Reactive arthritis is inflammatory arthritis developing after infection elsewhere in the body, typically genitourinary or gastrointestinal. The immune system activated against the infection cross-reacts with joint tissues through molecular mimicry. It appears 1-4 weeks after the triggering infection, primarily affecting large joints of the lower extremities. The classic triad is arthritis, urethritis, and conjunctivitis. It is associated with the HLA-B27 gene and is more common in young adults. Most cases resolve within 3-12 months, but some develop chronic arthritis.