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How is Blood Type Determined?

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Last week’s season finale of The Knick saw the cocaine-addicted Dr. John Thackery try to solve the mystery of blood types before another doctor could do it. I won’t spoil what happened for you here, but the episode did make me wonder: How are blood types determined these days?

First, some basics about your blood. There are 4 to 6 liters of it in the average adult body, and all blood is made of the same essential elements: Red blood cells, which, using a protein called hemoglobin, transport oxygen and remove carbon dioxide; white blood cells, which fight infection; platelets, which help blood clot; and plasma, which contains salts and proteins and is the fluid that transports the other components.

But though all blood is made of essentially the same stuff, there are some differences—which caused problems for patients receiving blood transfusions before 1901, when Austrian Karl Landsteiner discovered human blood groups (this earned him the Nobel Prize in 1930). According to the Nobel Prize website, mixing two different blood types “can lead to blood clumping or agglutination. The clumped red cells can crack and cause toxic reactions.” The problem is the immune system. Most blood contains antigens, which are substances that make the body produce antibodies. Usually antibodies are for things like viruses and bacteria, but in a wrong transfusion the immune system sees the new blood as an intruder that must be destroyed. Which can prove fatal.

Two blood group systems are important for transfusions, and Landsteiner was involved in the discovery of both. In the ABO Grouping System, there are four types of blood: Type A, Type B, Type AB, and Type O, which are determined by the presence or absence of certain antigens on the surface of red blood cells. According to the American Red Cross,

Group A – has only the A antigen on red cells (and B antibody in the plasma)
Group B – has only the B antigen on red cells (and A antibody in the plasma)
Group AB – has both A and B antigens on red cells (but neither A nor B antibody in the plasma)
Group O – has neither A nor B antigens on red cells (but both A and B antibody are in the plasma)

Immune systems are adapted to individual blood types. If someone from Group B donated blood to someone from Group A, the B antibodies would recognize the B antigen as a threat and blood clumping would occur. But if someone from Group B donated to someone in Group AB there are no B antibodies, so the immune system doesn’t recognize the intruder.

Additionally, there’s the Rh factor blood grouping system. The Rh antigen is either present (+) or absent (-) in the blood. Typically, Rh negative blood goes to patients without the antigen, and Rh positive blood goes to patients who have the antigen—but an Rh positive patient can receive Rh negative blood without any problems.

So there are eight blood groups you can belong to: A Rh+, A Rh-, B Rh+, B Rh-, AB Rh+, AB Rh-, O Rh+, and O Rh -, although doctors usually leave out the Rh and just say + or -. What blood type a person ends up with is determined by genetics.

To figure out a person’s blood type, doctors can use two methods: ABO Typing or back typing. In ABO typing, doctors take blood and mix it with serums containing the antibodies in Type A and B blood. According to the National Institutes of Health,

If your blood cells stick together when mixed with:

Anti-A serum, you have type A blood
Anti-B serum, you have type B blood
Both anti-A and anti-B serums, you have type AB blood
If your blood cells do not stick together when anti-A and anti-B are added, you have type O blood.

In back testing, known A and B cells are added to samples. If the blood clumps together only when B cells are added, the donor has type A blood. If the blood clumps together when A cells are added, the donor is Type B. And if the blood clumps when either type of cell is added, the donor has Type O blood. No clumping indicates Type AB.

Rh is determined by mixing in anti-Rh serum. If the blood cells stick together when the serum is added, the person is Rh positive; if not, the person is Rh negative.

Fun fact: Everyone knows that people with O Rh- blood are universal donors, but there are also universal receivers—they have the AB Rh+ blood type. The opposite is true for plasma donors. O plasma contains antibodies for both A and B, so it would cause an immune reaction in any other blood type. AB plasma doesn’t have either antibody, so it’s universal.

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Big Questions
Why Does Asparagus Make Your Pee Smell Funny?
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The asparagus has a long and storied history. It was mentioned in the myths and the scholarly writings of ancient Greece, and its cultivation was the subject of a detailed lesson in Cato the Elder's treatise, On Agriculture. But it wasn't until the turn of the 18th century that discussion of the link between asparagus and odorous urine emerged. In 1731, John Arbuthnot, physician to Queen Anne, noted in a book about food that asparagus "affects the urine with a foetid smell ... and therefore have been suspected by some physicians as not friendly to the kidneys." Benjamin Franklin also noticed that eating asparagus "shall give our urine a disagreeable odor."

Since then, there has been debate over what is responsible for the stinky pee phenomenon. Polish chemist and doctor Marceli Nencki identified a compound called methanethiol as the cause in 1891, after a study that involved four men eating about three and a half pounds of asparagus apiece. In 1975, Robert H. White, a chemist at the University of California at San Diego, used gas chromatography to pin down several compounds known as S-methyl thioesters as the culprits. Other researchers have blamed various "sulfur-containing compounds" and, simply, "metabolites."

More recently, a study demonstrated that asparagusic acid taken orally by subjects known to produce stinky asparagus pee produced odorous urine, which contained the same volatile compounds found in their asparagus-induced odorous urine. Other subjects, who normally didn't experience asparagus-induced odorous urine, likewise were spared stinky pee after taking asparagusic acid.

The researchers concluded that asparagusic acid and its derivatives are the precursors of urinary odor (compared, in different scientific papers, to the smell of "rotten cabbage," "boiling cabbage" and "vegetable soup"). The various compounds that contribute to the distinct smell—and were sometimes blamed as the sole cause in the past—are metabolized from asparagusic acid.

Exactly how these compounds are produced as we digest asparagus remains unclear, so let's turn to an equally compelling, but more answerable question:

WHY DOESN'T ASPARAGUS MAKE YOUR PEE SMELL FUNNY?

Remember when I said that some people don't produce stinky asparagus pee? Several studies have shown that only some of us experience stinky pee (ranging from 20 to 40 percent of the subjects taking part in the study, depending on which paper you read), while the majority have never had the pleasure.

For a while, the world was divided into those whose pee stank after eating asparagus and those whose didn't. Then in 1980, a study complicated matters: Subjects whose pee stank sniffed the urine of subjects whose pee didn't. Guess what? The pee stank. It turns out we're not only divided by the ability to produce odorous asparagus pee, but the ability to smell it.

An anosmia—an inability to perceive a smell—keeps certain people from smelling the compounds that make up even the most offensive asparagus pee, and like the stinky pee non-producers, they're in the majority.

Producing and perceiving asparagus pee don't go hand-in-hand, either. The 1980 study found that some people who don't produce stinky pee could detect the rotten cabbage smell in another person's urine. On the flip side, some stink producers aren't able to pick up the scent in their own urine or the urine of others.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
What Legal Authority Does Judge Judy Have?
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While Judith Sheindlin was a real, live judge—New York City Mayor Ed Koch appointed her to family court in 1982 and then made her Manhattan's supervising family court judge in 1986—she's not acting as one on her show. Neither are any of the other daytime TV judges (whether they passed the bar and served as actual judges or not).

TV court shows don't take place in real courtrooms and they don't feature real trials, though they are usually real cases—the producers often contact parties who have pending litigation in small claims court and offer them the opportunity to appear on TV instead. What you're seeing on these TV court shows is really just arbitration playing dress-up in small claims court's clothes.

Arbitration is a legal method for resolving disputes outside the court. The disputing parties present their cases to a neutral, third-party arbitrator or arbitrators who hear the case, examine the evidence, and make a (usually binding) decision. Like a court-based case, arbitration is adversarial, but generally less formal in its rules and procedures.

The power that Judge Judy and the rest of the TV arbitrators have over the disputing parties is granted by a contract, specific to their case, that they sign before appearing on the show. These contracts make the arbitrators' decision final and binding, prevent the disputing parties from negotiating the terms of the arbitration, and allow the "judges" wide discretion on procedural and evidentiary rules during the arbitration.

TV judges make their decision on the case and either decide for the plaintiff, in which case the show's producers award them a judgment fee, or with the defendant, in which case the producers award both parties with an appearance fee. This system seems to skew things in favor of the defendants and gives them an incentive to take their case from court to TV. If they have a weak case, appearing on the show absolves them of any financial liability; if they have a strong case, they stand to earn an appearance fee along with their victory.

If one party or the other doesn't like the arbitrator's decision, it can really only be successfully appealed if it addresses a matter outside the scope of the contract. In 2000, Judge Judy had one of her decisions overturned for that reason by the Family Court of Kings County. In the case B.M. v. D.L., the parties appeared in front of Sheindlin to solve a personal property dispute. Sheindlin ruled on that dispute, but also made a decision on the parties' child custody and visitation rights. One of the parties appealed in court, and the family court overturned the custody and visitation part of the decision because they weren't covered by the agreement to arbitrate.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

This post originally appeared in 2012.

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