A patient with polymorphic light eruption (PLE) asks about long-term preventive measures. The mechanism by which hardening (photohardening) therapy prevents PLE flares in subsequent summer seasons is:
- A Repeated low-dose UV exposure induces immunological tolerance (reduction in UV-sensitive Langerhans cells and regulatory T cell induction), desensitising the photoimmune response ✓
- B Tanning increases melanin which blocks UV absorption
- C Chronic UV exposure permanently depletes Th17 cells
- D Skin thickening from photohardening physically blocks UV penetration
Explanation
PLE is caused by a delayed hypersensitivity reaction to UV-induced photoallergens. Photohardening (graduated UV exposure using NBUVB or UVA1 in spring before natural sun exposure) prevents PLE by inducing immunological tolerance: repeated UV exposure depletes and functionally suppresses photosensitive Langerhans cells, induces regulatory T cells (Tregs) in the skin, and shifts the immune response away from the photoallergic reaction. Melanisation contributes minimally — the primary mechanism is immunosuppressive, not mechanical. This is analogous to allergen immunotherapy.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.