The most basic classification of the hundreds of ways to type human blood, the ABO system, is important because incompatible types of blood react with each other to form sticky globs of red blood cells that can cause serious and sometimes fatal blockages.
The classification was developed in 1901 by Karl Landsteiner, a Viennese-born pathologist and immunologist. For the first time, it explained why certain blood transfusions were beneficial while others caused severe problems.
The red blood cells of people with type A blood have a protein called the A antigen on their surface; people with type B blood have the B antigen. In type 0, the cells have neither A nor B antigens; in type AB, there are both antigens.
In one example of incompatibility, type A red blood cells (erythrocytes) have the A antigen, and the liquid part of the blood (the serum) contains antibodies against the type B cells; type B blood has the opposite configuration.
So if the two types are mixed, the antibodies from the serum react to the “foreign” types, making the cells clump together.