Unified by:- Absence of rheumatoid factor- Association with HLA-B27 - Pathologic changes in the ligamentous attachments rather than synovium- Involvement of sacroiliac joints

Ankylosing Spondylitis- destruction of articular cartilage and bony ankylosis, especially of the sacroiliac joint

Reactive Arthritis- Triad of arthritis, non-gonococcal urethritis/cervicitis, and conjunctivitis- prior infection: urethritis (Chlamydia) or diarrhea (Shigella, Salmonella, Campylobacter, Yersinia)- asymmetric pattern- ankles, knees, feet

Psoriatic Arthritis- affects peripheral and axial joints and enthesis- distal interphalangeal joints (RA: proximal)- asymmetric- 'pencil in cup' deformity