Blood Types and Compatibility for Donations

A, B, AB, and O

There are four basic blood types in the ABO typing system:

1. Type A blood has the Group A antigens and makes antibodies to fight Group B blood.
2. Type B blood has the Group B antigens and makes antibodies to fight Group A blood.
3. Type AB blood has both Groups A and B antigens but doesn't make antibodies for either one.
4. Type O blood doesn't have either type of antigen and has antibodies to both Group A and Group B blood.

Donating or receiving blood is complicated by the fact that there are four types of blood. Type O blood, since it doesn't have antigens for either type, can be donated to recipients with all four types of blood. The antibodies present in the donated blood do not cause problems in adult recipients.

Type AB, on the other hand, since it has both A and B antigens and also does not create antibodies for either antigen, can receive blood from all four types, but can only donate to other AB recipients.

The anti-A and anti-B antibodies are present from early in life, without any exposure to incompatible blood. Therefore, an immediate reaction will happen with transfusion with incompatible blood.

But, wait! There's more!

There's another antigen present on red blood cells that can affect how nice a recipient's blood is to the donor's blood. It's called the Rhesus factor (also known as the Rh factor).

Rh Factor

Besides the ABO typing system, there are protein antigens on red blood cells that can affect compatibility between a donor and a recipient. The most well-known one is the Rhesus factor (Rh factor).

Named for the Rhesus monkeys where it was first discovered (and the first test was developed), Rh factor refers to a protein antigen that can live on the red blood cells. Those who have the protein are known as Rh positive and those who do not have the protein are known as Rh negative.

Rh Antibodies

Antibodies will be created to fight the protein in recipients who do not have the proteins in their blood naturally. These antibodies can develop in an Rh negative person due to pregnancy with an Rh positive fetus or by an Rh positive blood transfusion.

A person with Rh- blood must not receive a transfusion from a donor with Rh+ blood. The recipient's body will develop antibodies if it is a first exposure, or if antibodies have already developed they will attack the Rh+ blood on contact.

Donating and Receiving

Previously, we discussed how a patient with Type O blood can receive A, B, or AB types through transfusion. Taking into account Rh factor means that O negative blood can theoretically be transfused to any type of patient. Type O- blood is known as the universal donor.

AB+ blood, on the other hand, is blood with all the proteins already in it. AB+ patients are known as universal recipients because their bodies will accept all types of blood.

What Type and Cross Match Means

Doctors on TV say it all the time: "Nurse, order a type and cross the patient."

You probably know that it has something to do with blood, but what does it really mean? First, you ought to know that "type & cross" is short for type and crossmatch. It refers to tests that blood typically goes through before a transfusion.

Typing

As we discussed, blood types are based on several different kinds of proteins and antibodies that can be present in any individual's blood. In the terminology, type simply refers to the testing process to determine a patient's blood type.

Cross-Matching

Just because the tests all match up and the patients appear to have compatible blood types doesn't mean a transfusion will always work. Crossmatching is a test where a bit of the patient's blood is introduced to a bit of the donor's blood to see how they get along.

Ideally, the blood samples will hit it off like old friends. If they mix well and settle in for a card game and a beer, all is well. On the other hand, if they start throwing punches, It's time to go back to the drawing board (or at least to the blood bank). If incompatible blood is transfused from one person to another, the reactions can be anything from anaphylactic shock to bleeding disorders.

Just to muddy the bloody waters a bit more, a positive test is not a good thing, but a negative test is. In typical medical fashion, the terminology of a positive or negative test is not referring to the preferred outcome, but to the presence or absence of a reaction. So, a positive test means that the blood did, indeed, have a reaction. Usually, that reaction is for the recipient's blood to attack and kill the donor's blood.

A raging battle in one's bloodstream significantly distracts from the blood's ability to actually perform its work.

A negative test, however, means that the two blood samples are truly keen on each other and will work together like old partners.