CD28 and autoimmune disease: It is a close homologue of CD28,122 but directly competes for CD28’s ligands, the co‐stimulatory markers CD80 and CD86, with higher affinity and avidity.123 CTLA‐4 is a potent immunoregulatory ligand, as seen in CTLA‐4 KO mice, which die of massive autoimmune disease driven by lymphocyte overproliferation.124, 125