CD28 and rheumatoid arthritis: Abatacept (ABA) is a fusion protein (CTLA4-Ig) that has proven useful in the treatment of RA; its CTLA4 portion binds CD80 and CD86, the CD28 ligands, on antigen-presenting cells, and competing with the engagement of CD28 on T cells, the drug influences the subsequent T-cell activation [6].