Rheumatoid arthritis is one of the most prevalent chronic inflammatory diseases. It primarily involves the joints, but should be considered a syndrome that includes extraarticular manifestations, such as rheumatoid nodules, pulmonary involvement or vasculitis, and systemic comorbidities.Rheumatoid arthritis is a chronic disease that carries a substantial burden for both the individual and society. Rheumatoid arthritis has an incidence of 0·5% to 1%, with an apparent reduction from north to south (in the northern hemisphere) and from urban to rural areas. Modern genetic technologies combined with large, well-characterised clinical cohorts have advanced our understanding of the genetics of the disease.Joint swelling in rheumatoid arthritis reflects synovial membrane inflammation consequent to immune activation, and is characterised by leucocyte infiltration into the normally sparsely populated synovial compartment. Cytokines and chemokines lead to the induction or aggravation of the inflammatory response by activating endothelial cells and attracting immune cells to accumulate within the synovial compartment.

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