Immunology of leprosy

Cell Medical Immunity (CMI) is determined by T lymphocytes and is responsible for defence against M. leprae. Most patients escape leprosy since they posses adequate cell mediated defence against M.leprae. A percentage get an early transient phase known as "indeterminate" from which most of them have spontaneous resolution; a minor percentage evolve in to definite types of leprosy. The type in the clinical spectrum of leprosy in to which patients evolve depends upon the degree of CMI against M.leprae. Patients with higher CMI evolve into the tuberculoid side of the spectrum and patients with lower CMI evolve towards the lepromatous spectrum. When the absence of CMI is absolute they develop lepromatous leprosy. In lepromatous leprosy the absence of CMI is only with reference to M.leprae and not a generalised depression of CMI covering other organisms also.

Lepromin test : This is an intradermal skin test performed with the antigen of M.leprae to demonstrate the presence of CMI towards Myco leprae. It is not of diagnostic value since it is positive in the mild form of other disease (tuberculoid) but is absent in the severe forms of the disease (lepromatous). It is of prognostic value and helps in classification of leprosy.