Delayed Hypersensitivity and the Lepromin Test

Q: What is the reason for delayed hypersensitivity in leprosy and what is lepromin test?
A: Delayed hypersensitivity in Leprosy is a type 4 hypersensitivity reaction which is a part of the cell-mediated immune response to the Mycobacterium antigens.
Humans generally have a high degree of immunity to lepra bacilli which manifests as both humoral and cell mediated immunity. The cell-mediated immune response is important in determining the type of leprosy that will develop in the patient.

A patient with adequate cell-mediated immunity will develop the tuberculoid type of leprosy and will have the delayed hypersensitivity reaction (which can be tested by the lepromin test) where as one with deficient cell mediated immunity will develop the lepromatous form of leprosy.

Lepromin test: It is a skin test similar to the Mantoux test for tuberculosis to detect the cell mediated immunity in leprosy.

Procedure: The test is performed by injecting 0.1 ml of lepromin into the forearm of an individual. The reaction is read at 48 hours and 21 days.

Early Reaction ( Fernandez reaction): The diameter of the red area(not induration) that develops is measured. A positive reaction has a diameter of more than 10mm. It is analogous to the tuberculin reaction but is poorly defined and carries little significance.

Late Reaction (Mitsuda) : At the end of 21 days if there is a nodule with a diameter of a more than 5mm it is considered positive. The late reaction does not indicate pre-existing delayed type hypersensitivity but is a measure of the cell mediated immunity induced by the injected Lepromin. Hence it distinguishes between people who can mount a response against the Lepra Bacilli and those who can't.

Importance of Lepromin test:
  • It is not a diagnostic test, nor does it indicate prior contact with the lepra bacilli.
  • It is used to classify the lesions of leprosy patients. Lepromin test is positive  in tuberculoid, negative in lepromatous and variable in borderline forms of the disease.
  • It is also used to assess prognosis and response to treatment in patients of Leprosy. Lepromin negative individuals are at a higher risk of developing multibacillary disease, those who are lepromin positive either don't get the disease( in most cases) or may develop paucibacillary illness (minority).
  • Another place where the lepromin test is employed is to identify people who may be employed in leprosaria. Lepromin positive individuals will have lesser chances of developing disease and hence can be employed here.




P.S: It is important to note that this deficiency of cell mediated immunity being talked about is specific to the lepra bacillus antigens. Patients suffering from lepromatous leprosy are not more susceptible to viruses parasites or other pathogens where CMI is important.




Answered by Pranav Hinduja

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