Rheumatoid arthritis:First-degree relatives of patients with RA have a 1.5-fold increased risk of developing RA compared with the general population. Monozygotic twin studies found a concordance rate for RA of 12–15%. An increased prevalence of RA is present in a subset of populations with the presence of HLA-DR4 (Western European descent) and HLA-DR1 or HLA-DR10 (Spanish, Basque, and Israeli descent). RA susceptibility is associated with the third hypervariable region of DR1b-chains from amino acids 70–74 referred to as the “shared epitope” (QKRAA, QRRAA or RRRAA) and is associated with both susceptibility and severity of RA.Multiple diarthrodial joints (with free motion) in a symmetrical distribution. In early disease, the MCP, PIP, wrist, and MTP joints are involved. Larger joints of the upper and lower extremities, such as the elbows, shoulders, ankles, and knees, are also commonly affected, although symptoms may appear later. Less common are cervical spine, temporomandibular, and sternoclavicular joint involvement. Joints that are very uncommon in RA include the distal interphalangeal (DIP) joints and thoracic and lumbar spin
