Blood Types

I can always remember my blood type, for a very silly reason: I’m A+, and in school that’s the grade I always aimed for. Today, I want to learn about what all these labels mean – what makes our blood different?


At core, everyone’s red blood cells are the same. What is different is the molecules attached to the surface of these blood cells: antigens and proteins. A person can have either A- or B-type antigens, or both, and which type a person has is determined by genetics. A certain gene can code for production of type A, type B, or no antigens. And, since a person has two copies of the gene (one received from each parent), they could have one copy coding for type A and the other copy coding for type B. This is what determines your blood type, as shown in this picture:

blood type inheritance

So, my red blood cells have type A antigens. This means the genes I got from my parents coded for A and A, or A and O.

The + or – next to a person’s blood type is determined by the presence or absence of the Rh protein on the red blood cells. A “+” means the person does have that protein, while a “-” means they do not. So, I am type A+: I have the type A antigen and the Rh protein on my cells.


This gets tricky when we talk about donating and receiving blood. A person will have antibodies in their plasma determined by whichever antigens are not on their cells. The antibodies are designed to detect and attack any foreign molecules that may have entered the body from outside: for example, blood with different antigens on it. I can accept blood from someone with type A antigens and Rh proteins, but not from someone who has type B antigens: my body would detect it and start an immune response.

So, someone with blood type AB+ can accept blood from anyone: they don’t have any of those antibodies in their plasma, so their body will recognize any blood as their own. They can only give blood to other AB+ people, though. On the other hand, someone with blood type O- can give blood to anyone, because their blood has none of the antigens or proteins that could trigger an immune response. They can only accept blood from other O- people, though, since their own plasma contains every relevant antibody. The chart below shows who can donate to and receive from whom:



So, it’s simpler than it looks. Two different antigens, and one protein, can code for eight different blood types. In the US, type O is the most common, while AB is the least common.

This gets me all excited about genetics again! My favorite part of that class was charting inheritance across generations. There’s something very satisfying about figuring out the likelihood of the different combinations…

So, what’s your blood type?

– Hannah




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