Blood Compatibilities: A Case of Overanalysis

It was a Friday morning when I came to my human anatomy class bringing with me stuffs I studied the previous night. I was so excited to share with my students some of the worksheets I researched for Human ABO blood groups.

I asked them to fill up first the table about the reactions that would happen (whether there would be clumping or none) if the ABO types will be applied with anti-A and anti-B sera. Of course, I excitedly discussed first the following:

Antigen – a protein that is found on the surface of the red blood cell.
Antibody – an anti-body that is produced by the protein. In no case shall the antigen and the anti-body be the same, otherwise, clumping will happen.

We imagined a blood extracted and two samples were placed separately in the same slide. We predicted the possible reaction of these samples to both Anti-A and Anti-B sera. We approached this by studying the antigen and antibody compatibility. Simply put as, A is not compatible with anti-A serum and so is B incompatible with Anti-B. We successfully completed the table such that agglutination (clumping) for Type A blood was seen in anti-A serum; B in anti-B serum; AB for both; and O showed no agglutination at all for both.

We then proceeded to determining the reactions when blood types are supposed to donate to other blood types. We were still caught following the antigen-antibody similarity approach to determine clumping. VJ, a student who usually came late in my class, arrived and all of a sudden reacted on how we determined clumping and non-clumping reactions. He asked me this question:

VJ: My logic is simple, sir. Is it not that when blood types are compatible, they mix and do not clump, right? How come that Type O can donate to type A and the latter cannot donate to the former, where in fact their blood can just homogeneously mix?

It was a question that stopped me and could not help but agree with the logic of the student. Back to the table, I wanted to prove that Type O is a universal blood donor and AB, the universal recipient. VJ and Dan, kept on insisting their points such that I encouraged them to prove their claim. Later on, the class became tupsy turvy.

This challenged me. Being a generalist has always been my excuse. But this is a proof of how much a prepared teacher can be blown away by some thoughtful questions. After the discussion, we went to the clinic for a demonstration of using the sphygmomanometer in getting one’s blood pressure. While they were there, I went home to get my bio books. It was just unfortunate, that during those times, my logic was completely crashed and I could not gather my thoughts about the problem. I let time passed by, but the grounded theory indeed hit me home. I thought of the situation, reflected and pooled again my blown concepts.

And so, I realized that it was just a case of over-analysis.

Here’s one simple thing that costs me the day: A contains an antibody that is anti-B. So any blood type with such antigen would cause clumping. B contains an antibody that is anti-A, too. See the table.


Type A has anti-B so it cannot receive blood from persons with blood types B and AB because they contain the B antigen. Type B has anti-A and cannot receive blood from types A and AB because they contain the A antigen. Type AB can receive blood form all types because it does not contain any antibody to fight A or B antigens. Type O contains anti-B and anti-B, that is why it is incompatible to receive blood from persons with types A, B, and AB.

Type A can donate to A of course: same antigen and same antibody. It can also donate to AB because AB does not have an anti-body that is against A. Type B can donate to B and AB for the same reason. Type AB can donate to type AB, of course. Type O can donate to all because it does not have an antigen, to which antibodies of Types A, B and AB would fight against.

That’s how simple it can be. Overanalysis destroyed my day!