Activin AB

Where it comes from

Made in

Wherever both chains are made at once: ovary, pituitary, placenta

Sent out by

With activins A and B

Acts on

• Pituitary
• Follicles

What it does

• Stimulates FSH like the other activins, and follistatin neutralises it as it does them ¹
• In cell work it switches on the bone morphogenetic protein branch, SMAD1/5, far more strongly than activin A does, which is the one hint that the mixed dimer is not simply a blend of the two ¹
• A minor species in people: a specific assay found it in bulk in cow, sheep and pig follicular fluid and at about a two-hundredth of that in human follicular fluid ²

In medicine

• Nothing is measured or treated: no assay used in care separates it from activin A and activin B
• The approved activin traps catch it as they catch the other two, so it is covered by treatment without ever being named

Beyond the body’s own use

• It can be told apart in the laboratory. A two-site assay cross-reacts less than 0.2 per cent with the other activins, the inhibins and follistatin, so the limit is clinical interest, not chemistry ²
• Followed through six whole pregnancies it was undetectable at every visit while activin A rose 22-fold, which is the clearest evidence that the heterodimer is not a circulating hormone in people ³