http://www.msn.com/en-us/health/medical/what-different-blood-types-mean-and-how-to-find-out-yours/ar-BBRxqmI?li=BBnba9O&ocid=iehp

Studying earlier experiments with animals, scientists had noticed that these foreign bodies were broken up in the human bloodstream by hemoglobin, which resides on red blood cells and carries oxygen through the body.

In 1901, Karl Landsteiner realized that similar reactions were occurring with human-to-human blood transfers. Common conditions, like shock or jaundice, could be coming from these failed transfers.

His work attracted moderate attention- a successful blood transfusion was made using his work in New York in 1907  … but in 1909, when Landsteiner was able to classify human blood into a variety of type, his work really took off. These groups are what are commonly known today as A, B, AB, and O groups.

Later in life, Landsteiner moved to New York. There in 1937, with forensic scientist Alexander Weiner, he discovered the Rhesus factor. Named after a similar property found in the Rhesus monkey, all blood has a Rhesus (Rh) factor, either positive or negative. Both of these discoveries would become crucial to how blood transfers work today.

While it would be several decades until the exact chemical makeup of antigens were discovered, with help from scientists like Elvin Kabat, Landsteiner laid down the basics.

The four major groups are decided by if they do or do not have two antigens, commonly known as A and B. Each of these antigens have an Rh factor, a positive or a negative. That's what leads to the commonly known blood types A+, A-, B+, B-, O+, O-, AB+, AB-.

A  person with blood type  O is called a universal donor, and is able to donate blood to a person of any blood type ... but is able to receive blood only from a person with the same blood type.