The Genetics of Blood Types

Knowing your blood type can be super important if you’re ever in need of the stuff. It can also have big implications in pregnancy. Here’s the basics about blood types, and why they’re so important.

Antigens and Antibodies

To understand why blood types are important, you have to know a little bit about the immune system. The immune system has many parts, but one key feature here is the antibody. Antibodies are little proteins your white blood cells make that stick to foreign intruders and make them easy targets to destroy. The antibodies recognize flags on the intruders called “antigens.” Your own cells have these flags too, but your immune system knows they belong to you, so it ignores them. It’s kind of like having a passport; have the wrong kind and you’ll be kicked out.

Your red blood cells have lots different antigens on their surface. Your blood type is just a description of which antigens you have. Though there are many groups of antigens, the main two groups we talk about when it comes to blood type are ABO and Rh. The first group gives us the letter or letters of your blood type, and the “Rh status” gives the positive or negative part (A+, O-, AB-, etc.). We focus on these two groups because they’re the ones that are most likely to cause trouble if they end up in the wrong person’s blood.

ABO Type

“A” and “B” are antigens that can be on the surface of a red blood cell, while “O” just means there are no A’s or B’s. Which antigens end up on your red blood cells is determined by which version of the ABO gene you have, but the ABO gene doesn’t actually make an antigen at all. Instead the protein it makes takes a different antigen (H antigen) and changes it.  The A version of the ABO gene converts the H antigen into the A antigen, while the B version converts it into the B antigen. The O version of the gene is actually missing a letter, so the protein it makes is a dud and can’t do anything to the H antigen.

The ABO gene is on chromosome 9, and we all have two copies of it; one from Mom and one from Dad. If Mom gives you an O version, and Dad gives you an A version, your blood type will be A, since you’ll only have A antigens. Get both an A version and a B version, you’ll have both types of antigens and your blood type will be AB.

Rh Type

The “positive” and “negative” at the end of your blood type is a completely separate entity from ABO. It’s determined by whether or not you have the Rhesus (Rh) group D antigen on your red blood cells (there are over 30 Rh antigens, but D is the important one here). The gene that makes the D antigen is called RHD, which is located on chromosome 1. Most people who have a negative blood type have a deletion of this entire gene on both copies of their 1st chromosome, so this antigen never even gets close to being made.

Blood Type Mismatching Problems

Everyone knows that blood type is important in giving and receiving blood, but keeping track of who can give and get what can be confusing. The best way to keep things straight is to remember that you can’t receive any blood that has a foreign antigen on it. If you have type B negative blood, the only antigen you have is B. Anything else is going to trigger your immune system to attack, so any blood with an A or “positive” type is out of the question. People with AB positive blood have all the antigens, so they can get blood from anyone. People with O negative blood, like myself, have none of these antigens, so our red blood cells can slide past anyone’s immune system undetected. In emergency situations, when there’s no time to check someone’s blood type, they’ll use O negative blood just to be sure the person won’t have a reaction.

Blood Type and Pregnancy

It turns out your Rh type is really important if you’re a pregnant woman. Women who have a negative blood type can have problems if their baby has a positive one (inherited from their father). During the pregnancy, mom and baby’s blood are separated by the placenta, but birth is a pretty violent process, so baby’s blood can get into Mom’s bloodstream at that point. When Mom’s immune system sees the baby’s positive blood, it will identify it as an intruder and attack. When this happens, we say that Mom has been “sensitized.” This isn’t much of a problem for the baby that just left the womb, but it could mean trouble for the next tenant. Once Mom’s immune system has identified an intruder, it makes more antibodies against that antigen so that it can attack it more quickly the next time around. If she has another baby with a positive blood type, those antibodies can cross the placenta and attack their blood during the pregnancy. This can give the baby a condition called “hemolytic disease of the newborn” (HDN), which can be life-threatening.

Preventing Problems

Luckily for Rh negative moms with Rh positive partners like me, we have a way to prevent this problem. It’s called RhoGAM. RhoGAM is an injection that’s given to moms anytime their baby’s blood may have entered their bloodstream (during bleeding, delivery, etc.). The RhoGAM is made from antibodies taken from the blood of donors who have themselves been sensitized. Some of them are moms who had babies with this problem before RhoGAM was developed and have been donating their plasma for many years. Their antibodies go around and attacks the baby’s blood cells that made their way into Mom’s bloodstream so her immune system never has a chance to see them and make her own. This prevents her from ever getting sensitized in the first place. The development of RhoGAM has saved 10’s of thousands of babies and has led this problem to be pretty rare in developed nations.

Rh isn’t the only antigen that can cause these kinds of problems, but it is the one that is most severe. There are actually a whole slew of other antigen groups that can do this, including Kell, Kidd, Duffy and even ABO. While these are less likely to cause HDN, there’s no RhoGAM-like treatment to prevent them yet.

Fun Fact! (or at least fun for a nerd like me)

You can’t always tell from your blood type which blood type genes you have, but sometimes you can tell by what blood types your kids end up with. Lets take my family as an example. My genes are straight forward; I’m O negative so I have two copies of the O version of the ABO gene, and zero copies of the RHD gene. My kids aren’t getting any antigens from me. My husband, however has an A positive blood type. This means he could either have two A versions of the ABO gene or an A version and an O version.  He could also have two copies of the RHD gene, or just one. But we do have kids, and they’re both A negative. Since they’re negative, the copy of chromosome 1 they got from their dad didn’t have the RHD gene on it. That means he must have only one copy of the RHD gene. (It also means I didn’t really need all that RhoGAM I had, but we couldn’t know that until after they were born). Now, both the kids are “A”, so we still can’t tell if he has an O version of the ABO gene to give. I guess I’ll have to convince him to keep having kids until we can figure that out. For Science!

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