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Antibody Tests

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What are antibody tests?

Antibody tests involve analyzing a patient’s sample (usually blood) for the presence or absence of a particular antibody (qualitative) or for the amount of antibody that is present (quantitative).

Antibodies are part of the body's immune system. They are immunoglobulin proteins that help protect people against microscopic invaders such as viruses, bacteria, chemicals, or toxins. Each antibody that is produced is unique. It is created to recognize a specific structure on an invading foreign cell or particle. The specific structure that is recognized is called an antigen. Antibodies attach to the antigens, creating antigen-antibody complexes (immune complexes) that serve as signals for the rest of the immune system to destroy the cell or particle.

There are five different classes of immunoglobulins (IgM, IgG, IgE, IgA, and IgD). The three most frequently measured are IgM, IgG, and IgE. IgM and IgG antibodies work together to produce short-term and long-term protection against infection. IgE antibodies are primarily associated with allergies.

The first time someone is exposed to a foreign substance, like a virus or bacterium, it may take the immune system up to two weeks to make an antibody blueprint and to produce enough of a specific antibody to fight the infection. This initial response consists primarily of IgM antibodies. Several weeks later, usually after the immediate threat has passed and the infection has resolved, the body creates IgG antibodies. It remembers the blueprint for fighting this microorganism and maintains a small supply of antibodies (a mixture of both IgM and IgG). The next time the body is exposed to the same foreign substance, it will respond much more strongly and quickly, to provide primarily IgG antibody protection.

Vaccines are designed to trigger production of antibodies prior to exposure to a potentially infective microorganism. Vaccines use either a weakened version of the microorganism (one that cannot cause infection) or an isolated protein that mimics an antigen structure on the surface of the microorganism. Thus, the vaccine provides a relatively safe initial exposure to generate the blueprint for future protection. Vaccines generate an initial immune response to create IgM antibodies and a secondary response that provides a supply of IgG antibodies. The antibodies generated by the vaccine provide long-term, rapid-response protection (termed immunity). Additional booster shots are sometimes given after the first vaccination to raise the concentration of antibodies in the blood to a level considered to be sufficiently protective (provide adequate immunity).

Appropriate antibody production and targeting depends on the body's ability to distinguish between itself and foreign substances and to correctly identify foreign substances that represent threats. Normally, a person's immune system learns to identify and ignore the antigens that are present on the person’s own organs, tissues, and cells. Sometimes, however, it will mistakenly identify a part of the person’s own body as foreign and produce autoantibodies. These autoantibodies trigger an inflammatory reaction that attempts to destroy the body’s own tissue in the same way it would try to destroy a foreign invader. An autoimmune response can affect a single organ (like the thyroid) or be systemic, affecting many tissues or organs. These autoantibody-induced responses result in conditions termed autoimmune disorders or autoimmune diseases.

Antibodies may also trigger immune responses to blood transfusions or organ transplantations. Although patients are given blood or organs that most closely match their own blood or organs to minimize the chance of an immune response, matches are not always perfect. Antigens in donated blood that is given during a blood transfusion may stimulate an immune reaction termed a transfusion reaction. All patients receiving donor blood must be watched carefully for symptoms of transfusion reaction. Antigens on transplanted body organs may stimulate an immune response that can lead to organ rejection. Transplant patients are treated with drugs to suppress their immune systems to avoid rejection of the transplanted organ.

Sometimes a person's immune system may respond to foreign substances that represent no threat and typically do not generate immune reactions in most people. Such responses are termed allergies (or hypersensitivities) and involve generation of IgE antibodies. The foreign substances that trigger allergic reactions include foods, pollens, molds, and animal dander. There are many different types of allergies, and allergic reactions may vary from mild irritations to severe life-threatening responses.

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