Blood Types

A, B, AB and O

Blood Types. Steve Dunwell / Getty Images

Blood transfusions can save lives, especially in patients with shock. However, you can't just go hooking up intravenous lines between any two people willy-nilly. Patients' blood comes in multiple blood types and different types can be less compatible than Apple and Android.

The most basic blood typing is to categorize blood based on its genetic makeup and a protein antigen that will be present on the outside of the red blood cells. This is known in the medical community as the ABO system and it uses markers for two antigens.

ABO Groups

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.

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

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 other proteins in the blood that can affect compatibility between a donor and a recipient. The most common is known as the Rhesus factor or the 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. So a patient with Rh- blood cannot receive a transfusion from a donor with Rh+ blood because the recipient's body 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 Does "Type & Cross" Mean?

Doctors on TV say it all the time: "Nurse, I need you to 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 cross match. It refers to tests that blood typically goes through before a transfusion.


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. Cross matching 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.

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