Everyone has one of four blood types — A, B, AB or O — which is inherited from your parents, like your eye color, dimples, or curly hair. While all blood is similar in its components (such as containing red cells, platelets, and plasma), it also has important characteristics that make it unique.1
Your blood type is determined by the presence or absence of two antigens — A and B — on the surface of red blood cells.2 A third antigen, called Rh factor, will either be present or absent. If your blood has it, then you're Rh positive.3 Antigens are substances that may trigger an immune response, causing your body to launch an attack if it believes they are foreign.4
Taken together, these factors determine the right type of blood for your body, should you need a transfusion. Receiving the wrong type can be catastrophic, even resulting in death. According to Blood Transfusions and the Immune System:5
"If incompatible blood is given in a transfusion, the donor cells are treated as if they were foreign invaders, and the patient's immune system attacks them accordingly.
Not only is the blood transfusion rendered useless, but a potentially massive activation of the immune system and clotting system can cause shock, kidney failure, circulatory collapse, and death."
Blood is a living tissue made up of red blood cells, white blood cells, platelets and plasma (which is more than 90 percent water).6 Your body weight is about 7 percent blood. Men have about 12 pints of blood in their body while women have about nine.7
The main role of blood is to transport oxygen throughout your body, although it also plays a role in fighting off infections and carrying waste out of your cells. Blood also:8
- Regulates your body's acidity (pH) levels
- Regulates your body temperature (increased blood flow to an area adds warmth)
- Supplies essential nutrients, such as glucose and amino acids, to cells
- Has specialized cells that promote blood clotting if you are bleeding
- Transports hormones
- Has "hydraulic functions," helping men to maintain an erection, for instance
It's not entirely true that you can't use blood from someone who has a different blood type than you. Everyone can receive type O blood, the most common type in the U.S., as it has neither A nor B antigens on red cells (and both A and B antibody in the plasma).9
Your blood type may be either positive or negative, depending on the presence or absence of Rh factor (about 85 percent of people are Rh positive). Generally, Rh negative blood is given to Rh-negative patients while those with Rh positive blood receive Rh positive blood in transfusions.10
Rh factor is generally tested during pregnancy, as an incompatibility between mother and fetus may cause the mother's body to attack the baby's "foreign" blood. (Rh immune globulin is an effective treatment that can stop this attack if found early on.)11
The American Red Cross has created the following chart to explain which blood types are compatible with others.
Source: American Red Cross, Blood Types
Blood types must be carefully matched as follows to avoid potentially deadly consequences. Here's a breakdown of the four blood types:12
- Type A — Only the A antigen on red cells (B antibody in the plasma). The second most common blood type.
- Type B — Only the B antigen on red cells (and A antibody in the plasma). Relatively rare, especially among Hispanics and Caucasians.
- Type AB — Both A and B antigens on red cells (both A and B antibody in the plasma). Very uncommon, only 7 percent of Asians, 4 percent of African Americans, 4 percent of Caucasians, and 2 percent of Hispanics have this blood type.
- Type O — Neither A nor B antigens on red cells (both A and B antibody in the plasma). The most common blood type, especially among Hispanics.
Again, it's very important that your blood type is tested to prevent health complications. Receiving the wrong blood type can be potentially life-threatening as it may cause transfusion reactions, a situation wherein donor red blood cells are destroyed by your immune system.13 Symptoms include:14
- Urticaria (hives)
- Dyspnea, or shortness of breath
It's thought that different blood types developed as a way to help protect humans from infectious disease. For instance, cells infected with malaria don't "stick" as well to type O or type B blood cells, which means a person with type O blood may get less sick if they're infected with malaria than someone with a different blood type.15 Perhaps not coincidentally, regions with high burdens of malaria, such as Africa, also have a high rate of type O blood.
The fact that certain blood types are incompatible is likely the result of a mutation. As reported by Live Science:16
"Blood type A is the most ancient, and it existed before the human species evolved from its hominid ancestors. Type B is thought to have originated some 3.5 million years ago, from a genetic mutation that modified one of the sugars that sit on the surface of red blood cells. Starting about 2.5 million years ago, mutations occurred that rendered that sugar gene inactive, creating type O, which has neither the A nor B version of the sugar.
And then there is AB, which is covered with both A and B sugars … But incompatibility is not part of the reason humans have blood types, says Harvey Klein, chief of transfusion medicine at the National Institutes of Health Clinical Center. 'Blood transfusion is a recent phenomenon (hundreds of years, not millions), and therefore had nothing to do with the evolution of blood groups,' he said."
You may have heard about diets based on your blood type, which claim that certain foods react in different ways in your body depending on your blood type. I personally do not advocate such diets. I actually attended a small lecture given by Dr. Peter D'Adamo before he published his book "Eat Right for Your Type."
I believe one of the main reasons why most support it is due to the fact that O is the most common blood type and calls for a severe grain restriction. If you are a blood type A like me, it can lead to severe problems.
I actually developed diabetes after following it for a short time. My fasting blood sugar shot up to 126. Not only did it include eating large amounts of fruit for breakfast, but it advocated mild exercising for blood type A. So, I cut down my exercise and increased my fruit intake, which resulted in a 20-pound weight gain and a diagnosis of diabetes.
This is one of the reasons I am so passionate about my nutrition plan — it is based on whole foods, nothing too extreme, and goes by the guiding principle to listen to your body and let it be your guide on which foods are best for you.
Someone in the U.S. needs blood every two seconds,17 so if you're up for doing a good deed, donating blood is a phenomenal choice. More than 36,000 blood donations are needed each day, but although about 38 percent of Americans are eligible to donate blood, less than 10 percent actually do so each year.18 The two most common reasons why people don't donate blood are fear of needles or simply not thinking about it.19
On the other hand, those who choose to donate most often do so to help others (which it does in spades, as one donation may save the lives of up to three people20). So, if you can spare an hour or so of your time, your donated blood may save the life of someone in an emergency (or the countless other scenarios in which blood transfusions are necessary).
Finally, if your iron levels are high, donating your blood is a safe, effective, and inexpensive solution, as one of the best ways you can get rid of excess iron is by bleeding.21
Q: What is the most common blood type?
A: According to the American Red Cross Blood Services, O positive is the most common blood type in the United States.22
Q: What is the rarest blood type?
A: The American Red Cross Blood Services reports that AB negative is the rarest blood type in the U.S. population.23
Q: How do blood types work?
A: Blood type is determined by the presence of specific proteins, or "antigens" in the context of blood typing. Testing for the presence of antigens will help determine your specific blood type.24
Q: Can your blood type change?
A: It's possible for your blood type to change if you undergo a stem cell/bone marrow transplant. The process may take several weeks, but eventually your blood type will transition to the donor's type.25
Q: Which parent determines the blood type of the child?
A: Parents pass down one of their two ABO alleles to your child. A parent with an O allele can pass it down, while a parent with AB alleles can pass down either the A or B allele. Overall, no one parent can determine the blood type as it entails a combination of both.26
Q: What blood types are not compatible for pregnancy?
A: Blood transfusions are applicable for various situations during pregnancy, such as the onset of mild anemia, early miscarriage and during or immediately after birth. You will be given the appropriate blood type to reduce the risk complications to you and your baby's health.27
Q: What blood type can O positive receive?
A: If your blood type is O positive, you can receive blood from the same type, as well as O negative blood.28