Long-lived plasma cells (LLPCs) and memory B cells (MBCs) that have undergone class-switching and somatic hypermutation to increase Ab affinity are products of GC reactions; as these cell types and the Abs they produce are thought to be absolutely critical for maintaining life-long protection against pathogens following exposure or vaccination, or for contributing to the development of a number of autoimmune diseases, the most recent developments regarding the function of ICOS have focused on how ICOS–ICOSL interactions contribute to GC-derived Ab production. The gene discussed is ICOS; the disease is autoimmune disease.