What does a lab tech do? Part deux

So let's say you are due for a major surgery in a few days; one thing that physicians have to consider is the possibility of significant blood loss. In order to prepare for this scenario it is important to have a couple of units of blood on hand in case a transfusion is necessary. But it is not as simple as transfusing whatever blood is available, certain steps have to be taken to ensure that the blood given is safe.


In the days before a surgery the lab - in this scenario, the blood bank - will receive a blood sample from the patient. The blood group of the patient will have to be determined before appropriate units of blood can be issued. Before I explain how the units are selected I should say something about red blood cell (RBC) antigens and their corresponding antibodies.

I think most people are aware of the basic blood groups. Those are A, B, AB, and O. There is an additional antigen involved which is a part of the Rhesus (Rh) group - also known as the D antigen. Depending on the presence or absence of this antigen you will see A-, A+, AB-, AB+, O-, O+ (O type blood just means you lack A and B antigens, there is no O antigen)

A red blood cell antigen is basically a protein or a sugar that extends from the surface of the RBC. If your blood type is A, than your RBC's have an A antigen; this means that you lack the B antigen and that B will be considered foreign by your immune system. A type A person who is exposed to blood cells that have the B antigen will reject those cells - the immune system will destroy those cells. This is why it is very important not to transfuse someone who is type A with blood from someone who is type B.

Now, if you centrifuge a tube of blood that contains anticoagulants you will get the following:

Anyway, the antibodies produced by the immune system are found in the plasma, and this is important in matching a unit of blood to a patient. So let's say the patient going for surgery is type A negative. This person has the A antigen and so has antibodies in their plasma against the B antigen (since it would be recognized as foreign). Furthermore, they lack the D antigen (hence A negative) and so will produce antibodies which attack the D antigen.

(note: erythrocytes are the same thing as red blood cells)

In order to determine the blood group of a patient you have to expose the patients RBC's to commercially prepared antibodies. If anti-A antibodies cause the blood cells to clump than the patient is group A; you can confirm this by exposing the patients plasma to blood cells that are known type A or B; an A type patient will have anti-B in their plasma, and so should their plasma will react with B cells.

So, we determine our patient to be A neg. We know the patient is Rh negative because their cells do not react with anti-D plasma antibodies. So what can we transfuse the patient with? A neg units would be the first choice. You can also transfuse with O neg cells since those cells have neither the A, B, or D antigens, and will not react with anti-A or anti-B. That is why they call O negative blood the "universal donor". (note: Blood cells are washed several times before transfusion to remove the plasma. So while O negative donors would have antibodies in their plasma against A and B antigens, those antibodies are washed away.)

Conversely, if you are going to be transfused with plasma, AB positive would the safest bet since that donor will not have any antibodies in his or her plasma against A,B, or D antigens. This is the same reason why someone who is group AB is considered a universal recipient for blood.

It can get quite confusing. Here is a good chart for blood transfusion:

There is one other check we perform, and that is to rule out the presence of any irregular antibodies. the ABO blood group system is only one of many, and people may form antibodies to any number of other blood groups if they are exposed to blood through previous transfusions or pregnancy. It is pretty rare, but people who routinely receive blood often do develop these antibodies. If they have, compatible donor blood will be chosen.

I hope all this makes a modicum of sense!