Tags on your blood cells
Every red blood cell wears tiny markers on its outside, a bit like name tags. Which tags you have decides your blood type.
There are two main tags, called A and B. If your cells have A tags, you are type A. B tags make you type B. Both kinds make you type AB. No A or B tags make you type O, like a zero.
Blood types were discovered in 1901 by a scientist named Karl Landsteiner. His discovery made it much safer for people to share blood.
Positive and negative
There is another tag called the Rh factor. If your cells have it, your type is positive. If they don't, it is negative.
So your full blood type might be A positive, O negative, AB positive or one of the others. That makes eight main types in all.
In the United States, O positive is the most common type. AB negative is one of the rarest.
Every type is a perfectly good type. Nobody's blood is better than anybody else's.
Passed down in families
You get your blood type from your birth parents. Each one passes along half of the recipe, and together the two halves decide your type.
That is why brothers and sisters can have different blood types, even with the same parents.
Your blood type usually stays the same your whole life.
Why it matters
Sometimes a person who is very sick or badly hurt needs more blood. Doctors can give them blood from someone else. This is called a transfusion (trans-FYOO-zhun).
The blood types have to match. If the wrong type is given, the body's guards think the new blood is a germ and attack it.
That is why doctors always check blood types very carefully before a transfusion.
O negative, the helper type
O negative red blood cells have no A, B or Rh tags. Because they have no tags, almost anyone's body will accept them.
So in an emergency, when there is no time to check a person's type, doctors often use O negative blood. People with O negative are sometimes called universal donors.
People with AB positive blood can get red blood cells from any type. They are sometimes called universal receivers.
Giving blood is a gift
Kind grown-ups give blood so it is ready for people who need it. This is called donating blood. Each donation is about a pint, which is two cups.
Afterward, the donor rests and has a snack and a drink. Their body makes new blood to replace what they gave.
One donation can be split into parts, like red cells, plasma and platelets, so it can help more than one person.
In most US states, people can give blood when they are 17, and in some states at 16 with a parent's permission. Until then, you can cheer on the grown-ups who give!