Antibodies defend the body against some bacteria, viruses, fungi, and other foreign substances, called antigens. Certain cells cause the body to produce antibodies during an active infection.
When you first become sick, few antibodies may be detected. Antibody production increases during the course of an infection. Antibody tests are often repeated several weeks after the first test is done, so that a comparison can be made to the first test. A rising level of antibody to a specific bacteria or virus tells the health care provider that you had an active infection.
The complement fixation test looks to see if the body has produced antibodies to a specific antigen -- in this case, the C.burnetii bacteria. If the antibodies are present, they attach to the antigen. This combination activates or "fixes" complement.
The test specifically looks for the antibodies to C. burnetii in the clear liquid portion of the blood called the serum. (The term for this technique is serology.) If you have persistently high levels of antibodies to C. Burnetii, you could have chronic Q fever.