Saturday, January 19, 2008

Apple #293: Blood Types

I donated blood yesterday. While I was there, some manager lady was inspecting the nurses collecting the blood, and she was talking really loud about all kinds of stuff. The nurses were short-staffed that day so there were long lines at each stage of the process, but of course the manager didn't step in to help, she just kept talking.

One of the things she said was, "Of course my blood type is boring old A positive. They've always got a supply of boring old A positive." I felt a bit sad at hearing this because my blood type happens to be A positive, and I was always sort of proud of that. You know, like A+.

I asked the nurse who was testing my blood whether what the manager lady had said was true, do they have too much A positive. He said yes, they always have the most of that because it's the most common blood type. O negative is the one they need the most, he said. He paused, then added, "But we always need people to donate, of any type."

This woman is donating blood. See how easy it is? You sit in a lounge chair, they give you free snacks and drinks, and everybody is really nice to you.
(Photo from the American Red Cross)

I decided I wanted to learn more about blood types. I remember learning something about it in school, but I had forgotten most of it.

  • If you give a person the blood of someone with a different blood type, the blood will clump. The red blood cells crack, which allows all the proteins inside leak out, and that's really bad because the hemoglobin that's kept in the red blood cells is toxic to every thing outside the red blood cells. So once hemoglobin is on the loose, so to speak, it can be fatal.
  • It wasn't until Karl Landsteiner figured out why blood clumps and all the rest happens that people were able to donate blood to each other successfully. Landsteiner, by the way, won a Nobel Prize for figuring out this problem.

Karl Landsteiner, the guy who figured out blood types
(Photo from Karl Landsteiner Geselleschaft)

  • So here's what he found out. Your blood contains a certain combination of antigens and antibodies (together, they're also referred to as agglutinogens).
      • Antigens: these are little knobs of protein, or they could be bacteria or a virus, and they're sort of like code-in-waiting. When they get stimulated, they make your body's immune system trigger a specific disease-fighting response.
      • Antibodies: these are the disease-fighting proteins that spring into fighting action when their related antigen shows up.
  • The blood types are a way to indicate what kind of antigens and antibodies are already in your blood. As you can see, the type name comes from the type of antigen.











A & B




A & B

  • By the way, the type generally referred to as O (the letter o) was originally referred to as 0 (zero), since it means that there are zero antigens on the red blood cells.
  • Knowing which antigens & which antibodies are present is important when it comes to deciding whose blood can get donated to whom.
  • As long as you've got the antibodies that do not correlate with the same antigens (type A antibodies present with type B antigens), you'll be OK. But if you give somebody an antigen for which they already have the antibody in their blood, the antibody will automatically start fighting against that antigen. Since the antigens live on the outside of the red blood cells, the antibody will eventually crack open the red blood cells, the hemoglobin will escape, and then all hell breaks loose.

Antigens on a red blood cell and antibodies attaching to them
(Drawing from Healthy, Wealthy, & Wise)

  • But if you give people a type of blood that will not make the antibodies fight against the antigens, all hell will not break loose. Here's a table that shows which blood types can work safely together:

Blood Group
Can give blood to
Can receive blood from
A and B
AB, A, B, 0
A and AB
A and 0
B and AB
B and 0
A and B
AB, A, B, 0
(table from Nobel

  • Because type O has no antigens, it won't trigger an antibody response. And since it has both types of antibodies, it can work just fine in the presence of either A or B antigens. That's why type O is known as the universal donor, because it can work when combined with any blood type.
  • Because AB has no antibodies, it doesn't have anything that will fight against whatever you introduce to it. So it can receive blood from anybody. However, if you give somebody AB blood, or both AB antigens, the antibodies that they already have will fight against the AB blood, clumping will result, hemoglobin escapes, hell breaks loose again.

  • There are now 20 different methods of typing blood. The ABO system is the one that is most commonly known.
  • The next most commonly known is the Rh factor system. This one refers to whether or not you have the Rh antigen present on the red blood cells.
  • Rh+ means you do have the Rh antigen, Rh- means you do not.
  • So you can give somebody who already has the Rh antigen either Rh+ or Rh- blood. But you can't give Rh- blood somebody with the Rh antigen, because the Rh- blood will develop antibodies to fight against the Rh+ blood.

(Image from

Commonly, the ABO blood typing system gets combined with the Rh typing system, so that blood types are expressed in terms of A's, B's, and O's and then with a positive or negative indicator afterwards. So someone like me with a blood type of A+ has A antigens, B antibodies, and is Rh positive.

The frequency with which the different blood types occur varies slightly depending on where you are in the world. But as more people marry other people from all sorts of places, the more uniform the blood type distribution will become.

At the moment, though, here is the frequency of occurrence of the various blood types in the United States, from most common to least common:

Type / Rh

Occurrence in the US

















(source: American Association of Blood Banks)

As you can see, Rh- is less common than Rh+.

For more information about blood type distribution on a global basis with lots of interesting maps, see Distribution of Blood Types


Adelaide spends much of Guys & Dolls demanding that Nathan Detroit get a license and a blood test -- to marry her, in other words.
(Photo of Frank Sinatra & Vivian Blaine as Nathan & Adelaide from

I had one more question about blood types. I thought that you used to be required to get a blood test before getting married so they could determine if you and your prospective partner had compatible Rh factors. If not, you might have children with birth defects. In fact, if I remember correctly, my fifth grade teacher taught us this. But I haven't heard people mention the blood test thing at all recently, so I wondered if doctors found a way around the Rh factor incompatibility.

It turns out that my idea about the Rh factor test was wrong. Blood tests used to be required to test, not for Rh factor, but for syphilis or rubella (a form of measles). The presence of either one of these diseases can lead to very serious birth defects, or they could be fatal to a fetus. The blood test would then be used to tell people whether they ought to go get vaccinated or quarantine themselves upon pregnancy.

Either my memory is faulty, or my fifth grade teacher didn't tell us about the syphilis part because it has to do with s-e-x.

Not many states require a blood test anymore because those tests now are done at pregnancy. A few states do still require a blood test, however, so if you're fixing to get married soon, check with your state marriage license office.

That said, Rh disease is possibility -- rare, but possible. If the mother is Rh- and the father is Rh+, the baby could also be Rh+, which would be incompatible with the mother's Rh- blood.

It's not as much of a concern now because doctors can give the mother an injection of a drug called Rhogam which removes the fetal Rh+ blood cells from the mother's Rh- blood stream before her body makes the antibodies. Or else it's possible to give the fetus a complete transfusion through the umbilical cord.

It's all pretty miraculous, isn't it?

Sources, Blood Groups, Blood Typing and Blood Transfusions
University of Utah, Genetic Science Learning Center, What are Blood Types?
Online Medical Dictionary, definition of antigens
CDC, National Immunization Program, Glossary, antigens
InteliHealth, Robert H. Shmerling, M.D., The Truth about Premarital Blood Testing, November 15, 2006
University of Pennsylvania Health System, Pregnancy Health Center, Blood Group (Rh) Incompatibility


  1. Hey, you've inspired me to give blood, which I haven't done since college. Just one of those things that doesn't cross your mind very often.

    Gotta link to the theme song for blood donation.

  2. Ah, yes, I was just thinking of that Pete Townshend song.

    And the college blood drive is where I first gave blood, so I usually think of that experience when I go to donate. Fortunately, I've never had to wait in as long a line as I did then.


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