Blood Components

Copyright © Karl Dahlke, 2022

As described in the previous chapter, the liquid component of blood is called serum, a straw-yellow solution of 90% water and 10% dissolved organic compounds and salts. Red blood cells are entrained in the serum, like fish in a river. The red color dominates, hence blood is red. Here is an illustration. Colors are not true, and in reality, red cells are crowded together, along with occasional white blood cells and platelets.

artery with red blood cells

When you donate blood, you give your whole blood, serum and cells and all. They may choose to separate out the red blood cells, there are advantages to doing that, as we shall see below, or they may simply store it as whole blood. In contrast, you can donate blood plasma, which is a more involved process, taking 3 to four hours instead of 15 minutes. You lie on a table with needles in your arms, while a machine separates blood serum from blood cells. The red blood cells are returned to your body, ready to ferry oxygen as before. The serum is retained for patients in need. Clotting factors are removed from the serum, so it doesn't clot in storage. The resulting liquid is blood plasma. This is appropriate for patients with clotting disorders, or liver disease, or patients undergoing certain types of major surgery. Most patients need the red blood cells however, thus there is a higher demand for whole blood. Feel free to donate if you're able.


Throughout history, people assumed that the heart pushed blood out to the body, then pulled it back, like the tides. This prevailing view was consistent with what we saw around us, the oceans, the seas. It's also how the lungs work, pushing breath out and taking air in, 20 times a minute, for as long as ye shall live.

In 1616, English physician William Harvey determined that blood circulates; it goes round and roun; it is not pushed out and pulled back like the tides. The heart pumps blood out through the arteries, red blood, fully oxygenated. You can feel your pulse in any major artery. The blood then returns, gently, through the veins, darker, relieved of its oxygen and laden with carbon dioxide. It reaches the heart and is pumped around again, and again. The heart is a uni-directional pump. It has valves, four valves in the human heart, which open and close to prevent blood from flowing backwards. Even an insect has a similar (though much simpler) uni-directional heart.

It's amazing how words shape our thoughts. Things that we know, concepts that are "self-evident", seem so only because of the words we are taught as a child. We know the earth is round because our parents have said things like, “He's been all around the world”, or, “The satellites circle the earth.” If this was not in our vocabulary, we would know the earth was flat, because that is what we see around us. This would be reinforced by phrases like, “the ends of the earth”, or, “the edge of the world.”

In the same way, we are use to the idea of blood circulating around the body because it's called the circulatory system. In elementary school we are taught the systems of the body, and the heart, arteries, and veins form the "circulatory system". “The blood goes around the body.” says your teacher. There is even a School House Rock song, Do the Circulation. So we know this is so. However, a thousand years ago, it might have been called the "tidal system". That would cement an idea in your head, and you would know, falsely, that the heart pushes blood out and pulls it back, like the tides. Language reflects what we know, but it also shapes what we know.


With an understanding of circulation, transfusions became viable. Put some blood in a person's vein, and it will return to the heart and circulate around the body, and quickly mix with all the rest of his blood. It seemed feasible, but they tried it with other animals, whence it was bound to fail. (There weren't a lot of volunteer blood donors in the 17th century.) Lamb's blood was the fluid of choice, and the patient experienced a hemolytic transfusion reaction every time. The practice was prohibited in France in 1668, and by the British Parliament in 1678.

Clearly we should be putting human blood into humans. In 1875, Leonard Landois demonstrated this in the lab. He placed red blood cells from one animal into the blood serum of another, and noted that the cells would clump together, or explode. The serum "rejected" the cells. He inferred, correctly, that the blood of two different species is incompatible, but this can also happen from one human to another, as we'll see below.

In 1901, Karl Landsteiner discovered two different blood antigens, which he simply called A and B. A blood cell could present the A antigen on its surface, or the B antigen, or both. He didn't understand the details of these molecules, he simply noted that different types of blood reacted with different types of serum in the lab, clumping and bursting, as described above. If a person is type A, his blood cells present A on the surface, and his serum will not attack his own blood. If it did, he would die before he was born. The blood plasma, the pale yellow liquid that carries the red blood cells about, tolerates the cells that were there from the beginning, but rejects cells of a different type. If cells, or whole blood, comes from a person of type B, the serum of type A will attack it, and the result could prove fatal. An AB cell fares no better. It still presents B, and is not tolerated. Cells that have neither A or B are called O, (think of O as 0), and these cause no trouble. Thus there are four blood types, O, A, B, and AB.

🔴 A🔴 🔴B A🔴B

It looks, from the above, like you can put O blood cells into a type A person, and you can, but blood cells were rarely filtered out and separated in the beginning of the 20th century. Whole blood was used, often directly from another person, as there was no refrigeration to store it. Let's see what happens if we put whole blood of type O into a person of type A.

The O cells are tolerated by the A serum, there's nothing wrong there. However, the O serum enters the type A person, and attacks his type A cells. The O serum has never seen A cells before, and treats them as foreign. When a transfusion involves whole blood, the types must match exactly.

It is often said that O is the universal donor, and it is, as long as you're talking about just the red blood cells; but if you are donating whole blood, O has to go to O. On several MASH episodes, an injured soldier is type AB, and they have no AB blood on tap, and are desperately looking for an AB donor in camp. It makes for a great story, and it's not unreasonable. Army units in 1952 didn't take the time to separate blood cells from serum, they used whole blood, hence types must match exactly. Furthermore, AB is rare, less than 4%, so they had to look around for a compatible donor, who was, inconsistently, a different person in various episodes.

The military prints the blood type on a soldier's dog tag, so that blood can be transfused immediately in case of a battlefield emergency. Simply fetch a pint of the same type and put it in the vein. In another MASH episode, a soldier assumed the identity of his dead compatriot, whose tour of duty was almost up, so that he could go home. He took the soldiers personal effects, and his dog tag. The tag had a different blood type however, and the soldier suffered a reaction when the wrong blood was administered.


In 1940 another blood type was discovered. It is called the rh factor, because the blood of a rhesus monkey was used to isolate it. + indicates the presence of this factor, and - indicates its absence. Type A+ presents A and RH, while A- presents just A. As a mathematician, I find this notation confusing, and a bit annoying. They should have called it C. Why not? We have A and B, and here is a third factor, independent of the other two, why wouldn't you just call it C? Then the 8 blood types would be: O, A, B, AB, C, AC, BC, ABC. Makes perfect sense. But instead, these types are: O-, A-, B-, AB-, O+, A+, B+, AB+.

There are more than 600 other antigens, which almost everyone has. If you happen to be missing one of these, your serum will reject the cells of almost everyone else, unless you can find another donor who is missing the same antigen - and agrees with your ABO type. You don't want to be in this position.


I'm going to ignore the RH factor in this section. It follows the same genetic rules as ABO.

At conception, a child receives a blood type gene from his father, and a blood type gene from his mother. Each of these could present A, or B, or neither. It is a roll of the dice.

For illustration, say the father is type A and the mother is type B. Note that the genes are not determined at this point. The father could be AO, that would present as type A, or he could be AA, which also presents as type A. The mother could be BO, or BB.

If the father is AA, the child always receives A from the father, there is no choice. If the father is AO, the child receives A or O, with a 50-50 chance, like the flip of a coin. The mother contributes B if she is BB, or B or O if she is BO.

If they have 8 children, you can probably reverse engineer their genes. Suppose all eight kids are AB. eight times, the father contributed A, and eight times, the mother contributed B. If either of them has an O to offer, it's pretty unlikely that they would not pass down an O gene even once. In fact the odds are 1 in 256. So we can be pretty certain the genes of the parents are AA and BB.

Suppose even one of the kids is type A. The Mom contributed an O, and she is BO. Similarly, if even one kid is type B, then the dad is AO. If a child is O then the Dad is AO and the Mom is BO. It has to be so.

Before DNA testing, blood type was sometimes used to rule out paternity. If the Mom is type A, and the child is type B, and Bill is type O, then Bill can't be the father. The B gene cannot appear out of thin air. Of course blood type can never establish paternity, only refute it. They did this in an episode of MASH, to prove Radar was not the father of a Korean baby.

Other Animals

Do dogs get transfusions, and if so, do they have blood types? We could ask this about any animal, but let's start with man's best friend. The answer to both questions is yes.

Dogs have more than 12 blood groups, but one antigen, DA1.1, is particularly potent. This is the antigen that is typically screened prior to a transfusion. Unlike us, the first transfusion is always safe; the dog reacts to the second transfusion of the wrong type.

As we see by this example, every animal has a different suite of blood antigens and blood types. The vet must understand these blood types, if she wants to administer a transfusion.

Star Trek

What is Mr. Spock's blood type? T negative, from the episode Journey to Babel. It is “rare, even for a Vulcan.” Spock gives a transfusion to Sarek, his father, who has the same blood type, while Sarek undergoes a delicate heart operation.

High School, then and now

Here is a memory from my youth, that could never happen today. High school biology, and we were learning about blood types. We were to determine our own types.

Each desk had two people, lab partners. Same setup as chemistry and physics. My partner was Doug Carter, a hilarious guy who use to sing Monty Python songs in the middle of class. Sometimes I'd join in but he was the instigator. We really had a lot of fun. Teacher wanted to be upset but we got A's, so he had to tolerate some of our antics. Doug was especially goofy during labs.

We had these pieces of paper with two circles and A B reagents in those circles. And we had lances, sealed so they were sterile. We were suppose to prick each other, then a drop of blood in each circle, watch for the reaction to A or B, and determine the blood type.

“I'm ready to jab you.” I offered.

“Nope.” said Doug. “I'm doing my own finger.”

He poked himself, and dripped blood in his circles, and determined his type, no problem. “Now hold out your finger.” he said.

I swear he loved the experience, practically a sadist. The lance felt like it went all the way to the bone. I had more than two drops; I bled plenty, thank you very much.

This was of course before aids. No chance of anything like this happening in today's schools.