Using Balloons to Teach Immunology

by David Scott, Ph.D.
American Red Cross
Rockville, MD

Set-Up

The students for which this demonstration is directed can be from elementary to middle to high school; this exercise can also be modified for use in undergraduate classes with additional levels of sophistication. The underlined words are definitions that can be learned through this exercise at higher-grade levels; they need not be memorized. As background, however, all students should have been introduced to the immune system at a basic level to understand its function to protect us from infection, for example. They will need to know that antibodies are materials (proteins called "immunoglobulins") that circulate in the blood and are designed to combine with, attack, and eliminate these invading organisms. Depending on the age group, the students should realize that an antibody has the potential to combine with two antigenic determinants and that the shape of an antibody molecule is similar to the letter "Y". Students are encouraged to form the letter "Y" with the trunk of their body and hands (and older students can even be reminded of the Village People's song "YMCA". Balloons of two different colors represent the invading organisms (antigens) in this demonstration. (A larger number of colors may be confusing.) I usually use red and yellow balloons represent different bacteria (or red blood cells since this exercise may follow a lab experiment showing blood typing or agglutination).

Six to eight student volunteers are requested to sep forward and to show everyone what an antibody molecule looks like. That is, they are to hold their arms out and make the shape of the letter "Y". They are informed that antibody can grab onto (have affinity for) certain foreign materials. The "grabbers" are their hands. The teacher then brings out a large bag full of twenty or so inflated balloons of two different colors and literally shakes them out over the students, who are instructed to grab as many balloons as they can. Remember that they are acting as antibody molecules. Depending on the age group, there usually is a flurry of activity in which students try to collect as many different colored balloons as possible (see illustration below). At this point, the instructor examines what has occurred and points out the following:

1. Antibodies can only combine with two antigens since they have a "valence" of two, that is two combining sites (see next item). Therefore, all students who are holding more than two balloons (under their arms, between their legs, etc.) are instructed to let go of them.
2. Students are reminded that this antibody combining region is represented by their hands, and so they must only hold onto the appropriate balloons with their hands. For example, students can be informed that a lobster might also represent an antibody molecule with the combining sites represented by the claws.
3. Students holding different colored balloons are then asked whether this is possible

   The teacher and/or audience should inform the students that an antibody combines with two antigens, but they are both identical (same color). An exchange of balloons can then occur at this point.

4. If the students have simply held onto only two balloons each at this time, they should be

   asked if any other kind of formation can occur. The teacher will then suggest that the individual antibodies might link up with a shared antigen. This is a slightly more complicated concept, but informs the students that an antigen may have repeating determinants and, therefore, two nearby antibodies can combine to crosslink a group of balloons (antigens). Then a group of students (for example, 3-4 students) might form a nice lattice, each holding one balloon, but connecting to another student holding the other side of that balloon. Thus, they form a human agglutination reaction!

5. Now that the students have formed individual lattices (breaking down into two groups binding balloons of identical colors, a red clump and a yellow clump), you can ask what might happen next. To eliminate these balloon "invaders" from the body, it is convenient at this point to have a small needle hidden in the palm of your hand. As theatrically as you may wish to do so, start punching holes in the balloons to show how the body also has components (called complement), which are able to directly lyse (literally poke holes in) antibody-coated invaders to eventually lead to their destruction and elimination. Alternatively, you can capture the antibody-coated balloons in a net to mimic uptake (phagocytosis or eating by cells) by white blood cells called macrophages, which destroy the invaders.

Concluding Remarks

This exercise, when coupled with some lab demonstrations, dramatically illustrates antibody structure, valence, specificity, and how the immune system works together with other components to eliminate potentially hazardous invaders. The active involvement of a group of students with the balloons stimulates a role play for deeper understanding of the process. This exercise has been performed successfully with students as young as ten to twelve years of age, as well as with adults including microbiologists and biology teachers.