Gut bacteria seem to vary according to the blood groups of their hosts, but the reason for this is not yet clear.

Your ABO blood type is determined by a type of sugar on the surface of your red blood cells . Type A individuals have a different sugar from type B individuals, while AB people have both sugars. O people – who are known as universal donors – have neither.

These sugars are called antigens and help tell your immune system that your blood cells belong to you and shouldn’t …