Why Can’t You Use Blood from Someone Who Has a Different Blood Type Than You?

Story at-a-glance
• Everyone has one of four blood types – A, B, AB, or O – which is inherited from your parents
• Your blood type is determined by the presence or absence of two antigens – A and B – on the surface of red blood cells. A third antigen, called Rh factor, will either be present or absent, making your blood type positive or negative
• If incompatible blood types are given during a transfusion, the donor cells will be attacked by the patient’s immune system, which may cause shock, kidney failure, and death
• Everyone can receive type O blood, the most common type in the US, as it has neither A nor B antigens on red cells

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.
Your blood type is determined by the presence or absence of two antigens – A and B – on the surface of red blood cells. A third antigen, called Rh factor, will either be present or absent. Antigens are substances that may trigger an immune response, causing your body to launch an attack if it believes they are foreign.
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:1
“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.”
What Exactly Is Blood?
Blood is a living tissue made up of red blood cells, white blood cells, platelets, and plasma (which is more than 90 percent water). Your body weight is about seven percent blood. Men have about 12 pints of blood in their body while women have about nine.2

Blood’s main role 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:3

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
Which Blood Types Are Compatible?
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 US, as it has neither A nor B antigens on red cells (and both A and B antibody in the plasma).
Beyond that, however, blood types must be carefully matched as follows to avoid potentially deadly consequences. First, a breakdown of the four blood types:4
• 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 seven percent of Asians, four percent of African Americans, four percent of Caucasians, and two 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.
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.
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.)
The American Red Cross has created the following chart to explain which blood types are compatible with others.

Source: American Red Cross, Blood Types
Why Are There Different Blood Types?
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.

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:5
“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.”
Does Your Blood Type Dictate Your Diet?
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. 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 myself, 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 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.
Facts About Donating Blood
Someone in the US needs blood every two seconds,6 so if you’re up for doing a good deed, donating blood is a phenomenal choice. More than 41,000 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.7 The two most common reasons why people don’t donate blood are fear of needles or simply not thinking about it.
On the other hand, those who choose to donate most often do so in order to help others (which it does in spades, as one donation may save the lives of up to three people). 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 youriron 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.


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