Immunoglobulins are proteins produced by the immune system to recognize, bind to, and neutralize foreign substances in the body. Immunoassays are tests based on the very specific binding that occurs between an immunoglobulin (called an antibody) and the substance that it specifically recognizes (the foreign molecule, called an antigen). Immunoassays can be used to test for the presence of a specific antibody or a specific antigen in blood or other fluids.
When immunoassays are used to test for the presence of an antibody in a blood or fluid sample, the test contains the specific antigen as part of the detection system. If the antibody being tested for is present in the sample, it will react with or bind to the antigen in the test system and will be detected as positive. If there is no significant reaction, the sample tests negative. Examples of immunoassay tests for antibodies include Rheumatoid Factor (which tests for the presence of autoimmune antibodies seen in patients with rheumatoid arthritis), or antibodies made in response to a vaccination (such as tests for antibodies to Hepatitis B to assure that the vaccination was successful).
When immunoassays are used to test for the presence of antigens in a blood or fluid sample, the test contains antibodies to the antigen of interest. The reaction of the antigen that is present in the person’s sample to the specific antibody is compared with reactions of known concentrations and the amount of antigen is reported. Examples of immunoassay tests for antigens include drug levels (like digoxin, vancomycin), hormone levels (like insulin, TSH, cortisol, and cancer markers (like PSA, CA 125, and AFP).