How Blood Grouping Works

How does blood grouping work and why do we have blood groups? All of us know the terms A- and B+ and so on, and is that the only classification that exists for grouping of blood types? How does all of this work?

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Before I begin, it will be useful to know a few additional terms in case you don’t already know them.

  • Anti-gen, is a molecule that is capable of causing an immune system response. It can be a protein, polysaccharide, lipid or nucleic acid.
  • Anti-body, is a protein that binds to an anti-gen, to allow our immune system to identify and deal with the anti-gen.
  • Plasma, is the yellowish liquid that remains after blood has been prevented from clotting by the addition of an anti-coagulant; separate from white and red blood cells.
  • Serum, is the clear liquid that remains after blood has clotted.

Blood grouping is based on the red blood cell (RBC) antigens and blood typing is a specific reaction pattern within a given blood grouping system. This statement will make a little more sense soon.

Of the 33 different blood grouping systems, representing over 300 antigens, the most commonly used and known system of blood grouping is the ABO system, which most of us are familiar with. Apart from the ABO system, there are others such as H-antigen, Rh system, MNS antigen system, Lutheran system and more.

The ABO system essentially refers to the following:​1​

Blood TypePlasma
Type AAgainst anti-gen BAnti-gen A
Type BAgainst anti-gen AAnti-gen B
Type ABNoneBoth A & B
Type OAgainst both A & BNone

This means, if Type A blood is transfused into a type B person, the anti-bodies in the type B person will act against the type A blood, and vice versa. Hence the need for blood type matching prior to transfusions.

On the same lines, Type O people therefore can only receive type O, since the anti-bodies in their systems will act against both anti-gens A and B, but can be given to anyone, since their RBCs contain no anti-gens. Type AB therefore cannot be given to everyone, but type AB people can receive from anyone as their plasma contains no anti A or B anti-bodies.​2​

Then there’s the matter of the negative and positive signs we see after our blood types – O+, AB- and so on. What does that mean?

That refers to an entirely different blood grouping system, the Rh system, which is the second most important system among the many that exist. As with all blood grouping system, the Rh system too is based on RBC antigens, this time 49 of them. Of these, the most important are D, C, E, c, and e. ​3​

While the system itself is one of the most complex, for the purposes of this article it suffices to say, the negative or positive suffix on an ABO status refers to the absence of presence of Rh anti-gen D or Rh(D).

To conclude, the term AB- refers to two blood grouping systems, the AB in that meaning the RBCs have both A and B anti-gens, and no anti-bodies in the plasma, and the negative sign indicates the absence of Rh anti-gen D. I hope this article helped clear your understanding of how the most common systems of blood grouping and typing work.


  1. 1.
    Mitra R, Mishra N, Rath G. Blood groups systems. Indian J Anaesth. 2014;58(5):524-528. doi:10.4103/0019-5049.144645
  2. 2.
    Yu Z, Kastenmüller G, He Y, et al. Differences between human plasma and serum metabolite profiles. PLoS One. 2011;6(7):e21230. doi:10.1371/journal.pone.0021230
  3. 3.
    Bethesda DL. Blood Groups and Red Cell Antigens. NCBI. https://www.ncbi.nlm.nih.gov/books/NBK2269/

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