Clinical meaning
Behcet disease (BD) is a chronic, relapsing systemic vasculitis of unknown etiology that can affect blood vessels of all sizes (small, medium, and large, both arteries and veins), making it unique among vasculitides. BD classically presents with the triad of recurrent oral ulcers, genital ulcers, and ocular inflammation (uveitis), but can involve virtually any organ system including the vascular, neurological, gastrointestinal, musculoskeletal, and pulmonary systems. The disease has a striking geographic distribution along the ancient Silk Road, with highest prevalence in Turkey (420 per 100,000), Iran, Japan, Korea, and Mediterranean countries. The pathogenesis of BD involves a complex interplay between genetic susceptibility, innate immune dysregulation, and environmental triggers. The strongest genetic association is with HLA-B51 (present in 50 to 70% of BD patients versus 15 to 20% of the general population in endemic areas), a class I MHC allele that presents intracellular peptide antigens to CD8+ cytotoxic T cells. HLA-B51 may promote BD by presenting specific microbial or self-peptide antigens that trigger cross-reactive immune responses, or by misfolding in the endoplasmic reticulum and activating the unfolded protein response, leading to...