Borderline Tuberculoid Leprosy.
In BT disease, immunologic resistance is strong enough to restrain the infection, in that the disease is limited and bacillary growth retarded, but the host response is insufficient to self-cure -. These patients are somewhat unstable— resistance may increase, upgrading to TT, or decrease, downgrading to BL.
The primary skin lesions of BT are plaques and papules. As in TT, an annular configuration is common, and both borders are sharply marginated, but annular lesions or plaques may have sharply marginated satellite papules . Hypopigmentation may be conspicuous in darkly pigmented patients. In contrast to TT, typically there is little or no scaling, less erythema, less induration, and less elevation, but lesions may become much larger , that is, well over 10 cm in diameter, a single lesion sometimes involving an entire extremity over time. Multiple, asymmetric lesions are the rule, but solitary lesions are not rare. Impaired sensation in skin lesions is the rule and nerve trunk involvement, enlargement, or palsies, usually asymmetric and affecting no more than two nerves, are common. Nerve abscesses, when they occur, are most often seen in males with BT disease .
Histologically, lymphocytic mantles are less well developed, Langhans cells are less common or absent, and any exocytosis is focal. Acid-fast bacilli and plasma cells are rare; if present, a DTH reaction should be suspected.