Tests for antigens and antibodies
Tests for antigens and antibodies are tests on samples of body fluids to detect antigens and antibodies in the body.
Every microorganism that enters the body has proteins on its surface, called antigens, that the immune system recognizes as foreign. When white blood cells of the immune system detect a foreign antigen, they produce antibodies that exactly fit on to that antigen and help to destroy the organism. Since every microorganism has unique antigens, and one antibody will lock on to only one particular antigen, tests for antigens and antibodies can identify microorganisms quickly and accurately.
Antibody tests are usually performed on samples of blood because antibodies are made by white blood cells. Antigen tests may be performed on any type of body fluid, such as urine or saliva.
Why are tests for antigens and antibodies done?
Antigen tests are useful when a rapid diagnosis is needed or when there are not enough microorganisms in a sample to be visible under a microscope. For example, finding the antigen of the bacterium that causes bacterial meningitis in a sample of the fluid that surrounds the brain can provide a diagnosis in minutes.
Testing for antibodies is useful for diagnosing a persistent infection, such as HIV infection (see HIV infection and aids), or for confirming your immunity from a previous infection, such as rubella. Antibody tests are not useful for diagnosing recent infections because the immune system takes about 10 days to make an antibody against a new microorganism.
How are tests for antigens and antibodies done?
Antigens and antibodies cannot be seen separately, but when they join together they form a large clump that can be seen by the naked eye. In antigen tests, small beads or red blood cells may be coated with an antibody that reacts with the antigens on one particular microorganism. The sample that is to be tested is then added to the antibodycoated beads. If the suspected organism is present in the sample, its antigens will become attached to the antibody beads and make them clump together.
In antibody tests, antigens from a known microorganism may be coated on to small beads. A sample of body fluid is then added to the beads. If the immune system has produced antibodies against the organism, the antibodies will attach to the antigen beads, producing visible clumping.
Although the clumps of antigen and antibody may be seen without a microscope, the test can be made more sensitive by attaching a fluorescent or radioactive marker to either the antigens or the antibodies before they are added to the test sample.
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