What’s your blood type?

Do you know your own blood type? Well, maybe, but do you know what it actually means? I bet most of you probably don’t. I’ll put my hand up and admit that while I’ve known vaguely that I was O negative, I’ve never really considered what that actually means. What’s the difference between my blood and other blood?

The reason I started thinking about this is that it turns out having a negative blood type is relevant to me now that I am pregnant. To understand why, you need to understand what the positive/negative business is actually all about. The NHS helpfully sent me lots of really great information on this, but being incurably interested meant that I felt compelled to go away and research it even more.

So, there are two parts to your blood type, the letter, which is either A, B, AB or O and the positive/negative side.

Easy as A, B… O?

The surface of red blood cells are covered in proteins. Most of these aren’t relevant here, but one specific type is. Agglutinogens hang out on the surface of red blood cells, flaunting themselves at passing immune cells and generally having fun. There are two types of agglutinogens, A type or B type, and they’re made by two slightly different enzymes. Both enzymes are coded for by the same gene in your DNA, so you can either have a gene that produces an enzyme that makes A-type agglutinogens or a gene that produces an enzyme that makes B-type agglutinogens. These enzymes are very specific craftsmen, they’re old and set in their ways, they either make A-type or B-type proteins, they can’t make both. However, seeing as you get one gene from your biological father and one from your biological mother, you can end up with one gene for each type, which in this case would give you both enzymes because the A gene and the B gene are co-dominant, neither one overrides the other. Therefore in this example you would have blood cells with both agglutinogens types dancing around the surface, this is type AB blood. There is also a fourth and final option, you can have a gene that produces a broken enzyme. In this case, you have no agglutinogens at all, and this is classified as O-type blood.

Most of the time, your blood type isn’t a big deal, the agglutinogens hang out on the surface. However, if different blood is added to your system, for example during a blood transfusion, it could become an issue. In your plasma, you have antibodies that are designed to recognise and drive out invaders, they’re basically the bouncers of your body. So, people with A-type blood will have antibodies that are primed to recognise B-type agglutinogens as undesirables. Conversely, B-type people have antibodies that evict A-type agglutinogens. People with AB-type blood have antibodies against neither, they are the most welcoming of blood types: they’ll let any old blood cells join in the party. People with O-type blood on the other hand have both types of antibodies, and are very difficult to please when it comes to transfusions. They only like other O-type blood to come in. It’s an exclusive club.

“If your agglutinogens don’t match, you’re not coming in…”

Ironically though, despite being very exclusive, or perhaps because they are so exclusive, O-type blood cells are very welcome in any other blood party. O-type blood cells have no agglutinogens at all, so they will be allowed in by the bouncers in A, B or AB types. This makes O-types universal donors. The key thing with a blood transfusion is making sure that the cells that are added to the body aren’t evicted by enthusiastic bouncers, so you need to match the A, B, AB and O blood types to avoid this.

Although all this is really important if you need a blood transfusion, it isn’t a big deal in pregnancy. The antibodies against agglutinogens don’t develop until after birth, so any blood that crosses from the Mum to the baby or vice versa won’t be recognised or attacked. However, the positive/negative business is a little different.

Why so negative?

As well as the above letter system, your blood type can also be classified as either Rhesus positive or Rhesus negative. Again, this applies to a particular type of protein hanging out on the surface of blood cells. In this case, it’s known as the Rhesus (Rh) D antigen. If you have RhD on your cells, you’re Rh-positive and if you don’t, you’re Rh-negative. Simple as that. And again, most of the time this really isn’t a big deal. However, if you are Rh-negative and some RH-positive blood finds its way into your system, a different bouncer, another antibody, will recognise it as foreign and tell it to shove off. In the case of transfusion you can avoid this by not giving positive blood to negative people. However, in pregnancy, it isn’t that easy.

At certain stages during pregnancy, and particularly when you give birth, some of the baby’s blood is likely to make it into your system. If your baby has Rhesus positive blood and you have negative blood, your body will react by creating antibodies against the baby’s red blood cells, causing something called heomlytic disease of the newborn. Although this may not necessarily harm your first child, it would mean that if you carried a second baby, and that one was positive as well, your body would recognise the positive blood as an invader, and attack.

In my case, for example, I am O negative. My husband is A positive. Incidentally this led to quite a bit of time spent having an inane argument along the lines of “my blood is smarter than your blood”, which I obviously countered with “yeah well, my brain is smarter than your brain”. I think you’ll agree I won that one. Anyway, since the positive gene is dominant, it’s highly likely that any children we have will also be positive. The only way we can get a baby with a negative blood type is if G is carrying a recessive negative gene, and even if he is, it’s still 50:50 on which type our babies would have.

Luckily, there is an awesome solution to this dilemma: stop your body producing antibodies against the baby’s blood. To this end, pregnant women with Rh-negative blood, like me, are offered an injection to prevent this happening. I’ve got mine due in another 6 weeks, and I’ll probably be back with a post on how exactly this awesome injection works (and whether it hurts).


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