There are four main blood types: A, B, AB and O. The system for classifying them came about in order to allow safe blood transfusions. Like all cells, red blood cells have molecules on their surface called ‘antigens’. People have different antigens, depending on their genes. We need to pay attention to red blood cell antigens because if you receive a transfusion from someone with different ones, your immune system will attack them, which can cause kidney failure and lung problems.
The Austrian doctor Karl Landsteiner identified the most common blood antigens in 1901, which he called A and B. He also found that some people had neither antigen (type O), and in 1902 two of his students discovered that some people have both antigens (type AB).
In 1937, Landsteiner and his colleague Alexander Wiener discovered another antigen, which they called ‘Rhesus factor’ because of its similarity to an antigen in rhesus monkeys. If you have this antigen, now known as RhD, your blood is ‘RhD positive’; if you don’t, your blood is ‘RhD negative’.
Since then, medics have found more ways of categorising blood types: there are currently 36 systems in total, involving 346 different antigens – most of which are extremely rare or don’t have particular consequences for blood transfusions.
The A and B antigens both evolved over 20 million years ago. Their exact purpose is unknown, but they may play a role in blood clotting and help to protect against certain diseases such as cholera.