Lepromin

The lepromin skin test is used to determine what type of leprosy a person has. It involves the injection of a standardized extract of the inactivated "leprosy bacillus",(Mycobacterium lepraeor "Hansen's Bacillus") under the skin. It is not recommended as a primary mode of diagnosis.

How the test is performed
An extract sample of inactivated Hansen's Bacillus is injected just under the skin, usually on the forearm, so that a small lump pushes the skin up. The lump indicates that the antigen has been injected at the correct depth. The injection site is labeled and examined 3 days and 28 days later to see if there is a reaction.

People with dermatitis or other skin irritations should have the test performed on an unaffected part of the body. If a child needs to have this test performed, it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar the child is with what will happen and why, the less anxiety he or she will feel. When the antigen is injected, there may be a slight stinging or burning sensation. There may also be mild itching at the site of injection afterwards.

Normal values
People who don't have clinical leprosy (= Hansen's Disease, or HD) may have little or no skin reaction to the antigen, or may have a strong reaction to it. Patients with "lepromatous" (Virchowian) HD have no skin reaction to the antigen. This is because "lepromatous" patients' immune system specifically recognises, not the Hansen's bacillus, but instead the plasmid (or "transposon", or "ultravirus") that it carries attached to the inside of its cell wall. In other words, this group of the population has a specific anergy to Hansen's bacillus itself, the "price" for recognition of the associated neuron invasion threat that the bacillus carries. The plasmid harboured by Hansen's bacillus contains a replicator gene which it must share with its procaryote host, and a neuron invasion gene. Virchowian ("lepromatous") patients' macrophages (Virchow cells) stop the plasmid escaping from the bacillus into their peripheral nerves which would result in nerve damage. How does this immune defense system work? Once the Hansen's bacilli are engulfed by Virchow cells, the Virchow cell effects detachment of the plasmid from the bacterial cell wall, so that the threat of nerve invasion is averted. The detached plasmid re-integrates into the Hansen's Bacillus chromosome, allowing the bacillus to divide unchecked (resulting in "lepromatous" = Virchowian - skin nodules).

By contrast, a positive Fernandez reaction is seen in the skin of patients with tuberculoid and borderline tuberculoid Hansen's Disease. Patient's with this form of HD have an immune system that does not recognise the molecular parasite (plasmid) carried at the surface of the Hansen's bacillus lodged in their skin, but, instead, recognises the bacillus itself as foreign. Thus, the plasmid, with its replicator gene, is not prevented from moving out of the bacillus into neighboring human nerves where it replicates, damaging those nerves and causing the tuberculoid HD clinical picture. The bacillus, meanwhile, is rendered unable to divide (having lost its replicator gene) and is destroyed by the immune system in the normal way.

Risks
There is an extremely small risk of an allergic reaction which may include itching and rarely hives.

Source
Lepromin skin test. Medline Plus Medical Encyclopedia. Update Date: 8/15/2003. By Daniel Levy, M.D., Ph.D., Infectious Diseases, Greater Baltimore Medical Center, Baltimore, Maryland, USA. Review provided by VeriMed Healthcare Network. Text in the US Federal Government public domain.