‘Augurs and understood relations have by magot pies and choughs and rooks brought forth the secret’st man of blood.’ –Shakespeare


Red blood cells carry oxygen to all the cells and tissues in our body. If we lose a lot of blood in surgery or an accident, we need more of it – fast. Hence the hundreds of millions of people flowing through blood donation centres across the world, and the thousands of vehicles transporting bags of blood to processing centres and hospitals.

It would be straightforward if we all had the same blood. But we don’t. On the surface of every one of our red blood cells, we have up to 342 antigens – molecules capable of triggering the production of specialised proteins called antibodies. It is the presence or absence of particular antigens that determines someone’s blood type. […]

If a particular high-prevalence antigen is missing from your red blood cells, then you are ‘negative’ for that blood group. If you receive blood from a ‘positive’ donor, then your own antibodies may react with the incompatible donor blood cells, triggering a further response from the immune system. These transfusion reactions can be lethal. […]

There are 35 blood group systems, organised according to the genes that carry the information to produce the antigens within each system. The majority of the 342 blood group antigens belong to one of these systems. The Rh system (formerly known as ‘Rhesus’) is the largest, containing 61 antigens.

The most important of these Rh antigens, the D antigen, is quite often missing in Caucasians, of whom around 15 per cent are Rh D negative (more commonly, though inaccurately, known as Rh-negative blood). But Thomas seemed to be lacking all the Rh antigens. If this suspicion proved correct, it would make his blood type Rh null – one of the rarest in the world, and a phenomenal discovery for the hospital haematologists. […] Some 43 people with Rh null blood had been reported worldwide.

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