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Mechanism of Action: Abatacept is a fusion protein that combines the extracellular domain of the molecule CTLA4 (CD154) with the Fc portion of a human immunoglobulin molecule. CTLA4 has very high affi nity for CD28. When abatacept binds to CD28 on the T cell surface, it prevents the second signal from being delivered, thus turning down the T cell response. Additional effects are decreasing the production of T cell derived cytokines including TNF.