a disease or infection; it is not the disease or infection. There are, in fact, antigen tests that can detect the actual pathogen by its signature antigen.

Some diseases can be diagnosed with an antibody or antigen test. In other cases, only an antibody or antigen test is available. A positive result means "yes," the test has detected the antibody or antigen. A negative result means "no," while borderline results are considered inconclusive.

If undergoing an antibody test, make sure you do so after the window period to obtain the most accurate result. Your doctor or clinic can tell what the window period is for the disease you're being tested for is.

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