In order to identify the blood type of an individual, we make use of the basic physiological elements found in the blood - the antigens on the erythrocytes and the antibodies found in the plasma.   In blood typing, we will mix the erythrocytes from an individual with known antibodies.  These antibodies are  raised from a monoclonal population of the B-cells that create each type of antibody - so we have separate bottle of Anti-A, Anti-B, and Anti-D antibodies (these solutions are also known as anti-sera).

As you'll find in lab, the actual typing of blood is quite simple - Take a drop of blood and mix it with one of the anti-sera listed above.   To help avoid confusion, each of the anti-sera contains a different color dye - blue for Anti-A, yellow for anti-B, and clear for anti-D.  Use a separate drop of blood, mix it with the next anti-sera, and so on. 

Interpreting the results often gets a little more complicated when you first attempt it.  The important thing to keep in mind:  If you see agglutination (clumping) - the antibody in question has found an antigen to interact with.   This is where you have to "forget" for a moment the fact that YOUR body makes antibodies in response to foreign antigens.  Since we're not relying on the antibodies your body made, if you see agglutination, that means your red blood cells have that antigen on them. In the cases illustrated below, we have two different drops of blood added into anti-B serum. 

btype.jpg (10292 bytes) notb.jpg (10687 bytes)

The sample on the left has agglutinated (or clumped), while the sample on the right shows no reaction.  Agglutination occurs as the antibodies bind to the antigen on the erythrocyte's membrane and cross-link several erythrocytes together.  They are then relatively heavy and drop out of solution.  The very sharp line of demarcation between the blood and the anti-sera is a good sign to watch for (blood cells that simply settle to the bottom of the tray don't show that very sharp distinction - that's seen on the right).  Since this is serum directed against the B antigen, the blood on the left must express the B antigen, while the blood on the right does not.  Therefore, the blood on the left is either Type B or Type AB blood (note that since I didn't show you the reaction to the anti-A serum, you can't decide if this person is type B or type AB).  The blood on the right cannot be type B or type AB, but may be either type O or type A (once again, you will need to see the reactions that occur when the erythrocytes are mixed with the anti-A before this can be determined). 

To make life a little easier, here is a table listing the reactions and what they mean:

If your blood agglutinates in: You are blood type:
Anti-A, but not Anti-B A
Anti-B, but not Anti-A B
Anti-A and Anti B AB
Neither Anti-A or Anti-B O
Anti-D Rh positive

The typing for the Rh factor (the D protein) uses antibodies directed against the D protein.  The typical blood typing tray contains wells for all three antisera (anti-A, anti-B, and anti-D) and you check all of them to determine blood type.  If you are positive for the Rh protein, you just denote a positive sign (+) after the identified ABO type (e.g. B+  indicates that agglutination occurred in both the anti-B and the anti-D sera.   Unfortunately, there are a number of variations in the D protein and it is possible to get false negatives (i.e. you are listed as a Rh negative person, but in fact are Rh positive).  In hospital labs, there are more sophisticated anti-sera for the D protein variations and blood typing there is less prone to such errors.  Using the single Anti-D sera as in lab will produce at least a couple of false negatives.  (Note:  There are also variations on the A and B antigens, but the anti-sera generally recognize those variations without difficulty). 

Special note:  It is very common for students to use the terms "clotting" and "clumping" (agglutination) interchangeably.  These two terms are NOT synonymous and I will NOT use them interchangeably on a test or in the lab. 


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