Reactive arthritis is a seronegative spondyloarthropathy developing 1-4 weeks after genitourinary (Chlamydia) or enteric (Salmonella, Shigella, Yersinia, Campylobacter) infection. Pathogenesis involves molecular mimicry in HLA-B27 positive individuals (60-80%). Bacterial antigens persist in the synovium, driving CD8+ T cell and Th17 inflammatory responses. Classic triad: arthritis + urethritis + conjunctivitis ('can't see, can't pee, can't climb a tree'), though complete triad present in ~30%. Arthritis is typically asymmetric oligoarthritis of lower extremities with enthesitis and dactylitis. Mucocutaneous features: keratoderma blennorrhagica (palms/soles — histologically identical to pustular psoriasis), circinate balanitis, oral ulcers. Generally self-limiting (3-12 months) but chronic in 15-20%, especially HLA-B27 positive patients.