The Letter Salad: Understanding Blood Types

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

An antigen is any molecule (could be a protein, carbohydrate, piece of nucleic acid, etc.) that can become a target of the immune response. The immune system is our bodies’ natural defense against bad stuff. 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 created by bacteria or viruses would be examples of non-self antigens. Non-self antigens typically become the target of an immunological reaction 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. Self antigens include all the proteins that our genetic material codes for. Under normal circumstances they do not become the target of the immune response (except in certain autoimmune conditions.

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 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 threat (ie: virus or 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 the membranes of their red blood cells. The most commonly talked about are the “A” and “B” antigens.

Remember that antibodies bind antigens.
If a person has type “A” blood than their body naturally produces type “A” antigens. 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. Alternatively, a person with type “AB” blood would have both “A” and “B” self-antigens on their red blood cells, and therefore would not produce antibodies to either one of these.

Type “O” blood lacks both “A” and “B” antigens. Therefore, people with type “O” blood 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 the blood of a type “O” patient attack the transfused blood; this leads to their destruction, which results in hemolysis (lysis of the blood cell). 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 red blood cells to any other human being because their cells do not contain any antigen (important caveat: this is not entirely true as there are other important antigens besides the "A" and "B" types). If they lack antigens on the surface of their red blood cells than the recipient of the blood cannot “attack” those cells. On the other hand, type "AB" individuals are universal recipients since they produce no antibodies; therefore they 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.