"Why do we have different blood types?" asks Doug from Norfolk.
The average adult human has around 30 trillion red blood cells, they make up a quarter of the total number of cells in the body.
We have dozens of different blood groups, but normally we're tested for just two - ABO and Rhesus factor. Adam and Hannah delve into the gory world of blood and the early history of blood transfusions, to discover why we have blood groups and what makes them so important.
Featuring interviews with Dr Jo Mountford, from the Scottish National Blood Transfusion Service and immunologist Dr Sheena Cruikshank from the University of Manchester.
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Presenters: Hannah Fry, Adam Rutherford Producer: Michelle Martin.