Ridley-Jopling Classification of Leprosy
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Direct answer
The Ridley-Jopling classification places leprosy along a spectrum of cell-mediated immunity, from tuberculoid (TT) at one end to lepromatous (LL) at the other, with borderline BT, BB and BL in between. Tuberculoid disease has few, well-defined lesions and no bacilli, whereas lepromatous disease has numerous diffuse lesions teeming with bacilli. The category guides prognosis, reaction risk and treatment, so every patient should be placed on the spectrum at diagnosis.
What you must remember
- TT (tuberculoid): one or a few sharply demarcated, hypopigmented or erythematous anaesthetic plaques with a raised active border; no bacilli and strong cell-mediated immunity.
- BT (borderline tuberculoid): a few lesions, less sharply defined than TT, with satellite lesions and scanty bacilli.
- BB (mid-borderline): numerous, punched-out lesions with intermediate immunity; the most unstable category, prone to reversal reactions.
- BL (borderline lepromatous): many papules, nodules and plaques with a high bacillary index, with weak cell-mediated immunity.
- LL (lepromatous): numerous, symmetrical, poorly defined papules, nodules and diffuse infiltration with abundant bacilli in macrophages, and loss of cell-mediated immunity.
- Indeterminate (I): a single or few ill-defined hypopigmented macules with few bacilli, which may resolve or evolve towards another pole.
- Bacteriological index: a logarithmic score from 0 to 6+ on slit-skin smears, rising from 0 in TT to 5+ or 6+ in LL.
- Reactions: reversal reactions in the borderline groups, and erythema nodosum leprosum at the lepromatous end.
Five categories on one spectrum
The categories are defined by immunity and bacillary load together, and the table sets them out from the tuberculoid to the lepromatous end. Candidates should memorise the direction of each change, since the bacillary index and the number of lesions move together.
| Category | Immunity | Skin lesions | Bacillary index |
|---|---|---|---|
| TT | Strong, lepromin positive | Few, sharply defined anaesthetic plaques | 0 |
| BT | Moderately strong | Few, less defined, with satellite lesions | 0 to 1+ |
| BB | Intermediate, unstable | Numerous, punched-out lesions | 2+ to 4+ |
| BL | Weak, lepromin negative | Many papules and plaques | 4+ to 5+ |
| LL | Poor, lepromin negative | Symmetrical diffuse papules and nodules | 5+ to 6+ |
Moving down the rows, lesions become more numerous and symmetrical while the bacillary index climbs, because the cell-mediated immunity that contains the bacilli weakens. Nerve involvement follows the same gradient, with TT producing early, asymmetrical nerve thickening and sensory loss, whereas LL involves nerves late and symmetrically. The table is best learned as one gradient rather than as five separate diseases, because most patients sit between two adjacent categories.
Where the exam catches students
Most traps involve confusing lesion count with bacillary load or assigning the wrong nerve pattern.
- Trap: equating the number of lesions with bacillary load Correct: TT may have a single lesion with a zero index, while LL has diffuse lesions with a high index, so the index measures bacilli.
- Trap: assuming TT is the most infectious form Correct: TT lesions contain no bacilli and it is the least infectious, while LL is the most infectious form.
- Trap: expecting early numbness in LL Correct: LL has late, symmetrical nerve involvement, while early asymmetrical nerve damage is the hallmark of TT.
- Trap: labelling a single hypopigmented patch as LL Correct: a single lesion suggests TT or indeterminate leprosy, and the bacillary index and skin test refine the category.
Frequently asked questions
What is the bacteriological index in leprosy?
The bacteriological index is a logarithmic score from 0 to 6+ based on acid-fast bacilli counted in slit-skin smears. It rises from 0 in TT to 5+ or 6+ in LL, and it helps place a patient on the Ridley-Jopling spectrum.
Which Ridley-Jopling types are most prone to reversal reactions?
Reversal reactions, also called type 1 reactions, occur mainly in the borderline categories, particularly BT and BB. They reflect an upgrade in cell-mediated immunity and cause swelling, erythema and neuritis that need urgent treatment.
Which leprosy type is the most infectious?
Lepromatous leprosy, LL, is the most infectious form because its skin and nasal lesions contain abundant bacilli in a host with poor cell-mediated immunity. Tuberculoid leprosy has no bacilli in its lesions and is the least infectious form.
Why does nerve damage in lepromatous leprosy appear late?
Nerve involvement in lepromatous leprosy is usually late and symmetrical, in contrast to the early asymmetrical nerve thickening of tuberculoid leprosy. The weak immune response in LL means little early immune-mediated nerve destruction, so damage appears later and symmetrically.