Originally posted on 27 March 2012, we are reposting this piece to make sure you are adequately prepared for The Walking Dead to return from hiatus on AMC Sunday, 10 February 2013 at 9PM (ET). We’ll have a third post in this series for you on Saturday. 

There are spoilers below. You’ve been warned. If you are even further behind watching The Walking Dead - Season 2 than me and can’t bear the thought of being spared the waste of those hours of your life, this is not for you.

I have recently started catching up on The Walking Dead – Season 2. At the end of Episode 1, Carl Grimes, son of our hero, Rick Grimes, is wounded in a hunting accident. Episode 2 is then devoted to keeping Carl from bleeding out. This means surgery and blood transfusions. Unfortunately, you can’t just stick blood from one person into someone else¹. The immune system will attack a blood transfusion as a foreign invader if the donated blood is not compatible with the recipient. These reactions can be fatal. Keeping blood banks stocked is difficult in our modern world. Trying to find appropriate blood donors for a kid with internal bleeding in a zombie-filled, post-apocalyptic wasteland poses a particular challenge.

Although there are 30 human blood group systems, the two main ones we worry about for blood donations are the ABO and Rh systems. In the ABO system, each individual has two versions of the ABO gene. These versions can express either an A antigen, a B antigen, or no antigen at all (O). This gives rise to the four possible ABO types in which the individual expresses only A antigen (A), only B antigen (B), both A and B antigens (AB), or no antigen (O). The Rh system actually considers many different antigens, but we focus only on the D antigen, which is important for predicting certain risks like hemolytic disease of the newborn. If an individual expresses this antigen, they are considered Rh+ for blood donation purposes. If they do not, they are Rh-.

Carl happens to have blood type A+ (ABO: A; Rh: Rh+). Fortunately for Carl, his dad, Rick, is also A+, as are 35.7% of Americans. Unfortunately for Rick, no one else in the show seems to be A+. Being heroic, Rick stands ready to be bled dry in order to save Carl. This is pretty useful to the plot of The Walking Dead. It severely limits the amount of blood Carl can receive, which means they can’t wait to perform surgery. It also means that Rick can’t go with Shane and Otis to get surgical supplies, which is key to later events.

It may be a good plot device, but how likely is it that Rick would be the only donor available?

We are used to thinking of blood type compatibility from watching medical dramas. Blood from blood banks is usually separated into its different components. When the actor playing a doctor on TV is yelling for two units of O Negative, they are asking for packed red blood cells. The patient is, essentially, only receiving the donor’s red blood cells.

In The Walking Dead, they are having to do a direct transfusion from Rick to Carl, which means that Carl is receiving all of Rick’s blood components directly from Rick’s veins. This raises additional issues with blood type compatibility. While Rick’s red blood cells express the A and Rh antigens, his plasma will have antibodies against the B antigen. A person with B- blood would have cells with the B antigen, but plasma with antibodies against the A antigen. Rh- people usually do not have antibodies against the Rh D antigen without some unusual prior exposure.

Which is a long way of saying that Carl could only receive a direct blood transfusion from an A+ or an A- donor. So, how likely is it that there would be no other compatible donors in the group?

By my count, at the point that Carl is bleeding into his abdomen, there were 11 potential donors in their survivor group, excluding Rick, Carl, and the surgical team of Hershel and Patricia. According to the American Red Cross, 33% of American caucasians are A+ and 7% are A-. The group also includes Glenn, who is of Asian descent (A+: 27%; A-: 0.5%), and T-Dog, who is of African-American descent (A+: 24%; A-: 2%)². From these data, the odds that the group won’t contain another compatible donor are 1 in 185³.

Many people, however, do not know their blood type. In this situation, the potential donors need to know that they are compatible. If a person does not know their blood type, they cannot be considered as a compatible donor. I can’t find reliable data about how many people actually know their ABO/Rh blood type. So, I tried several values for the “know your blood type” variable using two different models. In the “random” model, the 11 potential donors all have the same chance of knowing their blood type. In the “mixed model”, some of the characters are assumed to know their blood type due to their background. Rick and Lori seem to know Lori’s blood type. Shane is a public safety officer. Otis volunteers with EMTs. Hershel’s family seems very medically aware. The results of these models are graphed below.

I also modeled how the number of people in the group that we know are aware of their blood type affects the odds. Both these analyses show that at relatively low frequencies of general blood type awareness, it becomes quite likely that the group would contain another compatible donor.

In the already unlikely world overrun by zombies of The Walking Dead, this situation does not eliminate credibility, but it does strain it. Could they have chosen a better blood type for their storyline?

AB- is not only the rarest blood type, but can only receive direct transfusions of AB- blood⁴, due to the presence of antibodies against either the A or the B antigen, or both, in the donor plasma and possible reaction against the Rh D antigen. Even when all the characters know their blood type, there is only a 6% chance that there would be another donor in the group. For the record, AB+ and B- would also allow me to sleep easy and I would consider anything other than O+ and A+ (i.e., the one they chose) plausible. As if my suspension of disbelief wasn’t already being tested by the zombie plague. . .

Of course, if we are going to keep the plot device of having Rick being the only available donor for Carl⁵ and give Carl a rare blood type (e.g., AB-), we are going to have to worry about Lori’s blood type and how likely it is that father and son would actually match.

Look, it’s a post based on something from the horror genre. I’m required by both law and cultural convention to inject a last second, really obvious setup for the sequel.

NOTES

1. As Van Helsing does in Bram Stoker’s Dracula.
2. These numbers don’t quite match up with the 35.7% A+ numbers from above. I had to use different sources for the values and these things do wobble over time and from study to study.
3. This estimate has some error in it due to the details of the situation. Two individuals (Maggie & Beth) in the group are related, making their blood types non-independent and perhaps not representative of the wider population.
4. In the pinchiest of pinches, an AB- individual could try blood from an AB+ individual. Once. If the AB- individual has no prior exposure to the Rh D antigen, the one-time transfusion may not cause a transfusion reaction. Maybe.
5. If we have no available donors, Carl would die in the first ten minutes, the rest of the episode would just be a funeral, and the rest of the season’s plot line would be hosed.