What Are Blood Types?

The most common blood types are A, B, AB, and O.

Every 2 seconds, someone in the U.S. needs more blood. But blood can’t be made in a lab. It has to come from another person, and not just any person. He or she has to have the same blood type as the person who’s going to receive it.

Whether you’re donating blood for someone else or receiving it, testing is needed. The test shows what you’re blood type is. Then a doctor can be sure the person who needs it gets the right blood type. If the person gets the wrong type, the body could reject it.

Your blood contains:

• Red blood cells, about 25 trillion of them, that carry oxygen to the rest of your body
• White blood cells that help protect you from infections
• Platelets that form clots to stop bleeding
• Plasma, a yellow liquid that transports nutrients and carries away waste products

Your red blood cells have special markers on the surface that determine your blood type. These markers (antigens) help your body recognize cells that belong inside you. If you receive red blood cells with different antigens, your immune system will treat them just like it does disease cells that enter your body. It will create special proteins called antibodies to destroy the foreign cells.

There are 4 major blood types:

• Type A red blood cells have A-type antigens.
• Type B has B antigens.
• Type AB has both A and B antigens.
• Type O has neither A nor B antigens.

Aside from antigens, your red blood cells can have a special protein called Rh factor. If you have it, it means your blood is Rh positive (+). If you don’t have it, you’re Rh negative (-).

Combining A and B antigens with the Rh factor gives you 8 different blood types:

• A+/A-
• B+/B-
• AB+/AB-
• O+/O-

These aren’t the only blood types. In fact, there are about 30 others, but they’re all very rare.

Just like your skin tone and eye color, your blood type is something you’re born with. It runs in your family and is determined by your parents’ blood types. You’re also more likely to share the same blood type with people that have the same ethnic background that you do. For instance, a rare blood type called Ro is 10 times more common in black people than it is in white people.

Blood transfusions -- when more blood is added to your body -- are very common. You may need one if you lose a lot of blood during surgery or as a result of being injured. Your body could also stop making enough healthy blood on its own.

For a transfusion to be safe, the doctor must be sure your blood type matches that of the donor. If the blood type doesn’t match, your immune system will attack it. That is unless it’s O- blood.

This type of blood is like a wild card. Because it can match with any blood type, it’s in high demand at hospitals. Health care providers use it in emergencies and to help premature babies and people with cancer. If you have type O-, you’re what health professionals call a universal donor. You can safely give blood to anyone. Only 7% of people have this blood type.

If you have AB+ blood, you’re a universal recipient. You can receive blood from any donor and your body will not attack it. AB blood also makes you a universal plasma donor. You can donate the plasma part of your blood to all blood types because it doesn’t have A or B antigens.

The rarest major blood type is AB-. Only 1% of the population has it.

A simple blood test can tell what type of blood you have. You’ll get this test before you:

• Donate blood
• Have surgery
• Need a blood transfusion
• Volunteer to donate an organ

You also get a blood test when you’re pregnant. If you’re Rh-negative and your baby is Rh-positive, it’s not safe for your blood to mix together. Even though mothers and fetuses don’t share blood, a small amount of the baby’s blood can mix with the mother’s. If you’re Rh-incompatible, your red blood cells can start to attack your baby’s red blood cells. But if doctors know about your different blood types, they can give you shots to keep your baby safe.