How does bloodletting work?

Excess blood cells forming conditions such as polycythemia are often treated by blood letting.So also in iron poisoning.Exchange transfusion used in many disaese also involve initial blood letting and transfusion with another persons blood.As such a normal person may not have any benefits from blood letting,so dont do it for such benefit.But if a person letting blood for donation is to be encouraged as it saves life of the recipient and does not harm the normal donor.Body has remarkable power to replace lost blood quickly.