Abatacept is a selective inhibitor of T cell activation. A fully human fusion protein consisting of the extracellular domain of cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) together with a modified Fc portion of human IgG1, it competitively binds to CD80 or CD86, inhibiting T cell activation and potentially affecting cytokine production as well as cellular aspects of autoimmunity. Abatacept is effective in treatment-resistant rheumatoid arthritis in adults and now a multicentre trial in Europe, Latin America and the USA (Nicolino Ruperto and colleagues. Lancet 2008;372:383–91; see also Comment article, ibid: 348–50) has shown that it …