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O negative blood type is referred to as the "universal donor", however this applies to blood products only. Tissue transplantation is a more complex case requiring the matching of several different factors referred to as major and minor histocompatibility complexes. This answer will only approach blood transfusions as the immunology of tissue transplantation is extremely complex.

Human red blood cells possess a particular marker on their surface (a glycoprotein know as the H antigen) which allows the body (specifically the immune system) to recognize them. There are 3 versions of this marker and we term them A, B and O. The A and B forms are modifications of the O form which is shared between all 3 types. As the O form is common to A, B and O blood types it will not be recognized as foreign within the body.

The negative refers to the Rhesus type which is a separate marker on the red blood cell. In this case, some people have the marker (Rhesus positive) and some do not (Rhesus negative). If the Rhesus factor is present on blood cells that are given to a person who does not naturally express the Rhesus factor their body will reject those cells. If however the blood cells do not have the Rhesus factor, there is nothing to recognize and they will be accepted by the body.

There are many other antigens on red blood cells which vary between individuals however the H antigen and the Rhesus factor are the most important. In a person who has never received a blood transfusion it is satisfactory to match only these 2 markers. People who have had previous transfusion however may have built up immunity to other less important antigens. These patients require more extensive cross matching of any blood they are to receive to prevent rejection.

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13y ago

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