CD28 and rheumatoid arthritis: T cells are an important target for immunotherapy in different autoimmune diseases due to an aberrant expression of their co‐stimulation molecules that leads to the loss of self‐tolerance.12 The importance of the costimulatory molecules CD28 and CTLA‐4 in the pathologic mechanism of RA has been demonstrated by genetic associations and the successful clinical application of CTLA‐4Ig for the treatment of RA.19