Many clinical laboratory tests utilize the basic immune principles of antigen-antibody detection. Almost anything can be tested for antibodies against that substance can be developed. The most significant use of antibody-antigen testing has been in the blood bank, where numerous antisera detect molecules ("antigens") on the red blood cell surface. This forms the basis of detecting major and minor blood types and detecting if the patient has formed an antibody to a specific RBC antigen. Similarly, MHC/HLA antigens are tested for when typing is done for tissue transplants.
Some oldest tests are serological tests based on antibody/antigen reactions. A screening test for syphilis, the rapid plasma reagin (RPR) test, detects a patient's antibodies complexing with cardiolipin antigen. Many other bacterial, viral, and fungal infections can be tested using the same basic principles. One of the oldest tests for syphilis was a complement fixation test, known as the Wasserman test.
Besides antibody/antigen reactions, other basic immune system concepts are also utilized in laboratory testing. The flow cytometry technique can detect any membrane markers on immune cells. CD4 T cell and CD8 T cell counts are done on patients with HIV. Many leukemias can also be diagnosed based on the knowledge of membrane markers on the immune cells.
