Having found out about the ABO-system blood transfusions went a lot better. Still some patients died during the transfusion. Years later scientist found another blood antigene on the blood => The Rhesus Antigene
All humans have the Rhesus-Factor(Antigene) or they don‘t. Humans with the Rhesus-Factor are called „Rhesus positive (Rh+)“ or „Rhesus negative (Rh-)“.
Giving Rh+ blood to Rh- Patients makes the blood clot too, as the body builds antibodies against the Rh+ blood.