Immune responsiveness to Mycobacterium leprae and other mycobacterial antigens throughout the clinical and histopathological spectrum of leprosy.
Immunological responsiveness was studied throughout the clinical and histopathological spectrum of leprosy (Ridley–Jopling scale) by the methods of lymphocyte transformation, leucocyte migration inhibition and delayed skin hypersensitivity, 
 
The response to Mycobacterium leprae showed by all methods a continuous decrease from strong responses in the polar tuberculoid (TT) group to virtually negative responses in the polar lepromatous (LL) group. There was a good agreement between the in vitro methods and the lepromin skin test, giving support to the latter as useful tool in the evaluation of immune responsiveness to M. leprae in leprosy patients. 
 
The immune response to BCG and PPD on the other hand, decreased only slightly towards the lepromatous pole of the spectrum, confirming the high degree of specificity of the immune defect in lepromatous leprosy. Patients grouped histologically as subpolar tuberculoid (TT/BT) reacted particularly strongly to BCG and PPD. 
 
As it is likely that the methods used mainly measured T-lymphocyte function, the clinicopathological manifestations of leprosy appear to reflect the strength of the cellular immune response against M. leprae. Thus the findings give strong support to the concept of a host-determined, immunological diseases pectrum as expressed in the Ridley–Jopling classification.