A Clinical Application for Immunology

In class this Tuesday, we spoke briefly about immunological assays and how they have become widely used in the clinical world. Because I recently worked in a clinical lab using these techniques, I thought I would share the large part that they actually played.
Before coming to UCD, I worked for a clinical lab that concentrated on organ transplantation. When most people think of immunology and organ transplants, we think of anti-rejection drugs and antibody cross-matching (such as blood typing). As a part of the Infectious Disease department, our job was to make sure that organ donors did not have any infectious diseases (such as HIV and Hepatitis). It may not be inherently obvious as with cross-matching, but immunology plays an important role in this process. To do this, we used a process called Enzyme Immunoassays, or EIAs. This process is basically an antibody-antigen reaction.
When testing for these infectious diseases, the easiest, fastest, and most generally reliable method is to look in the blood for the antibody. A manufactured protein, similar in structure to the disease antigen is affixed to the bottom of a plate. When the serum or plasma of a patient is added, after a period of incubation, any antibody present in the patient’s blood will become bound to the antigen. Everything else is washed away, leaving only the antibody-antigen complex. This complex can be conjugated to a color development reagent, allowing visible representation of the amount of antibody in the blood.
This method of testing is very fast and efficient. If needed to be done quickly, all of the tests required by the FDA for whole organ transplantation can be done within six hours. (This is very handy for someone on a list waiting for an organ.) It is also possible to perform this kind of test on multiple patients at one time, as they can be done on 96-well plates. Unfortunately, some down-sides become apparent with this method of testing. For one, it relies on antibodies made to viruses, instead of the actual virus. There is a period time called the “window period” in which the virus is in the body, but our immune system has not been able to make a detectable amount of antibody. It is rare, but possible that an organ donor could have died without knowing they had recently contracted a disease, and their blood was drawn for testing during this window period. A solution for this is a method of Nucleic Acid Testing (NAT) called Transcription Mediated Amplification which detects the genetic material of viruses using a technique similar to PCR. This can detect the presence of a virus much earlier than EIA testing, but is not available for all infectious diseases.
Another drawback to using EIAs to detect diseases is the phenomenon of cross-reaction, which we also discussed in class. As you might imagine, there are proteins that are similar in structure to antibodies. Cross reactivity is one of the main causes of false positive results in EIAs. For example, if a blood sample is hemolyzed (the red blood cells have lysed), the protein fragments released could become bound to the antigen and the patient would appear to be reactive. There is not much that can be done about this, especially if a person has a medical condition that causes them to produce proteins similar to the antigen being tested for. As more nucleic acid testing becomes available, it will be easier to detect diseases, and fewer organs will have to be discarded because of false positive results.
If anyone happens to comment on this or have a question, I'll be out of town for the rest of the month :)