Blood types (unraveled facts)

When scientists in the late 1800s mixed blood from different people in test tubes, they noticed that sometimes, the red blood cells stuck together but because the blood generally came from sick patients, scientists dismissed the clumping as some sort of pathology, not worth researching. Nobody bothered to see if the blood of healthy people clumped. It took the efforts of Karl Landsteiner to ascertain that that mixtures of healthy blood sometimes clumped too.

fig 1: Karl Landsteiner

Landsteiner set out to map the clumping pattern, collecting blood from members of his lab, including himself. He separated each sample into red blood cells and plasma, and then he combined plasma from one person with cells from another.

Landsteiner found that the clumping occurred only if he mixed certain people’s blood together. By working through all the combinations, he sorted his subjects into three groups. He gave them the entirely arbitrary names of A, B and C. (Later on C was renamed O, and a few years later other researchers discovered the AB group. By the middle of the 20th Century the American researcher Philip Levine had discovered another way to categorise blood, based on whether it had the Rhesus (Rh) blood factor. A plus or minus sign at the end of Landsteiner’s letters indicates whether a person has the factor or not.)

When Landsteiner mixed the blood from different people together, he discovered it followed certain rules. If he mixed the plasma from group A with red blood cells from someone else in group A, the plasma and cells remained a liquid. The same rule applied to the plasma and red blood cells from group B. But if Landsteiner mixed plasma from group A with red blood cells from B, the cells clumped (and vice versa).

The blood from people in group O was different. When Landsteiner mixed either A or B red blood cells with O plasma, the cells clumped. But he could add A or B plasma to O red blood cells without any clumping.

fig 2: Blood types

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It’s this clumping that makes blood transfusions so potentially dangerous. If a doctor accidentally injected type B blood into my arm, my body would become loaded with tiny clots. They would disrupt my circulation and cause me to start bleeding massively, struggle for breath and potentially die. But if I received either type A or type O blood, I would be fine.

Landsteiner didn’t know what precisely distinguished one blood type from another. Later generations of scientists discovered that the red blood cells in each type are decorated with different molecules on their surface. In my type A blood, for example, the cells build these molecules in two stages, like two floors of a house. The first floor is called an H antigen. On top of the first floor the cells build a second, called the A antigen.

People with type B blood, on the other hand, build the second floor of the house in a different shape. And people with type O build a single-storey ranch house: they only build the H antigen and go no further. This means that different blood types arise because of different molecules on the surface of red blood cells.           (credits: bbc.co.uk)