Karl Landsteiner was born in Vienna, Austria, in 1868. He was essentially raised by a single mother, as his father died when he was only six. He finished medical school at age 23 and then began traveling to study under other famous scientists of the day. He often couldn't find research jobs (which were his passion) and he would make his living by doing autopsies at "deadhouses," which we call morgues today. But he always persevered and no matter what his circumstances, he would carve out a space to do research. Somewhat reclusive and pessimistic by nature, he felt at home in the laboratory and made it the focus of his life.
One of the topics he researched and found interesting was human blood. Doctors had tried giving people transfusions (a dose of another person's blood) but it was actually just pure luck if it worked. Sometimes the patient became much better and other times the patient had a fatal or deadly reaction. Before he discovered that people had different blood group types, people routinely bled to death from ulcers, accidents, and childbirth problems. No one knew that there were four different types of blood (A, B, AB, and O) and that if you gave a person the wrong type of blood, they could die from a reaction between their blood and the donor's blood. If a person receives the wrong type blood, a terrible reaction begins. This can start a chain reaction of other problems, in which the red blood cells will begin to react with the new blood cells, and then the cells will actually lyse, or fall apart, in the blood vessels. This releases hemoglobin that can damage the kidneys, which can lead to death.
In 1901, Landsteiner discovered that different people's blood had different characteristics that made it "incompatible" with other people's blood that didn't carry those same traits. He discovered the A, B, and O blood types. His discovery of the differences and identification of the groups that were alike made it possible for blood transfusions to become a routine procedure. This paved the way for many other medical procedures that we don't even think twice about today, such as surgery, blood banks, and transplants.
Landsteiner is known as the "melancholy genius" because he was so sad and intense, yet he was so systematic, thorough, and dedicated. He wrote 346 papers during his long career contributing to many areas of scientific knowledge. He is considered the father of Hematology (the study of blood), Immunology (the study of the immune system), Polio research, and Allergy research.