Your blood type is one of the many things you inherit from your parents. But what does it mean, and how does it affect your medical care? Do you have a rare blood type?
Like eye color and hair color, we also inherit our blood type from our parents. Doctors use a few different classification systems to group our blood into types, but ABO system and our Rh status are the most important. Your blood can be type A, B, AB, or O and each one of those types can be either Rh positive or Rh negative, giving eight possible blood types.
Many of us may not even know what type of blood we have so … does it matter? What does having different blood types mean for us?
What does your blood type mean?
According to the Red Cross, someone in the US needs a blood transfusion every two seconds. Your blood type affects how healthcare professionals respond if you should need a blood transfusion. Let's find out why.
The presence (or lack) of A and B antigens on the surface of your red blood cells determines your ABO blood type. An antigen is typically any substance that triggers a response from your immune system, usually an attack to ward off the foreign antigen invader.
Our red blood cells are covered in hundreds of known antigens—like sugars or proteins—that our immune system ignores. These are sometimes called self-antigens. But if our immune system encounters blood antigens it doesn’t recognize, even if they're totally normal blood antigens for somebody else, it could register those antigens as uninvited guests and launch an attack.
Someone with type A blood has only the A antigen on their red blood cells while someone with type B blood has only the B antigen. Those with type AB blood have both and those with type O blood have neither. So not all blood types mix well together.
Anyone can receive O-negative blood no matter what blood type they have.
We call people with O-negative blood the universal donor. Anyone can receive O-negative blood no matter what blood type they have. That is why emergency room doctors always give O-negative blood to trauma patients when there’s not enough time to determine blood type. On the other end, a person with AB-positive blood is known as the universal recipient because if you have AB-positive blood, you’re in luck: should you need a blood transfusion, you can take any blood of any type. But all other pairings are not so universal. People with type O-negative blood, for example, can only receive blood from other type O-negative people. People with type B-positive blood are a little more accepting: they can receive blood from anyone with type O or type B blood, positive or negative.
What does having negative or positive blood mean?
Rh status indicates whether you have the Rhesus D protein on your red blood cells (positive status) or not (negative status). We inherit our Rh status genetically just like our ABO type and any ABO blood type can be either Rh positive or negative.
If you happen to have a child like mine who is obsessed with zoo animals, you may recognize the name “rhesus.” It's a type of adorable monkey known for its ability to live around humans. With a little digging, I learned that this antigen was originally named after the rhesus monkey because when it was first discovered in 1937, scientists thought it was similar to an antigen found in the red blood cells of rhesus macaques. This turned out later not to be true, but by then the use of the term was so widespread that the name stuck. (This reminds me of how planetary nebula in astronomy actually have nothing to do with planets.)
If someone with a blood type that does not produce the Rhesus D antigen (that’s someone with Rh-negative blood) is given blood that does contain the D antigen (that’s Rh-positive blood), the person’s immune system may produce anti-D antigens to fight those proteins off. So Rh-negative blood can be given to either Rh-positive or Rh-negative patients but Rh-negative patients can only receive blood that is also Rh-negative.
Anyone with Rh-negative blood who has been pregnant may remember receiving the largest shot they’ve ever seen. It's so large that nurses administer it in— Well, let’s call it a "juicier" spot—the rump. Normally, there is no blood mixing between mother and fetus, but there are times when it can happen, like during childbirth. So to be safe, the lucky Rh-negative mother gets a shot (the FDA-approved version is called RhoGAM) to safeguard against this anti-antigen production.
How rare is my blood type?
The Standford Blood Center gives estimates for how rare versus how common different blood types in the ABO system are. Most common are type O-positive and type A-positive which each make up around 35 percent of the population. Another way to think about this percentage is that in a group of three people, one of them is likely to be O-positive. The rarest blood types are B-negative (only one in 67 people have B-negative blood) and AB-negative (only one in 167 people have AB-negative blood). O-negative blood, that universal donor that we need for emergency situations, only circulates in one in fifteen of us.
These percentages vary a bit with ethnicity. According to the Red Cross, 8 percent of white people have type O-negative blood, and that percentage decreases to 4 percent for Black people, 4 percent for Hispanic people, and only 1 percent for Asian people.
O-negative blood, that universal donor that we need for emergency situations, only circulates in one in fifteen of us.
Looking just at Rh status, only 15 percent of the population has Rh-negative blood. There are some very interesting theories (which I won't link to—this is a science podcast after all!) about how the relative rarity of Rh-negative suggests it was derived from an alien race.
As I like to tell my students, could it be a coincidence that I have a rumored alien blood type and I worked for NASA where I could potentially have covered up alien secrets? (The answer is yes, it is a coincidence.)
Can your blood type affect how you get illnesses?
We’ve discussed on earlier episodes how your blood type can affect whether or not mosquitos like to bite you—type O blood is twice as attractive to the pests than type A blood. But more research is needed to determine if blood type affects whether or not you're more prone to attracting head lice.
Doctors have been under tremendous pressure to figure out what causes some people to be asymptomatic carriers of COVID-19. The evidence is still conflicting.
More recently, doctors have been under tremendous pressure to figure out what causes some people to be asymptomatic carriers of COVID-19 while others have much more severe symptoms requiring hospitalization and even leading to death. One potential differentiating factor they’ve investigated is blood type. As I record this podcast, the evidence is still conflicting.
Doctors have been under tremendous pressure to figure out what causes some people to be asymptomatic carriers of COVID-19 while others have much more severe symptoms requiring hospitalization and even leading to death.
Researchers in China first reported in March 2020 that patients with type A blood were more likely to contract the virus. They observed that of approximately 2,200 patients hospitalized for COVID-19, 38 percent of them had type A blood, which was higher than the percentage (31 percent) of those with type A blood in a sample of 27,000 healthy people. They found the opposite for those with type O blood—they made up a higher percentage (34 percent) of the healthy group than they did for the sick group (26 percent).
Similar results were observed in a study of New Yorkers—people who live where one of the largest and earliest surges in the United States occurred—and in a study of COVID-19 patients in Italy and Spain. The latter study even found a gene variant that was linked to severe illness and a greater risk of death due to COVID-19. The variant resides in an area of the genome also associated with blood type determination and with the functioning of the immune system. Those researchers found those with type A blood were 45 percent more likely to encounter respiratory failure while people with type O blood had a 35 percent lower risk of such an outcome.
Another recent study, however, found no link between blood type and the need for intubation or covid-related fatality when examining over 1,200 patients from multiple institutions and hospitals. They did find a link between blood type and the likelihood of testing positive for COVID-19: among those with symptoms, people with type B-positive and AB-positive were more likely to have a positive test result, while those with O-type blood were less likely to get that positive result.
The field of COVID-19 research is still very new and so more studies are needed to determine whether or not blood type plays a role.
How do I find out my blood type?
Curious about your own blood type? Your doctor may have your blood type on record. Pregnant women are often typed to determine their Rh status and you’ve likely been typed if you have received a blood transfusion.
Most blood banks will tell you your blood type when you donate blood.
But blood typing is not standard practice for most patients. There are at-home do-it-yourself blood typing kits and most blood banks will tell you your blood type when you donate blood. Now, some will even tell you if you have antibodies for COVID-19.
If you’re interested in donating blood, contact your local hospital or visit the Red Cross for information.