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 leprae or "Hansen's Bacillus") under the skin. It is not recommended as a primary mode of diagnosis.

Positive reaction means. A) 10mm or more induration after 48hrs B) 5mm or above nodule after 21 days. 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 upward. The lump indicates that the antigen has been injected at the correct depth. The injection site is labeled and examined at 48hours and 21 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.

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. This is because lepromin only tests for infection, not for ongoing disease. It is believed that most people exposed to Mycobacterium leprae are not infected and thus would not respond, or are infected but self-resolve or never manifest overt symptoms and therefore would respond to the lepromin skin test. Paradoxically however, patients with "lepromatous" (Virchowian) HD, the most severe and transmissible form, have no skin reaction to the antigen. This is because an effective immune response to the bacterium is a result of a cellular immune response (T-cell mediated) rather than a humoral response (B-cell/antibody). Lepromatous HD, the more severe and disfiguring form is a result of the patient's immune response being mainly humoral in nature. Antibodies, the main effectors of a humoral response, are ineffective against M. leprae because of the unusually dense and waxy nature of the mycolic acid containing bacterial cell wall, and so the bacterium proliferates, causing the cutaneous disfigurements and peripheral neuropathologies characteristic of the disease. The reason there is little or no response to the lepromin test is that a positive response to lepromin is due to "delayed type hypersensitivity" which is T-cell mediated, and it is the failure of a robust T-cell response that results in the onset of lepromatous leprosy in the first place. However, given the severe nature of lepromatous leprosy, a skin test is unnecessary, and the definitive test, a biopsy, readily reveals the bacterium within lesions as well as the characteristic histopathology of HD. Moreover, lepromatous HD is typically diagnosed on clinical presentation alone.

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