What's the fuss ABO-ut blood types?

“I’m a B+.”

“I’m an O-”

“I’m a U. Oh, are you not talking about grades?”

Do you know what’s worse than listening to bad jokes? Getting the wrong blood type in a transfusion. You hear doctors talk about it. You see it on all of your medical records. It’s plastered everywhere thanks to those advertisements that the blood banks around Singapore keep putting out. But if you have no clue why your blood type even matters, you are about to learn (by force.)

Yes, this is an important topic, so please pay attention. Let’s not fool around!

Antigens and antibodies

I really hope you know what red blood cells are. For the uninitiated, RBCs are cells that have the important job of carrying oxygen around in your body so that all of your organs can get some and your cells can respire instead of dying. They’re what make blood red, and they achieve their important job making use of a little something called haemoglobin (that’s what binds to oxygen and causes the red colour, and it’s so important that these cells lose their nucleus just to hold more haemoglobin).

But that’s not what we’re looking at today. Instead of what’s inside of them, we’re going to consider what’s on the outside of a RBC – antigens, specifically the A and B antigens. These are basically molecules made of proteins and sugars that serve as little identifiers of your RBCs, embedded into their cell membranes. These antigens allow the body to identify your RBCs as your own, making sure that foreign cells in your body are removed quickly (and not your own).

To this end, antibodies are present in your blood that target all antigens that are not present on the membrane. If you have A antigens, for instance, your blood contains B antibodies. If you have both, your blood has no antibodies. Et cetera.

The antigens themselves arise as a result of genes that you receive from your parents, either the IA, IB or IO alleles (variations of a gene). The first one codes for the A antigen, the second for the B antigen, and the last one for no antigen at all. You receive one copy of the gene from each of your parents, making up your final genotype; if either IA or IB are present (or both), you end up producing those antigens for your RBCs. IO codes for nothing, so if you have that with one of the other two alleles, you’ll just produce the antigen corresponding to the allele. If you have only IO alleles from both parents, you’ll just produce no antigens at all. Sorry, your RBCs are NPCs (no hate to our type O readers out there, you are more than your red blood cells!).

Apart from that, there’s the Rh (Rhesus) blood type system, which contributes the “+” or “-” you often see next to your blood type. This indicates the presence or absence of Rh(D) antigens respectively, which are attached to the surface of RBCs alongside the ABO antigens.

The Bombay O type is an example of a unique blood type where all blood cells are missing the “H” antigen. This antigen is a precursor for the standard ABO antigens and its absence results in what appears to be a type O trait, but is in fact very different. This is why parents who both have blood type AB (IA IB genotype) could end up having a child with type O blood – not because of a strange paternity/maternity crisis, but because they could both be carriers of the allele that fails to code for H antigen. Their child would thus not produce either the A or the B antigen, appearing to be type O.

There are other blood type classification systems as well, such as the MSN system, and you can check these out if you’re interested.

You are NOT my type

So, why is this a problem? When you donate blood, you aren’t actually just donating a homogeneous liquid solution. Your blood is separated by blood banks into three components: RBCs (which contain the antigens), White Blood Cells, and plasma (which contains the antibodies). Normally, when hospitals talk about having a “blood shortage”, they’re referring to RBCs, where the most important thing to verify is that the antigens in the donated cells will not cause an immune response by binding with the antibodies of the recipient’s plasma. If the donor’s blood has antigens that the recipient's antibodies bind to, the consequences could be fatal. With the antibodies designed to bind to multiple complementary antigens each, a vast number of RBCs stick (or agglutinate) together before being destroyed. Such a massive immune response comes with fever, shock, and possibly a rapid death.

Similar immune responses can arise due to mismatches in Rh type, or even in cases like Bombay blood where an immune response can be triggered by the antigens in any standard ABO RBCs. In Bombay blood, anti-H antibodies are produced, resulting in these standard RBCs (that all contain H antigens) getting attacked. Similarly, Rhesus disease occurs when the baby has an RhD positive blood type while the mother has an RhD negative blood type, so the mother’s blood contains antibodies that attack the child’s RBCs. Unfortunately, this often results in jaundice and anaemia for the baby, and can even lead to stillbirth.

This is why knowing your blood type is so important. Hospitals need it in case of a blood transfusion so that the correct type of blood is given that will not kill you. This is also why you might have heard of type O blood being the “universal donor”, since they possess no antigens and are thus unlikely to trigger an immune response no matter who they are donated to. AB cells, by contrast, can only go to AB type recipients – since both antigens are present on their RBCs, recipients with any antibodies in their blood at all will suffer an immune response. Unfortunately, in a classic case of inequality, blood type O patients can only receive from blood type O donors, while blood type AB patients can receive blood of all blood types.

You might think, isn’t that a little unfair? Why are AB types so selfish? Well, thirteen-year-old me thought the same and sent a strongly-worded enquiry to Google, filled with the rage of any social justice warrior.

Before you start flaming your blood type AB friends, let me clarify that AB types do pull their weight too. You see, although their red blood cells may cause a reaction, their plasma contains no antibodies. Plasma donation can be just as vital to hospitals in cases where traumatic injury results in the loss of clotting factors in the serum, which means the body cannot signal platelets to stop bleeding. When it comes to plasma donation, individuals of type AB get their turn to be the universal donors where their plasma can go to any recipient without fear of their RBC antigens reacting with it while O blood type goes to be the universal recipient who can receive plasma from anyone. Isn’t it nice how nature keeps a balance here?

In conclusion…

For those of you who have trouble remembering who can donate to who, here’s a helpful tip: everyone can donate to themselves, everyone can donate to type AB, and type O can donate to everyone.

I hope it’s been a helpful introduction to both antigen-antibody interactions and the blood types for you. Keep working hard, and don’t stop reading and learning!

Image Credit:

• https://www.science.org.au/curious/people-medicine/understanding-different-blood-types