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Understanding Blood Types

Antigens || Antibodies || Blood Types || Leave a Comment || Search

In order to understand the different blood types we first have to understand two key words: antigen and antibody.

An antigen is a molecule (could be a protein, carbohydrate, piece of nucleic acid, etc.) that can become a target of the immune response. Antigens can be further broken down into two categories: self and non-self.

Non-self antigens are those molecules that are foreign to our body. For example, proteins on bacteria or viruses would be examples of non-self antigens. Non-self antigens typically become the target of the immune response because the body recognizes them as foreign, and attempts to eliminate them.

Antigens are either
"self" or "non-self".
Non-self antigens
are the target of
the immune response

Self antigens, on the other hand, are molecules that our body makes naturally. For example, self antigens include all the proteins that our genes code for. Under normal circumstances they do not become the target of the immune response (except in certain autoimmune conditions, see rheumatology).

Antigens, both self and non-self are displayed by nearly every cell in the body on what are known as major histocompatibility complexes (MHCs). Immune cells then interact with these MHCs to determine if the antigen attached to it is self or non-self. If it is non-self, an immune reaction begins; if it is self, the immune system remains quiescent

bind antigens

The second term we must understand is "antibody". Antibodies are proteins that our immune system makes. Their sole purpose is to bind to antigens and neutralize them.

Antibodies are formed against viruses, bacteria, parasites, etc. In addition, antibodies are the basis behind many vaccines. The vaccine is designed to induce antibody production against specific components of the virus/bacteria it is trying to protect us against.

So how do antibodies and antigens relate to blood types? First, every person has specific antigens on their red blood cells. In other words, every person has self-antigens that stud the surface of their red blood cell membranes. The most commonly talked about are the "A" and "B" antigens.


Red Blood Cells
If a person has type "A" blood than their body naturally produces type "A" antigen. Since humans do not normally produce antibodies against their own self antigens (this would be effectively attacking your own body), people with type "A" blood do not produce type "A" antibodies; however, they would produce type "B" antibodies. A person with type "AB" blood would have both "A" and "B" self-antigen on their RBCs, and therefore would not produce either "A" or "B" antibody.

Type "O" blood lacks both "A" and "B" antigens. Therefore, these people produce antibodies against both "A" and "B" antigens. This is why giving someone with type "O" blood a transfusion of type "A", "B", or "AB" blood results in a transfusion reaction. In essence, the antibodies floating around in a type "O" patient's blood attack the transfused antigens on the donor red blood cells. This leads to their destruction (aka: a hemolytic, literally meaning "lysis of blood", transfusion reaction). To review:

Blood Type Antigens Produced? Antibodies Produced?
O None Type A and B
A A Type B
B B Type A
AB AB None

Using the same logic, a patient with type "O" blood is a universal red blood cell donor. They can donate their RBCs to any other human being because their cells do not contain any antigen (caveat - this is not entirely true as there are other important antigens besides the "A" and "B" types). If they lack antigen the red blood cells cannot be attacked by the recipient's antibodies. Type "AB" patients are universal recipients since they produce no antibodies, and therefore can receive "A", "B", "AB", or "O" blood.

Before transfusing blood it is important to do several tests to ensure that the appropriate blood is given. The first test is the "type and screen." In this test the patient's ABO/Rh status is determined and an antibody screen is done. The second test is the "type and cross", in which the patient's blood and donor blood are tested for compatibility.


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