FAS and autoimmune disease: SAA is believed to be an autoimmune disease attributed to hematopoietic cell destruction by activated cytotoxic T lymphocytes (CTLs), causing HSPCs impairment through the Fas/FasL pathway and secreted perforin and granzyme B.[19,20,21] NK cells are an important component of the innate immune system and play a core role in the regulation of adaptive immunity, which could inhibit antigen-presenting cells (APC), such as DCs and Tregs in patients with autoimmune diseases.[22,23,24]