Investigating blood types

The topic of blood types is often taught in school science lessons but experimenting with real blood may not be possible for many good reasons—because of the concerns of parents, the need for comprehensive risk assessments to prevent infection or the transmission of blood-borne disease, or the reluctance of students to use their own blood.

In this practical activity, simple chemical solutions are used to simulate blood types. The activity can be used in lessons, for a science club or as part of a forensic science day for students of many ages.

The science of blood

Blood is a sticky red fluid containing several kinds of cell suspended in a watery liquid called plasma: red blood cells, white blood cells and platelets (figure 1). Many chemicals are also suspended or dissolved in the plasma, including proteins, sugars, fats, salts, enzymes and gases. Each person’s blood has certain inherited characteristics that distinguish it from the blood of other people.

Until the 1980s, blood was primarily differentiated by ABO blood typing, which relies on the presence of three substances on the outside of red blood cells, called antigens. Although for forensic purposes, this technique has since been replaced by other methods such as DNA fingerprinting, for clinical purposes, ABO blood typing is still used before giving someone a blood transfusion to prevent complications such as rejection.

The presence or absence of A and B antigens on red blood cells determines a person’s ABO blood type. This leads to the identification of four main blood types: A, B, AB (when both antigens are present) and O (when neither antigen is present). A third important blood antigen is the Rhesus (Rh) factor, or D antigen. People with the D antigen are Rh positive, and those who lack it are Rh negative.

In order to type a person’s blood, antibodies (called agglutinins and sometimes referred to as antiserums) are added to a few drops of blood. These agglutinins bind to the antigens on the surface of the red blood cells, causing the cells to aggregate or clump. If clumping occurs in a blood sample, then that associated antigen is present. Once all antigens have been tested, the blood type can be deduced (table 1).

ABO blood typing experiment

Materials

Each group will need:

• Spotting tiles (dimple tray)
• Two pipettes, one for each blood sample
• 2.0 mol dm^-3 hydrochloric acid solution in a dropping bottle labelled ‘Anti-A’
• 2.0 mol dm^-3 sulfuric acid solution in a dropping bottle labelled ‘Anti-B’
• Identified ‘blood samples’ (aqueous solutions made thicker with glycerol and dyed with food colouring to resemble blood), labelled by blood type:
  
  • ‘O’ = distilled water
  • ‘A’ = 0.1 mol dm^-3 silver nitrate solution
  • ‘B’ = 0.1 mol dm^-3 barium nitrate solution
  • ‘AB’ = a 50:50 mixture of 0.1 mol dm^-3 silver nitrate and barium nitrate solutions

• Unidentified blood samples, made from the same solutions as the identified blood samples, labelled ‘victim 1’, ‘victim 2’, etc.

Procedure

Explain the scenario to your students: there has been an accident and you need to know the ABO blood type of the victims before they can be given blood transfusions. It is the students’ job to use the blood samples and work out the type of blood each victim has.

1. Using a clean pipette, put one drop of one of the identified blood samples into each of the first wells of the first two rows of your dimple tile. Complete the rows with the other blood samples, as shown in figure 2.
2. Add a drop of Anti-A antiserum to each dimple in the first row and record your observations in table 2. If you’re not sure of a result, add another drop of Anti-A antiserum.
3. Add a drop of Anti-B antiserum to each dimple in the second row and record your observations, also in table 2. If you’re not sure of a result, add another drop of Anti-B antiserum.
4. Use your results to conclude how each blood type (O, A, B and AB) reacts to the antibodies.
5. Take a clean dimple tray and test the victims’ blood using the same method. Record your observations in table 3.
6. Use your results to assign the correct blood type to each victim.

What is happening?

These tests mimic how different blood types react with agglutinins, by using simple chemistry. With older students you may wish to discuss this chemistry, pointing out the differences between the antibody-antigen reaction which is being modelled and the simple displacement reaction that is actually happening.

In this experiment, instead of clumping blood cells, the (white) precipitates make the solutions clump in the spotting tiles.

Barium nitrate(aq) + sulfuric acid(aq) → barium sulfate(s) + nitric acid(aq)

Silver nitrate(aq) + hydrochloric acid(aq) → silver chloride(s) + nitric acid(aq)

The clumps that form are dark red, instead of white, because of the food colouring present.
 

Acknowledgement

There are a few alternatives to this experiment. The solutions used in this version came from a group of science technicians from the Association of Science Technicians in Independent Schools in Western Australia (LABNETWEST)^w1.