Why do we have blood types?

Admittedly, I was inspired for this post by the google doodle of the day which recognizes Karl Landsteiner who is credited with distinguishing the major blood groups.

I go past the Canadian Blood Donor clinic on the train on my way to work everyday. It seems like they are constantly asking for type O blood, and for good reasons. One, type O is still the most common blood type with about 39% of the population (Canadian Blood Services). And two, type O blood can be given to almost anyone. I say almost because while type O can be accepted by individuals with type A, type B, and type AB, there is a second “type” on everyone’s blood of + or -. So technically 39% of the population has O+ blood. And O+ blood cannot be given to anyone who is – regardless of their letters.

So what exactly makes up a blood type?

Many cells have antigens on them. You can kind of think of an antigen as a barcode. It helps your body to identify what type of cell it is. This is really useful when it comes to bacteria as your body can recognize bacteria that doesn’t belong and work to get rid of it. It does this with antibodies.

If you have type A blood than you do not have the type A antibodies. This is good, as it means that your body will not attack your own blood. Type B and type AB are the same. (Type AB means that you have both A cells and B cells in your blood.) They don’t have the antibodies for the type of cell they have. Type O is a little different. There are no antigens on type O cells (other than potentially the + or -). But that means that individuals with type O have the antibodies for type A and type B.

Just like some people aren’t very good at dealing with change, our bodies aren’t either. I have O+ blood. Which means that my body will recognize the antigens on type A and type B blood cells as something different, and different is bad, so it will respond with the appropriate antibodies. This causes ABO incompatibility which is potentially life threatening.

So, type O with no antigens is the universal donor. It can be given to anyone regardless of blood type (except of course + or -). That’s why blood banks always want type O blood. If you are in need of blood, the best blood type to have is type AB. Type AB doesn’t have either of the A or B antibodies so they can accept blood from anyone; they are the universal recipient.

While this is common medical knowledge today, it was not in the past, before Landsteiner’s work. This meant that getting blood from someone else was a much more dangerous thing in the past.