Introduction: Mast cells are responsible for secreting cytokines and other chemical mediators, involved in immuno-inflammatory responses. However, their role has not been clearly defined in the immunopathogenesis of leprosy. Aims and Objectives: To count the number of mast cells in leprosy granulomas and to correlate their numbers with the serum levels of IL-4 in borderline spectrum of leprosy. Materials and Methods: Thirty cases of newly diagnosed, untreated patients of borderline tuberculoid (BT), borderline borderline (BB) and borderline lepromatous (BL) leprosy were included in the study. Skin biopsies were taken from the margins of the skin lesion of leprosy and stained with toluidine blue for quantification of mast cells. An average number of mast cells per granuloma was calculated after examining the granulomas in each skin biopsy. Concentration of serum IL-4 was measured quantitatively in the collected venous blood samples by ELISA method. Results: The mean of average number of mast cells increased from BT towards BL leprosy. Statistically significant difference was seen between BL and BT/BB leprosy. However, no significant difference was found between BB and BT Hansen’s. A positive correlation was found between mast cell numbers and serum levels of IL-4 since both showed an increasing trend from BT through BL leprosy. Conclusion: We found a progressive rise in mast cell count and serum levels of IL-4 across the borderline spectrum of leprosy. This indicates that mast cells play a role in progression and dissemination of leprosy by proliferating at the site of inflammation and secreting an array of chemical mediators especially IL-4.

Keywords: Borderline Leprosy; Mast Cells; Granuloma; Interleukin 4; Tuberculoid; Lepromatous.