What Are the Five Types of Antibodies?

Image of an antibody

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Antibodies are specialized proteins made by the immune system. They help the body fight against infections and disease by "recognizing" viruses, bacteria, and infected cells. Each antibody binds to a specific antigen associated with a danger signal in the body. This antigen is also known as the antibody's target.

In addition to responding to different targets, antibodies also come in different types. These types are known as isotypes or classes. During the course of an infection, antibodies against a single antigen (target) will be produced as a variety of different isotypes. The type of antibody produced depends on where they are needed in the body.

What are the Five Types of Antibody?

Antibody isotypes, or antibody classes, define the role of the antibody in the body. All classes are named using the convention Ig*, where Ig stands for immunoglobulin and * is the designation for the specific isotype.

There are five different antibody isotypes seen in humans: IgG, IgA, IgM, IgE, and IgD.

IgG is the antibody isotype that most people think of when they're talking about antibodies. It is the antibody that is built by immunization. It activates an immune cascade that can eliminate some forms of infection. IgG can also neutralize certain toxins.

There are multiple forms of IgG that circulate through the body and respond to infection. IgG is made up of only a single Ig subunit. It is the only type of antibody that can cross the placenta during pregnancy. That is how infants can take advantage of their mother's immune response. Her IgG can attack their infections.

IgA is the antibody isotype that is found in usually mucosal areas, such as the mouth and the vagina. It can also be found in saliva, tears, and breast milk. IgA is formed by two Ig subunits bound together. When IgA binds to a target, it can stimulate inflammation. In mucosal areas, IgA can also keep pathogens from sticking to epithelial cells. The production of IgA against inappropriate targets is associated with certain autoimmune diseases, such as celiac disease.

IgM is one of the first types of antibody to be produced after a pathogen has entered the body. Since it is made up of five Ig subunits bound together, it has very high avidity. In other words, it sticks very strongly to its target. IgM is very important in the early stages of an infection. IgM sometimes appears when an infection becomes reactivated, such as with a herpes outbreak. It can also appear when someone is reexposed to a disease they've previously gotten rid of.

IgE is the antibody that is responsible for the allergic response. It is mostly found in the lungs, skin, and mucous membranes. When IgE binds to an allergen, it starts the histamine reaction. It's the histamine reaction that causes the symptoms of an allergy attack. This single subunit antibody also helps to protect the body from parasitic worms.

IgD is important in the early stages of the immune response. Bound to B cells, it does not circulate. Instead, it signals those cells to become active. This can help to stimulate inflammation. IgD is the least well understood type of antibody, and its functions are still being discovered.

The Role of Antibody Isotypes in Diagnostic Testing

Many STD tests, and tests for other diseases, look for an antibody response to the disease rather than the pathogen itself. For a long time, this was particularly true for viral diseases and bacteria that were difficult to grow in culture. However, this has changed as technology has improved. For instance, there are now nucleic acid tests, such as LCR and PCR, which test directly for the pathogen's DNA. As such tests become more widely available, they serve as a useful alternative to antibody testing.

One condition that is still tested for using antibodies is herpes. Herpes antibody tests can also be used to distinguish between new herpes infections and infections that have simply gone unnoticed. How? Scientists can look for two different isotypes of antibodies the body may be producing. Positive IgM tests usually mean that the infection is recent. IgG tests speak to a longer-term infection. This is because it takes longer for the body to produce IgG than it does to produce IgM.

Did you know? Type-specific STD herpes antibody tests aren't trying to figure out what isotype of antibodies your body is producing. Instead, type-specific herpes tests look at whether the anti-herpes antibodies your body is making react to HSV-1 or HSV-2.They are looking for different target types rather than different antibody types.

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