Explore how your immune system seeks out and disables pathogens and prepares your body for future attacks.
COVID-19 Learning Note: The novel coronavirus that causes COVID-19 has not previously been introduced to the human population, so no one in the world has prior immunity to it. This Snack models how your body develops immunity to foreign invaders by producing antibodies that attack them. The goal of a vaccine is to have people create these antibodies without ever being infected.
- One printout of these tabletop-sized Antigen and Antibody templates (for individuals and small groups) or these full-sized templates (for a larger group).
- Five sheets of cardstock or construction paper, each in a different color (so one sheet of green paper, one of yellow, one of blue, one of orange, and one of purple, for instance)
- Five pieces of white cardstock or construction paper (or two pieces, if using the tabletop templates)
- Black markers (for tracing)
- Plastic garbage bag
- An open space to work, such as a large table or on the floor
- Optional: Masking tape (to hold down the templates if they move around while you’re tracing)
- Cut out the five antigen templates from your printouts.
- Trace cutout shapes onto colored paper and cut along the traced lines to make two of each antigen using one color of paper per antigen. (Two or more copies should fit on one sheet of colored cardstock or construction paper.) When you’re done, you should have two green copies of Antigen A, for instance, two yellow copies of Antigen B, two blue copies of Antigen C, and so on. You can use any colors you want, except white. You’ll need the heavy white paper for your antibodies.
- Cut out the five antibody templates from your printouts.
- By tracing and cutting, make one copy of each antibody using your sheets of heavy white paper.
Place all the antigens and antibodies on a flat surface, such as a table or the floor (this surface represents the body). Move all of the antibodies to one side and all of the antigens to the other.
Antigens (represented by the colored shapes) are proteins found on the surface of pathogens, such as viruses, bacteria, and other foreign invaders to the body. What do you notice about the antigens? Are there any similarities among them? Any differences?
Antibodies (represented by the white shapes) are proteins produced by B cells, which are specialized cells produced by your immune system. What do you notice about the antibodies? Are there any similarities among them? Any differences?
Slide the antibodies across the surface and connect them to their matching antigens. Are the matches always perfect? Can an antibody connect to more than one antigen? Can an antibody connect to more than one type of antigen? What happens as a result of the antigens connecting to the antibodies?
Attach as many antibodies to antigens as you can, and notice that eventually all the antigens become trapped in interconnected groups.
Phagocytes—another type of cell in your immune system—are attracted to connected collections of antibodies and antigens like these, and recognize them as trash. Your plastic garbage bag is your phagocyte! Have the bag engulf, ingest (gobble up), and eliminate these large globs of material. How might this process help the body fight an infection?
This activity is a simple model of the adaptive immune response, one part of the human body’s immune system response. While this is not the first step in a real immune response, it is an important one that is unique to humans and higher vertebrates, and allows for the body to target specific pathogens and remember them in preparation for future contact.
Pathogens can invade your body through breaks in the skin, or through mucous membranes in your eyes, nose, and mouth, creating internal infections. While bacteria often grow in the fluids between your cells, and can reproduce and spread through the body via the bloodstream, viruses have a different strategy. Viruses cannot reproduce on their own, so they insert their genetic material into your cells and use them as virus-making factories. The newly copied viruses then exit the cells and spread throughout the body.
In response, the body’s immune system launches a cascade of complex processes that end up with the antigen from the outside invader binding with a matching antibody. This joining takes place in the lymph nodes, on the surface of a specialized immune cell called a B cell. Because there are only a few B cells with antibodies that match any given antigen, the first contact with a specific antigen initiates a response that might take several days to become effective.
Once the match takes place, the B cells divide rapidly. Some become antibody-making factories called plasma cells, and some become memory cells, which retain the “memory” of that particular antigen for the future.
Plasma cells produce and release millions of antibodies into the bloodstream and lymphatic system. Those antibodies seek out and bind to specific antigens, disarming them, and stopping further spread of the pathogen. As you may have noticed in the activity, the fit isn’t always perfect, but in the body, it continues to improve as the B cells make more and more antibodies.
Because the unique Y-shape of the antibody creates two binding sites for antigens, multiple antigens and antibodies can clump together, creating globs of cells called agglutinations. These agglutinations attract phagocytes that find, ingest, and digest them, eliminating the dangerous pathogen and infected cells from the body. This process of antibody production and “cleaning”—represented in the activity by the plastic bag “gobbling up” the globs of material—continues for a few days until the pathogen is removed.
This activity matches just five kinds of antibodies and five kinds of antigens. In reality, there are millions of different kinds of each. Animals with adaptive immune responses have evolved the ability to not only target specific pathogens, but also to create memory cells that remember the pathogens they’ve been exposed to. When a familiar pathogen reenters the body, the immune system is prepared, the antibody launch is rapid and profuse, and the pathogen is often quickly eradicated. We call this “having immunity.”
How does the rest of the immune system work?
How do vaccines work?
Vaccinations—such as the ones children get to protect them from smallpox, measles, mumps, and chickenpox—work by preparing the immune system for an attack by a virus.
Vaccines are developed by using bits of damaged virus (often just the outer coating, without any internal genetic material) that contain viral antigens but cannot make us sick. When these materials are injected into the body, the immune system alerts the body’s B cells to recognize the antigens introduced and create memory cells. If the virus finds its way into the body at a later date, the body responds as if it is a second exposure and the immune response is rapid, often eliminating the virus before we even notice any symptoms.
Because viruses contain genetic material, they are capable of mutating and evolving over time. Their ability to quickly reproduce leads to equally rapid changes in the virus. Some viruses, such as influenza, change so rapidly that scientists develop a new vaccine every year based on which strain they think will be most likely to affect the public.
Consider how you might adapt this activity so the simulation includes the body having the “memory” of a virus through vaccination, thereby creating a rapid immune response that generally avoids illness. What steps would you add to the procedure? What new materials might you use?
You can do this activity either in small groups using the tabletop templates as explained above, or with the whole class using the full-sized templates. To do the activity as a class, divide students into “antibodies” and “antigens." Distribute cutouts so that each “antibody” student has one antibody and each “antigen” student has two antigens. Have the antigen and antibody groups move to opposite sides of the room. Have the antigen group hold out their arms (like pathogens with antigens on their “surface”). Then have the students representing antibodies move across the room and find their corresponding antigens. When all the matches have been made, you can act as the phagocyte, using the trash bag to clear away the clumps of antibodies and pathogens until the “body” (the classroom space) is clear of any simulated pathogens.
As you complete this activity with students, consider what additional information they may need. Also, consider at which times you might have students assess whether aspects of this simulation accurately model the immune response. How might students change the model to make it more accurate? Where are the holes in the model that are hardest to fix?
Connecting to Teaching Standards
This activity and its extensions are focused on the crosscutting concepts of systems and system models, structure and function, and cause and effect. For older students, the intricacies of the protein structures that allow antigen/antibody binding and the resulting creation of memory cells offer an elegant entry into the exploration of these crosscutting concepts. For younger students, the simple idea that antigens can match with antibodies to keep us from getting sick may be enough to emphasize these concepts.
At the elementary level, having students gain a basic understanding of how our body uses antibodies to protect us from viruses would provide sufficient depth of study, while at the same time engaging students in an interesting topic. Highlighting viral and bacterial diversity would reinforce the disciplinary core ideas (DCIs) around biodiversity in nature, and introduce learning around the resilience of a species in an ecosystem and the idea of adaptive survival.
At the middle school level, this activity relates to the DCI that, in multicellular organisms, groups of specialized cells work together as members of organs and organ systems in incredibly complex ways. Additionally, teachers can emphasize the evolution of the adaptive immune response as advantageous to survival, highlighting DCIs around natural selection.
At the high school level, teachers may choose to dig deeper into the feedback mechanisms in the immune response process in order to emphasize homeostasis within an organism. They may also want to emphasize the hierarchical organization of body systems, how the immune system is intricately tied to other systems, and how viral evolution might impact immune response.