What was one of the main challenges with blood transfusions before the 20th century?

One of the main challenges with blood transfusions before the 20th century was the mismatch of blood types. Prior to the discovery of blood groups by Karl Landsteiner in 1901, medical practitioners did not understand that not all blood was compatible. This lack of knowledge often led to severe reactions or fatalities during transfusions when incompatible blood was mixed, making it a widespread risk. It wasn't until the identification of the ABO blood group system that the safety and effectiveness of blood transfusions significantly improved, allowing for proper matching of donors and recipients.

While challenges such as finding donors, pain during the procedure, and the risk of infection from unsterilized equipment were relevant issues in the context of medical procedures, the critical issue of blood type mismatch was a unique factor that directly impacted the success and safety of blood transfusions at that time.