# Leprosy Pathology > Leprosy pathology for MBBS Pathology: Ridley-Jopling spectrum, lepra reactions type 1 and 2, nerve predilection, NLEP multidrug therapy. - Canonical URL: https://prepelephant.com/topics/mbbs/pathology/leprosy-pathology - Exam / course: MBBS · Subject: Pathology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Leprosy Pathology", PrepElephant, https://prepelephant.com/topics/mbbs/pathology/leprosy-pathology ## Direct answer Leprosy is infection with Mycobacterium leprae, an unculturable, acid-fast bacillus of cooler tissues, whose entire clinical spectrum is set by host cell-mediated immunity — from tuberculoid leprosy (single anaesthetic lesions, strong immunity, paucibacillary) through borderline forms to lepromatous leprosy (symmetrical plaques and nodules, systemic involvement, multibacillary). Nerves are the signature target: dermal and peripheral nerve involvement at cool superficial sites produces the anaesthesia, claw hand, foot drop and ulcerations that define the disease's deformity. The lepra reactions — type 1 (reversal, cell-mediated, neuritis with oedema of lesions) and type 2 (erythema nodosum leprosum, immune-complex, systemic) — are the emergencies of the disease, and India, which achieved elimination as a public-health problem in 2005, still reports more new cases than any other country. ## What you must remember - **Ridley-Jopling spectrum:** tuberculoid (TT) — few well-defined anaesthetic lesions with thickened nerve, smears negative; borderline tuberculoid (BT) — more lesions with satellites; midborderline (BB); borderline lepromatous (BL); lepromatous (LL) — symmetrical plaques, nodules and infiltration, smears strongly positive; indeterminate lesions precede them all. - **Why cooler sites:** M. leprae grows best at temperatures below core body temperature — skin, nasal mucosa, testes, anterior eye, superficial peripheral nerves; hence Leonine facies, nasal collapse, testicular atrophy and iritis in LL. - **Histology across the spectrum:** TT — well-formed epithelioid granulomas invading dermal nerves, no visible bacilli; LL — sheets of foamy Virchow macrophages packed with bacilli (globi), nerves replaced. - **Nerves first:** ulnar nerve at the elbow, median at the wrist, common peroneal at the fibular neck, posterior auricular and great auricular — the exam list; thickened, tender nerves with anaesthetic skin are leprosy until proven otherwise. - **Type 1 (reversal) reaction:** cell-mediated upgrade, usually after treatment starts in borderline disease — existing lesions redden and swell, neuritis threatens function; treat with corticosteroids urgently. - **Type 2 reaction (erythema nodosum leprosum):** immune-complex systemic reaction of LL/BL after therapy — crops of tender nodules, fever, iritis, orchitis, neuritis; treat with thalidomide or corticosteroids. - **Multidrug therapy (WHO/NLEP):** paucibacillary — rifampicin monthly plus dapsone daily for 6 months; multibacillary — adds clofazimine, 12 months; monotherapy is forbidden to prevent resistance. - **Diagnosis:** slit-skin smears from ear lobes and lesions for the bacillary index, biopsy with modified Fite stain, and loss of sweating and sensation in lesions. - **Indian frame:** elimination (under 1 case per 10,000 population) achieved in December 2005, yet India still reports a majority of the world's new detections. ## A patch that does not sweat A 30-year-old farm labourer reports a hypopigmented patch on his back that has not sweated for months and cannot feel the pinch of a pin; a nearby cut has healed painlessly. Examination shows a well-defined anaesthetic patch with a thickened cutaneous nerve at its edge and an enlarged, tender ulnar nerve at the elbow. This is tuberculoid leprosy in its purest teaching form — strong cell-mediated immunity localising infection to one patch and one nerve, smear-negative because immunity is outrunning bacterial multiplication; the biopsy would show epithelioid granulomas tracking down a dermal nerve with no visible bacillus. Six months of paucibacillary multidrug therapy cures the infection; the anaesthesia is permanent, so deformity management begins where antibiotics end. At the other end of the same spectrum, a man presents with years of symmetrically infiltrated ears, brows and nasal mucosa, a saddle-nose collapse, glove-and-stocking anaesthesia and nodular skin loaded with bacilli: lepromatous leprosy, the anergic end, where smears teem with globi-filled Virchow cells and therapy runs 12 months. Borderline patients sit between and are the ones whose disease reacts — within weeks of starting drugs, a BT patient's lesions swell angry red with neuritis (type 1, steroids the same day or the nerve is lost), while an LL patient spikes fevers with crops of tender nodules and orchitis (type 2, thalidomide). Reactions, not the infection, take most nerves and eyes — the sentence that organises the chapter. ## Where students slip The spectrum is memorised but reversed under stress; anchor with immunity — more immunity means fewer bacilli and fewer lesions (TT), less immunity means bacilli everywhere (LL). Anaesthesia is attributed to skin involvement alone; the lesion is anaesthetic because dermal nerves are invaded — the granuloma in TT is perineural by definition. Type 1 and type 2 reactions are swapped; type 1 is cell-mediated swelling of old lesions in borderline disease, type 2 is immune-complex nodules with fever in lepromatous disease — thalidomide belongs to type 2, steroids urgently to type 1. Finally, "elimination" is quoted as eradication; elimination meant prevalence below 1 per 10,000, and transmission continues. ## Frequently asked questions ### What determines the tuberculoid-to-lepromatous spectrum in leprosy? The strength of host cell-mediated immunity — strong in tuberculoid (paucibacillary, granulomas, negative smears), weak in lepromatous (multibacillary, foamy Virchow cells with globi). ### What is a type 1 lepra reaction and its emergency? Reversal reaction — cell-mediated, in borderline disease, lesions redden and swell with acute neuritis; immediate corticosteroids prevent permanent nerve damage. ### What is the WHO multidrug therapy for paucibacillary and multibacillary leprosy? Paucibacillary: monthly rifampicin with daily dapsone for 6 months; multibacillary: adds daily clofazimine with monthly supervised rifampicin and clofazimine for 12 months. ### Why does Mycobacterium leprae favour skin, nasal mucosa, testes and superficial nerves? It replicates best at temperatures below core body temperature, so cooler superficial tissues bear the multibacillary burden.