PF4 and autoimmune polyendocrinopathy: In patients with APS, anti-β2-GPI–IgG can bind to circulating β2-GPI, creating stable complexes with platelet product factor 4 (PF4), a small chemokine belonging to the CXC chemokine family that is also known as chemokine (C-X-C motif) ligand 4 (CXCL4) [10].