About Antigen/Antibody Tests
Infectious diseases are caused by tiny microorganisms such as bacteria, viruses, fungi, and parasites. Proteins, called antigens, are carried on the cell surface of all infectious organisms; the immune system recognizes these foreign invaders, and in response forms proteins called antibodies to destroy or control them.
Antigen/antibody tests rely on the fact that there is a specific antibody for each antigen, thus, each one can be used to detect the presence of the other. For example, a known antigen may be added to a blood sample; if the corresponding antibody is present in the specimen, the two proteins will bind together. These tests are usually performed on serum (the liquid portion that remains when blood clots and the cells are removed); but occasionally, samples of other body fluids, tissue, or stool may be used. Several different techniques may be employed, including the following:
Precipitation reaction. When an antigen-antibody reaction takes place, the antigens and antibodies cross-link to form a lattice like structure that precipitates out of the solution, settling on the bottom of the beaker. Observation of this precipitate can confirm the presence of a specific infectious agent.
Agglutination reaction. Agglutination refers to a clumping that occurs when an antigen comes into contact with its corresponding antibody. This process mimics what happens in the body when antibodies cause antigens to agglutinate, promoting easy removal of the infectious organism. Observation of an agglutination reaction aids in the detection of specific antibodies, such as those produced in syphilis or salmonella infections.
Complement fixation refers to a reaction in which an antigen binds with an antibody, forming a combination that causes complement (a complex group of blood proteins) to become fixed at the same site. Detection of this complement fixation reaction leads to identification of the original antigen.
Immunofluorescent assay is a technique in which specific antibodies are tagged with a fluorescent dye. When these antibodies bind to antigens from a particular organism, they appear as green, glowing particles under a fluorescent microscope, thereby revealing the presence of the infectious agent.
Enzyme-linked immunosorbent assay (ELISA) uses tagging to identify unknown antibodies or antigens in a serum sample. In this technique, the antibodies are tagged with certain enzymes (proteins that speed up chemical reactions in the body). When an antigen and antibody bind to each other, the enzyme causes a reaction that produces a color change, thereby identifying the unknown microorganism.
Radioimmunoassay is a similar technique to ELISA that tags antibodies with radioactive material.
Purpose of the Antigen/Antibody Tests for Infectious Disease
- To aid in the diagnosis of viral, bacterial, fungal, and parasitic infections
- To monitor the course of an infection or autoimmune process
- To evaluate how protected you are against a microorganism (your immunity)
Who Performs Antigen/Antibody Tests
- A doctor, a nurse, or a laboratory technician draws the blood sample.
Special Concerns about Antigen/Antibody Tests
- Cross-reactions can occur, in which one antigen reacts with antibodies developed against another antigen, leading to false-positive results.
- False-negative results can occur in the early stages of certain infections, prior to the development of antibodies.
Before the Antigen/Antibody Tests for Infectious Disease
- No special preparation is needed for a blood test. Preparation varies for other procedures according to what type of tissue or fluid sample is required; your doctor will provide specific instructions.
What You Experience
- In most cases, a sample of your blood is drawn from a vein, usually in your arm, and sent to a laboratory for analysis.
- Occasionally, a sample of another body fluid, tissue, or stool is obtained and sent to a laboratory for analysis. These procedures may be noninvasive or invasive, depending on what type of sample is needed.
Risks and Complications of Antigen/Antibody Tests
- Possible risks vary according to what type of tissue or fluid sample is required.
After the Antigen/Antibody Tests for Infectious Disease
- Post-test care varies according to what type of tissue or fluid sample is required.
- Immediately after a blood sample is drawn, pressure is applied (with cotton or gauze) to the puncture site. Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.
Antigen/Antibody Test Results
- The blood specimen or other sample is sent to a laboratory for analysis.
- Your doctor will make a diagnosis based on these results, as well as your medical history, symptoms, physical exam, and findings from any other tests, such as a culture. Appropriate treatment will be initiated, depending on the specific infection.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media