Fighting Infections with Your Immune System

Background Information for Teachers

Our immune system is made up of different cells and organs in our body that normally defend us against harmful microorganisms (microscopic organisms). We are all constantly exposed to microorganisms, some of which can make us sick. Any microorganism that makes us sick is called a pathogen ( "germs" in everyday language). Pathogens include harmful bacteria (like Salmonella or E. coli that cause food poisoning), microscopic fungi, viruses (ranging from measles to the flu), and more. When pathogens make us sick, we call it an infection.

Our immune system goes through a process called the immune response to fight off pathogens. When a pathogen attacks for the first time, it triggers a primary immune response. If the same pathogen attacks again after that, a secondary immune response is triggered. White blood cells (also called leukocytes) do most of the work during an immune response. There are different types of white blood cells that each carry out a different function. Some white blood cells find the pathogens in our body and must tell them apart from human cells. This process is referred to as self/non-self recognition (if it fails, it can cause an infection or an autoimmune diseases where the immune system attacks the body instead of pathogens). Other types of white blood cells then make antibodies, tiny particles that grab, or bind onto, the pathogen (Figure 1). When antibodies bind to the pathogen, this stimulates other white blood cells to destroy the pathogen. Lastly, some white blood cells leave a memory cell behind. Memory cells "remember" the specific pathogen encountered, so they can mount a larger and faster immune response if they encounter the same pathogen again. This way our body can build up immunities to certain diseases.

A good way to build up immunities against certain diseases is using vaccines. Vaccines contain dead or modified pathogens for a specific disease, so they cannot make us sick. However, our immune system still recognizes them as invaders and triggers an immune response, producing matching antibodies that can destroy the pathogen. When the real pathogens attack, our body is already prepared and can quickly react and neutralize the invaders.

Image Credit: Teisha Rowland, Science Buddies / Science Buddies
Figure 1. Illustration of antibodies (blue) binding to a pathogen (red). This simplified diagram is not to scale.

In this lesson plan, students will use magnets, iron filings, and salt to make a model of the immune system in the human body and investigate how antibodies and memory cells help the body fight off an infection.

Additional Background Links

• What does our immune system do?, Science Museum
• Immune System, Ducksters.com
• Memory in the Immune System, The Partnership in Education
• Difference between Primary and Secondary Immune Response, Microbiology Notes

Prep Work (15 minutes)

1. Cut three 1 inch by 1 inch squares of magnetic tape for each group. Leave the paper strip on the tape so the adhesive is covered.
2. Cut three 4 inch by 4 inch pieces of plastic wrap for each group.
3. If you do not have a scale available for each group, set up one or two scales in the classroom that can be shared between groups.

Engage (15 minutes)

1. Start by exploring how the human immune system works and explain how our body fights pathogens.

   Does anybody know why we get sick sometimes? What exactly causes an illness such as a cold, upset stomach, or sore throat?

   Diseases are caused by pathogens. These are tiny microorganisms such as bacteria, viruses, fungi, or parasites that make us sick when they get into our bodies.

   How does our body respond to pathogens?

   Luckily, our body's immune system can protect us from the invasion of pathogens. The immune system is made up of specialized organs, cells and tissue that all work together to destroy invaders. The immune system's reaction to an invading pathogen is called the immune response. When the immune system fights an invader for the first time it is called a primary immune response. If the same pathogen has attacked before, it is called a secondary immune response.

   How does our immune system destroy pathogens?

   This is done by specialized cells in our blood stream called white blood cells. There are different types of white bloods cells that all help destroy the pathogen. Some of them produce antibodies, which are specifically designed to bind, or attach, to one type of pathogen. Once the antibodies bind to the pathogens, other types of white blood cells take over and destroy them.

   How does our immune system tell the difference between invading pathogens and beneficial microorganisms (like the "good" bacteria in our digestive system) or cells that belong to our body?

   It is important for our immune system to differentiate pathogens from the regular cells in our body. This is why antibodies are specifically designed to only attack one type of pathogen. Antibodies function similarly to a lock and key system. They can only bind to pathogens that have a "marker" that matches up perfectly. This way, only the targeted cells will be destroyed, and the good cells are ignored. However, sometimes this system fails causing autoimmune diseases, in which the immune system attacks our own body.

   Can anybody think of what would happen if one specific pathogen attacks us a second time? Do you think we still get sick?

   Once our immune system encounters one specific pathogen, it is able to "remember" it with the help of a special type of white blood cells called memory cells. The memory cells stay in our body and remember what kind of antibodies need to be produced to match the pathogen. When a second attack by the same pathogen occurs, they can quickly respond and produce antibodies to fight the pathogen. This way, our body can build up an immunity to certain diseases. Sometimes we might not get sick at all, or the illness might not be as severe. This is also how vaccines work, by introducing our immune system to dead or weakened pathogens so they can develop antibodies for it.

2. Explain to your students that they will build a model of our immune system and simulate an immune response after an attack by a pathogen.

   Can you think of a way to model our immune system? What could mimic our body? How could we simulate the antibody binding to the invading pathogen? What could we use as pathogen?

   Let your students come up with their own ideas. If they have trouble, point them towards the materials they will use for the activity. Ask them specifically, what the salt, the iron filings, the jar, or the magnets could represent. End the discussion by explaining to them that in their model of the immune response, the jar represents the human body. The salt represents human cells and the iron filings represent many copies of the same pathogen. The human body is infected with the pathogens. Each piece of magnetic tape represents many antibodies made by the immune system to fight the pathogens. Be sure to let students know that their model is not to scale, because in reality the cells would be much larger than the pathogens, which would be much larger than the antibodies. (Also, normally many antibodies bind onto a single pathogen.)

   With our model, how could you simulate the effect of memory cells?

   Whereas during a primary immune response no memory cells are present, they already exist when a secondary immune response happens. As memory cells remember which antibodies to make for a specific pathogen, they can make these much faster than during a primary immune response. We can simulate this in our model by adding more antibodies to our immune system model. This allows us to see if more pathogens are killed with or without the presence of memory cells.

Explore (30 minutes)

Walk the students through the experimental procedure. (A slideshow is available that you can use to guide your students through the experiments.)

Setting up the Immune System Model

1. Make the "antibodies," as shown in Figure 2.
   1. Label the pieces of magnetic tape 1, 2, and 3 by writing large, bold numbers on the paper backing.
   2. Put each magnetic tape square in the middle of a plastic wrap square, with the adhesive side (covered by paper) facing up.
   3. Pull the ends of the plastic wrap together around each magnetic tape square, twist them together, and secure them with a twist tie.
   4. Make sure there are no openings in the plastic wrap surrounding any of the magnets (so that salt or iron filings cannot get through and reach the magnetic tape).

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 2. Completed antibody models.

2. Measure the mass of each antibody model and record these values on the student worksheet.
3. Measure 1 cup of salt and pour it into the glass jar. Then measure 1 tbsp. of iron filings and add it to the jar, as shown in Figure 3, left.
4. Put the lid on the jar tightly, and then mix the salt and iron filings together by flipping the jar upside down and then right-side up again about ten times, or until the iron filings appear evenly dispersed throughout the salt (Figure 3, right).

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 3. After mixing the iron filings and salt together, the iron filings should appear only as specks throughout the jar.

Simulating the Primary Immune Response

5. Take one piece of magnetic tape and put it in the jar, resting on top of the mixture as shown in Figure 4. Then put the lid back on the jar tightly and flip it ten times.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 4. Magnetic tape in the jar.

6. Carefully remove the magnetic tape from the jar.
   1. You may need to carefully tilt the jar to grasp the tape.
   2. Only grab it by the twist tie or twisted plastic, and try not to touch the iron filings.
   3. As you take it out of the jar, gently turn the piece of tape upside down, so that any salt trapped in the twisted plastic wrap falls back into the jar.
   4. If necessary, gently shake the tape to remove the extra salt, but do not shake it so hard that you remove the iron filings.
7. Examine the magnetic tape (shown in Figure 5). Does it look like more iron filings are stuck to the tape than salt?

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 5. Magnetic tape after being removed from the jar.

8. Measure the mass of the magnetic tape, including any attached salt or iron filings.
   1. Place a piece of paper onto the scale in case any iron filings fall off.
   2. Zero the scale.
   3. Measure the mass of the magnet and record this value on the student worksheet.
9. Hold the magnetic tape over the piece of paper, untie the twist tie, and carefully open up the plastic wrap so the salt and iron filings fall onto the paper.
10. Use the paper to carefully funnel the iron filings and salt back into the jar and secure the lid. Mix the iron filings and salt again by flipping the jar ten times.

Simulating the Secondary Immune Response

11. Now you will simulate a second attack with the same pathogen. In the model, this represents the immune system's memory cells making more antibodies when they encounter the same type of pathogen again.
12. Repeat steps 5–10, but this time put all three pieces of tape in the jar, as shown in Figure 6. You will also need to measure and record the combined mass of all three magnetic tape pieces with and without attached salt and iron filings.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 6. All three pieces of magnetic tape in the jar.

Reflect (20 minutes)

1. Discuss your results as a class. Let each group present their results and interpret their findings.

   What happened when you added the magnetic tape to the jar and mixed it with the salt and iron filings?

   The iron filings were attracted to the magnet and stuck to it. While some salt might have gotten stuck along with it, most of the material stuck to the magnets was iron filings.

   How is this similar to a real antibody response in our body?

   In our body, antibodies are only supposed to bind to invading pathogens, while ignoring regular cells. This is similar to the magnet only binding the iron filings (pathogens), while the salt (regular cells) is not affected. Note: Some salt might get stuck to the magnet together with the iron filings, although it should be a negligible amount. This does not mean that in our body antibodies also attack non-invading cells—the exception being if you have an autoimmune disease.

   How is the secondary immune response different from the primary immune response?

   In the secondary immune response, antibodies are produced much faster due to the action of memory cells that were created during the primary immune response. This means more antibodies are available to fight off the second attack by the same pathogen. In our model, we simulated this by adding more magnetic tape (antibodies) during the secondary response.

2. Calculate the mass of iron filings (pathogens) removed by the magnetic tape (antibodies) in both the primary and secondary immune responses. Do this by subtracting the initial mass of the tape(s) (no iron filings) from the final mass (including iron filings).
3. Use the worksheet to plot your results in a bar graph.

   In which scenario, the primary or secondary immune response, were more iron filings removed by the magnetic tape? Can you explain why?

   During the secondary immune response, many more iron filings (pathogens) should have been removed by the magnetic tape(s) (antibodies), because there were more antibodies available to pick them up.

   Looking at your results, do you think it would take more or less time to fight off the disease the second time?

   Since there are more antibodies available to bind to pathogens during the secondary immune response, it should take less time to fight off the disease.

   Based on your experiments, can you explain why memory cells are important for our immune system?

   Memory cells store the information about a specific pathogen, so that when it attacks again, our body is prepared to produce the matching antibodies much faster. Without memory cells, our body would not be able to gain immunity to diseases.

   How would your results look different if you could model an autoimmune disease? Why?

   If you have an autoimmune disease, your immune system is not able to differentiate between pathogens and your own body's cells. This means that your white blood cells will also attack cells that belong to your body, which can result in infections or even damage of your tissue. In your model, this would mean that your magnetic tape (antibodies) would also bind to the salt.

   Based on what you have learned about the immune system, can you explain how vaccines protect us from certain diseases?

   Vaccines have the purpose of triggering the primary immune response to a specific pathogen. They contain dead or weakened pathogens for a specific disease, which cannot make us sick. However, our body still recognizes the pathogens as invaders and builds up antibodies. When the real disease tries to attack, our body is already prepared and can quickly react and neutralize the pathogens.

Assess

You can use this quiz to assess student learning after the activity:

• Online quiz, assignable in any LMS
• Quiz (pdf) and answer key (PDF)

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Physician

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Career Profile

Physicians examine patients and perform diagnostic tests to detect illnesses such as autoimmune diseases. They develop treatment plans to minimize disease symptoms and counsel patients on necessary lifestyle changes due to their condition. Physicians also administer vaccines and keep track of your vaccination records. Read more

Biochemist

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Career Profile

Biochemists study chemical aspects of antibodies to find out how they function. They help to develop new drugs and vaccines, and design and execute tests that are able to detect autoimmune diseases. Read more

Medical & Clinical Laboratory Technician

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Career Profile

Medical & Clinical Laboratory Technicians examine and analyze body fluids. These analyses include white blood cell counts or looking for pathogens or abnormal cells. They also prepare vaccines and serums in the laboratory and test for disease immunities. Read more

Lesson Plan Variations

This activity can be expanded or modified in a number of ways. Here are a few options:

• This model could be tweaked to model differences in the immune response. For example, if students want to investigate antibodies that are more effective, they could try using larger or stronger magnets. Alternatively, they could make more plastic-wrapped magnetic tape squares and try more than three such antibody models at a time. Is there a certain number of antibodies at which their results change?
• Students may have noticed that the antibody models capture a small amount of salt. In the model, this would represent the antibodies attacking human cells in an autoimmune response. Students could investigate the autoimmune response by quantifying how much salt was captured compared to the total amount captured. To separate the salt from the iron filings, they could spread them both over a piece of paper and hold a plastic-wrapped magnetic square directly above them to pull out the iron filings and then deposit them onto another piece of paper. Students could then weigh each separately on a scale. How much of each (iron filings and salt) is there? What percentage does each represent of the total?