Immediately after leprosy treatment initiation, changes in leprosy lesions also commence. The histopathological and bacilloscopic characteristics of the regressing lesions undergo continuous changes over years or decades. It is important to recognize these changes as they allow for the assessment of whether a particular lesion is in regression or if there are signs of disease reactivation. Interpretation of the findings will depend on a close correlation among histopathological patterns, bacilloscopic characteristics, and clinical data. This chapter discusses the main factors that allow the recognition of the histopathological characteristics of regressive phenomena from initial phases to late or residual phases, the changes typical in regressive leprosy granulomas and their importance for assessing treatment effectiveness, the reaction phenomena triggered after treatment initiation on regressing lesions, and the evaluation of leprosy recurrence. Further, this chapter includes a discussion on the main differential diagnoses of leprosy regression and relapse.
Keywords: Hansen's disease, Leprosy, M. leprae, Regression, Relapse.