How do blood transfusions work? – Bill Schutt

In 1881, doctor William Halsted
rushed to help his sister Minnie, who was hemorrhaging after childbirth. He quickly inserted
a needle into his arm, withdrew his own blood,
and transferred it to her. After a few uncertain minutes,
she began to recover. Halsted didn’t know
how lucky they’d gotten. His transfusion only worked
because he and his sister happened to have the same blood type— something that isn’t guaranteed,
even among close relatives. Blood types hadn’t been discovered
by Halsted’s time, though people had been experimenting
with transfusions for centuries— mostly unsuccessfully. In 1667, a French physician
named Jean-Baptiste Denis became the first to try the technique
on a human. Denis transfused sheep’s blood
into Antoine Mauroy, a man likely suffering from psychosis, in the hopes that it would reduce
his symptoms. Afterward, Mauroy was in good spirits. But after a second transfusion,
he developed a fever, severe pain in his lower back,
intense burning in his arm, and he urinated a thick, black liquid. Though nobody knew it at the time, these were the signs of a dangerous
immune response unfolding inside his body. This immune response starts 
with the production of proteins called antibodies, which distinguish the body’s
own cells from intruders. They do so by recognizing
the foreign proteins, or antigens, embedded in an intruder’s
cell membrane. Antibodies latch onto the antigens, signaling other immune cells to attack
and destroy the foreign cells. The destroyed cells are flushed
from the body in urine. In extreme cases,
the massive break down of cells causes clots in the bloodstream that
disrupt the flow of blood to vital organs, overload the kidneys,
and cause organ failure. Fortunately, Denis’s patient
survived the transfusion. But, after other cross-species
transfusions proved fatal, the procedure was outlawed across Europe, falling out of favor
for several centuries. It wasn’t until 1901
that Austrian physician Karl Landsteiner discovered blood types, the crucial step in the success
of human to human blood transfusions. He noticed that when different types
were mixed together, they formed clots. This happens when antibodies
latch on to cells with foreign antigens, causing blood cells to clump together. But if the donor cells are the same
blood type as the recipient’s cells, the donor cells won’t be flagged
for destruction, and won’t form clumps. By 1907, doctors were mixing together small amounts
of blood before transfusing it. If there were no clumps,
the types were a match. This enabled them
to save thousands of lives, laying the foundation
for modern transfusions. Up to this point, all transfusions
had occurred in real time, directly between two individuals. That’s because blood
begins to clot almost immediately after coming into contact with air— a defense mechanism to prevent
excessive blood loss after injury. In 1914, researchers discovered
that the chemical sodium citrate stopped blood coagulating by removing
the calcium necessary for clot formation. Citrated blood could be stored
for later use— the first step in making large scale
blood transfusions possible. In 1916, a pair of American scientists
found an even more effective anticoagulant called heparin, which works by
deactivating enzymes that enable clotting. We still use heparin today. At the same time, American and British researchers
developed portable machines that could transport donor blood
onto the battlefields of World War I. Combined with
the newly-discovered heparin, medics safely stored
and preserved liters of blood, wheeling it directly onto the battlefield
to transfuse wounded soldiers. After the war, this crude portable box
would become the inspiration for the modern-day blood bank,
a fixture of hospitals around the world.