A person can find out their blood type by undergoing the ABO test, according to WebMD. This test identifies whether a person has an A, B or O antigen. In addition, medics perform an Rh test that identifies whether the blood is rhesus positive or negative.
Blood type tests usually take place when a woman is pregnant or prior to a transfusion. In addition, blood typing is done for people prior to them donating blood or organs, according to Mayo Clinic. Blood tests also take place during DNA tests and some criminal procedures, though individuals may also request blood tests if necessary.
During these tests, a lab technician identifies whether the A, B or O antigen is present, according to WebMD. If a person has the A antigen, their antibodies reject B blood. Similarly, if someone has the B antigen, their antibodies reject A blood. In rare cases, people have both A and B antigens, which allows them to receive A, B and O blood. People with O antigens can only accept O blood, however those with O negative blood are considered "universal donors" as their blood type can be used by anyone. Blood typing is very important, as receiving the wrong antigens can result in illness or death.
In addition to identifying the blood antigen, finding out about Rh sensitivity is necessary, according to WebMD. Everyone has either Rh negative or positive blood. Individuals with Rh negative blood can only receive Rh positive blood once, according to Medscape. After this, their body produces antibodies against the Rh factor. This is particularly important for Rh negative pregnant women who may carry an Rh positive fetus. Unless they receive Rh immunoglobin, their body rejects future fetuses with Rh positive blood.