Blood typing is a type of laboratory test which is used to determine someone's blood type by reacting a sample of blood with various reagents. This test is performed prior to a blood transfusion to determine which kind of blood can be used in the transfusion, and it is also part of the battery of tests performed to prepare for an organ transplant. It may also be a useful diagnostic tool for certain kinds of medical issues.
Humans have been experimenting with blood transfusion for a very long time, but it was often unsuccessful in the early days. Some transfusion recipients appeared to benefit, while others sickened and sometimes died. It wasn't until 1901, when Karl Landsteiner discovered blood types, that people understood why some transfusions took while others did not.
A person's blood type is determined by a set of inherited antigens. One very well known typing system is the ABO system, in which people can have A, B, AB, or O blood. The Rh blood grouping, which includes Rh+ and Rh- blood, is another blood group system. There are numerous others, all of which can be tested for, and may be tested for among people with unique or unusual genetic inheritances.
When typing is performed, a sample of blood is taken from the patient and exposed to reagents. If a reaction occurs, it means that the blood has antibodies to a particular blood group, which means that it cannot belong to that blood group. Several different techniques can be used to narrow down someone's type through a series of reactions to various reagents.
Reactions can be observed under a microscope. Incompatible blood will clump or react in other ways, suggesting that an antibody reaction is happening on the microscope slide as a result of exposure to antigens in the reagents. Some high school science classes perform ABO typing as part of an introduction to blood types and genetic inheritance, using kits which allow people to check for the presence of basic antibodies.
Some people are surprised to learn that even with typing, incompatibilities can still occur. This is because full testing for every known blood group is generally not performed, because it is expensive and time consuming. People can belong to the same blood group and still have incompatible blood within the framework of another blood grouping system. This makes it possible for someone to have rare antibodies which could cause an adverse reaction to a transfusion even after screening to rule out potential bad donors on the basis of common antibodies.