Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory polyarthritis driven by autoreactive T cells, B cells producing rheumatoid factor and anti-CCP antibodies, and pro-inflammatory cytokines (TNF-alpha, IL-6, IL-1). The inflamed synovium (pannus) invades cartilage and bone, causing erosions and deformity. Treatment follows a treat-to-target strategy aiming for remission (DAS28 < 2.6) or low disease activity within 6 months, escalating every 3 months if targets are not met. The ACR/EULAR algorithm: (1) Methotrexate first-line (anchor drug), 10-15 mg weekly escalated to 25 mg; (2) If inadequate at 3 months: add TNF inhibitor (adalimumab, etanercept) or targeted synthetic DMARD (JAK inhibitor); (3) If first biologic fails: switch mechanism (anti-IL-6 tocilizumab, B-cell depletion rituximab, T-cell modulator abatacept). Methotrexate combined with biologics reduces immunogenicity. Early aggressive treatment prevents irreversible damage.