Donating blood is typically thought of as a commensal act, benefiting the recipient at no cost beyond time and inconvenience to the donor. Some even view it as a parasitic act, wherein the recipient benefits at some health cost to the donor. But it seems reasonable to posit the opposite.
Iron has been suggested as the reason why women have notably lower incidence of cardiac events than men, until menopause that is. Why? Because some blood loss every 28 days helps to reduce iron levels in the blood. Similar logic holds with metabolic syndrome and type 2 diabetes.
Additionally, treatment-resistant high blood pressure responds — unsurprisingly — quite well to blood loss, as a bit less blood means, all other things being equal, proportionately lower pressure.
But this is speculation. Is there any legitimate evidence of the health benefits of blood donation?