Many clinical laboratory tests utilize the basic immune principles of antigen-antibody detection. Almost anything can be tested for if antibodies against that substance can be developed. Perhaps the greatest use of antibody-antigen testing has been in the blood bank where numerous antisera are utilized to detect molecules ("antigens") on the red blood cell surface. This forms the basis of detecting major and minor blood types, as well as 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 of the oldest tests are serological tests based on antibody/antigen reactions. A screening test, the Rapid Plasma Reagin test (RPR) for syphilis detects patient's antibodies complexing with cardiolipin antigen. Many other bacterial, viral, and fungal infections can be tested for using the same basic principles. One of the oldest test for syphilis was a complement fixation test, known as the Wasserman Test.
Besides antibody/antigen reactions, other basic concepts of the immune system are also utilized in laboratory testing. The technique of flow cytometry can detect any number of membrane markers on immune cells. CD4 T cell and CD 8 T Cell counts are done on patients with HIV. Many leukemias can be diagnosed based on the knowledge of membrane markers on the immune cells as well.