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 a five different antibody isotypes seen in humans:
- IgG is the antibody isotype that most people think of when they're talking about antibodies. 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, and it is the only type of antibody that can cross the placenta during pregnancy.
- IgA is the antibody isotype that is found in 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.
- 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. It is very important in the early stages of an infection.
- IgE is the antibody that is responsible for the allergic response. When it binds to an allergen, it starts the histamine reaction that leads to allergy symptoms. 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.
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, now that nucleic acid tests such as LCR, which test directly for the pathogen, have become widely available, they serve as a useful alternative to antibody testing.
However, type-specific tests aren't trying to figure out what isotype of antibodies your body is producing -- they don't distinguish between IgG and IgM. Instead, type-specific herpes tests look at whether the anti-herpes antibodies your body is making react to HSV-1 or HSV-2.