This tutorial will take you through blood typing, what it means and how we predict which blood transfusions will work.
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Blood transfusions are the most common kind of organ transplant. You're taking tissue from one person and putting it into another person.
And everybody knows, when you transplant one person's tissue into another they need to match. But why?
Transplanted tissues need to match because your body defends itself against cells that it thinks are foreign.The immune system identifies foreign cells by the molecules on their surface, or CELL SURFACE ANTIGENS. If you give someone red blood cells with foreign antigens, the person's immune system will create molecules called ANTIBODIES that bind to the antigens. Then they attract white blood cells to eat the foreign cells, or activate other poisons to break them apart.
Here's a foreign cell with cell surface antigens that your immune system identifies as foreign...
and here it is being attacked by the antibodies your immune system produced!
Images from Microsoft clip art
So what are the cell surface antigens and the antibodies that we need to look at, to give someone the right red blood cells?
Red Blood Cell Surface Antigens
Red blood cells, like all cells, have lots of different kinds of molecules on their surfaces. When your blood type is tested, though, we look at two kinds: a set of sugar molecules called the ABO group and a protein, the RH factor or D antigen.
Here's a red blood cell with all the antigens we normally look for - A, B, and Rh. This cell is AB-positive or AB+ blood.
Page 3ABO antigens
These are sugar molecules on the surface of the cell. They are the most immunogenic of the RBC surface antigens -- the ones most likely to cause the immune system to form antibodies.
These sugar molecules are also found on some bacterial cells and in some foods, so everybody is likely to encounter them. That means that your patient has probably already developed antibodies against the ABO antigens, and is ready to attack them the minute they get into the bloodstream! If you give someone blood with the wrong ABO type, a serious reaction can occur right away.1
There are three major forms of the ABO antigen, and which type you have depends on the alleles you inherited from your parents. If you have the A allele, you make Type A antigens. If you have the B allele, you make type B antigens.
If you inherited an allele for type A from one parent and an allele for type B from the other, you will have both A and B antigens and will have type AB blood.
What if you got an O allele? That allele doesn't produce either A or B antigens.
The O allele is recessive. That is, if you have one O allele and an A or B allele, the A or B will show up. You'll create A or B antigens. But if both of your parents gave you O alleles, you won't make either A or B antigens and your blood will be called type O. You might remember that more easily by thinking that it's like a zero - neither of the antigens.
1Dean, L. (2005). The ABO blood group. National Center for Biotechnology Information (US).
Time to solve a problem! A man has type A blood, so what could his genotype be like?
If you were to look in this man's blood for antibodies against the cell surface antigen, would you expect to find:
Which of the blood types listed below can you safely give to this man?
Rh factor or D antigen
There are many different forms of the Rh factor, but the one we pay most attention to in routine blood typing is the D form. A person who has the D form is called Rh-positive and a person who doesn't have the D form is called Rh-negative.
If you get a gene for the Rh factor from either one of your parents, you will have Rh-positive blood. If neither of your parents gives you a gene for it, you won't be able to make the antigen and your blood will be Rh-negative.
The Rh factor is the second most immunogenic of the RBC surface antigens, but it is less commonly encountered through the diet or on bacteria, so patients don't usually develop antibodies against it unless they are exposed to Rh-positive blood.
That doesn't mean it's not important! If you gave a person mismatched Rh factor blood, they might not have an immediate negative reaction this time, but they would develop antibodies against the Rh factor - and the next time they encountered Rh-positive blood, they would get sick.
Page 6Let's apply it! A woman's blood is A-positive. What cell surface antigens are on her red blood cells?
What antibodies against red blood cell surface antigens will be in this woman's blood?
What blood type/s could you give this woman?
There's a simple way to think about giving someone blood transfusions.
Never Give Any Antigens They Don't Already Have
Apply this by choosing the right terms in the sentences below.
A man with type AB-negative blood has A / B / Rh antigens.
The man does NOT have A / B / Rh antigens.
You can give him blood with A / B / Rh antigens, but not with A / B / Rh antigens.
Never Give Any Antigens They Don't Already Have
Here are a bunch of transfusions. Are they safe, or is the patient in danger?
Mr. A has AB-positive blood and was given A-positive blood. safe / not safe
Mrs. B has A-negative blood and was given A-positive blood. safe / not safe
Ms. C has 0-negative blood and was given AB-positive blood. safe / not safe
Mr. D has B-negative blood and was given O-negative blood. safe / not safe
Good work figuring out transfusions based on the ABO/Rh antigens!
You might wonder, what about all those other antigens on the red blood cells? Don't they ever cause a reaction?
The answer is yes, but not as often, because not as many people have developed antibodies to them. It isn't a common enough problem to justify checking every blood sample for all of them. What they do instead is to screen the patients to see if they have antibodies against those minor antigens. Most people don't; so you only have to check the blood you're giving to the few people who do have those antibodies.1
You've now finished the tutorial on how to decide what blood type is safe, assuming your patient doesn't have any of the rare antibodies. You should be able to choose any blood type and decide which blood you can give and which you can't. Happy studying!
1 Sapatnekar, S. (2015, March 1). Molecular Typing for Red Blood Cell Antigens - AACC.org.
Copyright Patricia S. Bowne firstname.lastname@example.org