MadSci Network: Immunology |
O positive is indeed the most common blood type. The only testing that is routinely performed is for the major blood groups A, B, and O, as well as for Rh (positive or negative). The reason we test for A, B, or O is because people have antibodies to the major antigens which their own red cells do not express (ie, people with type A have antibodies to type B antigens, etc.). However, there are hundreds of "minor" blood group antigens that are not routinely tested for because they do not generally cause a problem if a recipient of one type receives a transfusion of another type. This is because people do not have preexisting antibodies to these minor antigens. These minor antigens can cause a problem, however, in patients who get multiple transfusions. For example, if a patient gets a unit of red cells that expresses minor antigen "x", and that patient's own red cells do not have antigen "x", then that patient may develop antibodies to antigen "x". The next time that patient gets transfused with a unit of red cells expressing minor antigen "x", the antibodies may lyse those cells. These reactions to minor antigens are generally not life-threatening, but if the patient lyses the cells, it defeats the whole point of the transfusion. We don't screen for these antigens because 1) there are hundreds of them, and 2) they generally are not life threatening and 3) we would generally pick up a potential reaction during testing that is done prior to transfusion called a cross-match. Cross matching is performed like this: Whenever a unit of red cells is being prepared for transfusion into a patient, we mix serum from the recipient with cells of the unit that we want to transfuse. If there is a reaction, we can generally identify that the patient has developed one of these antibodies and can usually identify what that minor antigen is. So if that patient needs blood, we now can screen donated units for the presence of antigen "x", and then pick ones which do not express antigen "x", thus avoiding a reaction. These are especially important for antigens which are very common in the population. If a patient does not express antigen "x" and has antibodies to antigen "x", but most of the population does express antigen "x", you can see why it might be hard to find compatible blood. It may be the case that a unit of blood that you donated was screened in this way, and that perhaps you were found to lack an antigen which is commonly expressed in the general population. This would make your blood important for people who have antibodies to antigen "x". Giving blood is very important and a wonderful gift of life. Recently, I needed a blood transfusion so I thank you very much!!
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