Activin A was initially identified in ovarian fluid as a stimulator of FSH secretion from anterior pituitary cells. Activin A is a member of the TGF-β1 family which includes TGFs, activins, Nodal, growth and differentiation factors (GDF), and bone morphogenetic proteins (BMPs). Activin A is a dimer of the beta A subunits of human inhibin; the beta A subunit binds the beta B subunit of inhibin to form Activin AB. The beta B subunit also exists as a heterodimer to form Activin B. Activin A binds two type I and two type II serine/threonine kinase receptors at the same time. The type II receptor kinase is constitutively active and phosphorylates the type I receptor after the formation of the ligand-receptor complex. Five type II receptors have been identified: ActRIIA, ActRIIB, BMPRII, TGFβRII, and MISRII. ActRIIB binds to Activin A, BMP-2, BMP-7, GDF-8, and GDF-11. The type I receptors are named activin receptor-like kinases 1 to 7 (ALK1-7), and Activins bind to ALK4 and ALK7. Follistatin (FS) is a natural Activin A antagonist; FS neutralizes its biologic activities by preventing Activin A interaction with its receptors. Activin A has pleiotropic functions. It is a morphogen during embryonal development, regulating chondrogenic and osteogenic differentiation of mesenchymal progenitor cells (MPC). Activin A has been associated with neuroprotection, apoptosis, and fibrosis. Also, high levels of Activin A have been found in synovial fluid from patients with rheumatoid arthritis, inflammatory bowel disease, and gout. In addition, it has been speculated that Activin A, in collaboration with TGF-β1 and IL-25, promotes Th9 generation and the development of allergic pathology. Mature human Activin A shares 100% amino acid sequence identity with bovine, feline, mouse, porcine, and rat Activin A.