An antibody defends the body against bacteria, viruses, fungus, or other foreign body (antigen). Certain cells tell the body to produce antibodies during an active infection. In the initial stage of an illness, few antibodies may be detected. Antibody production increases during the course of an infection. For this reason, such tests are often repeated several weeks after the first test is done. This test specifically looks for the antibodies in the clear liquid portion of the blood, which is called the serum. (This technique is called serology.)
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