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Blood Type 101

Do you know your blood type?

Many people don’t, but knowing your blood type can help you make a lot of health and wellness decisions over the course of your life, so it’s good to know the basics.

What is blood?

Blood is made up of cells floating in plasma (a liquid made of proteins and salts).

  • Red blood cells carry oxygen
  • White blood cells fight infection (antibodies)
  • Platelets help blood clot

What is blood type?

Blood type is defined by proteins, or molecules in your blood. These molecules are classified as either:

  • Antigens, which trigger immune responses, or
  • Antibodies, which stop intruders in the body

The combination of antigens and antibodies determines your blood type, which you inherit from your parents. There are 8 different combinations depending upon your group and Rh factor.


Blood groups are based on the combination of antigens and antibodies in your blood.

  • A has the A antigen and B antibody
  • B has the B antigen and the A antibody
  • AB has both antigens but neither antibody
  • O has neither antigen but both antibodies

Rh factor

This is a protein your blood either has or doesn’t.

  • If you have it, you’re positive (which is much more common)
  • If you don’t, you’re negative

What blood types are most common?

O positive is the most common blood type, and AB- is the rarest.

Why does blood type matter?

When different types of blood mix, clotting can occur because some blood types have antibodies that attack other kinds of blood. This can be fatal, and often was before blood types were discovered in 1930.

These incompatibilities matter when:

1. You need a blood transfusion

It’s best when a donor and recipient are an exact match, but they don’t have to be as long as they are compatible. In general:

  • People with Type O blood can donate to anyone
  • People with Type AB blood can receive blood from anyone (and can donate plasma to anyone)
  • People with Rh negative blood can receive only from an Rh negative donor
blood type chart

Graphic: Canadian Blood Services

2. You’re pregnant and you’re Rh negative

If your baby is also Rh negative, you are fine. If, however, your baby is Rh positive and you are Rh negative, problems can occur. This scenario doesn’t happen often, but your doctor can test you and, if needed, treat you with a few simple injections or a blood transfusion.

Who can donate blood?

Most people who are in good health, are over age 16 and weigh 110 pounds can give blood. You can review all the eligibility requirements here.

What kinds of blood are most needed for donation?

Both O negative and O positive are in high demand. The need for O negative blood is high because it’s used most in emergencies.

How do I find out my blood type?

Ask your doctor or donate blood.

Visit the American Red Cross website to use this interactive donation chart and to learn where to donate in your community.

screenshot of interactive blood donation chart on the American Red Cross website

Graphic: American Red Cross

Ashley Brantley

Ashley Brantley

Ashley Brantley has been writing about food, culture and health for more than a decade, and has lived in three of Tennessee’s four major cities (Memphis, Nashville and Knoxville). As Senior Copywriter at bohan Advertising, she is a writer, editor and social media strategist.

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