Today, April 29th is the Day of Immunology, which undoubtedly deserves some extra attention this year. Right now, millions upon millions of immune cells are doing their best around the globe to fight off SARS-CoV2. If you think about the enormous amount of viruses, bacteria, fungi and parasites that can cause disease, consider how remarkably well the immune system does its job.

Here are the 10 top reasons your immune system is awesome: 

1. Your immune system will eat almost any intruder!

How come our immune cells can recognize so many different pathogens?

When an intruder first enters our bodies, it meets the first line of defense: phagocytes.

Phagocytes patrol the body and look for pathogens. When they find one, they use specialized proteins to hold on to them and eat them. This process is called phagocytosis and it was discovered over 110 years ago. In fact, a phagocyte can eat something that is much larger than itself. It will attempt to eat anything it deems dangerous, and does not give up lightly. Phagocytes can chase bacteria by following chemicals that bacteria leave behind, as you can see here. Once inside the phagocyte, the pathogen is bombarded with acid and damaging enzymes that rip it apart.

2. Your immune system can capture bacteria with DNA!

Did you know that DNA is very sticky?

It’s sticky enough to use as a tiny fishing net for bacteria!

Specialized types of phagocytes that are called neutrophils patrol your blood. These immune cells are filled with proteins and enzymes that are adapted to killing pathogens. In some cases, a neutrophil will set off a process where it releases its own DNA along with several destructive enzymes. Bacteria are trapped in the web of DNA and killed by the enzymes.

3. Your immune system can explode bacteria!

Neutrophils are not the only exploding component of the immune system. Our immune system has another way of exploding bacteria.

This is done by the complement system, a collection of proteins that exists in blood and other body fluids. The complement system does three main things. First, it attracts phagocytes to the site of infection so that they can come and eat the invading pathogen. Next, certain complement proteins bind to bacteria and make them easier for the phagocytes to eat. And finally, complement proteins assemble into a “membrane attack-complex”. This complex is shaped like a tube and it inserts itself into the surface of bacteria, essentially covering the bacteria with tiny holes – leading the bacteria to explode!

4. Your immune system is fairly tolerant!

Despite all its very aggressive killing behavior, your immune system is actually very tolerant. We are all constantly covered by bacteria, viruses, fungi, archaea, protozoa and helminths. Together they are our microbiome and we need many of them to stay healthy. The interaction between the microbiota and the immune system is very complex, yet carried out in a way that benefits both parties. The nice microorganisms help keep pathogens away by outcompeting them, while they are allowed to live on us (in tightly regulated and designated places).

5. Your immune system will know a healthy cell from a sick one!

It is not only invaders that can make you sick. Sometimes our own cells cause diseases such as cancer. Our immune system has specialized cells that look for cancer cells called T cells and Natural Killer cells. They patrol the body and check the surfaces of cells, looking for hints that a cell has become cancerous, infected by a virus or bacteria or become dysfunctional in some way.

The immune system heroes! From a Lifeology course for kids on COVID and the immune system, illustrated by Abrian Curington.

Every cell in your body carries specialized proteins on their surface that show they belong to you. This tells the immune cells not to attack them. If a cell is infected with a virus or has become cancerous, these proteins will change or be missing from the cell surface. If T cells or Natural Killer cells find such a cell, they will eliminate it by instructing it for controlled cell death.

What happens to the dead cell?

You guessed it – a phagocyte shows up to eat it.

6. Your immune system can create indefinite variations of antibodies!

You have probably heard about antibodies lately, but what are they?

Antibodies are Y-shaped molecules that are produced by a type of immune cell called B cells. Antibodies float along in the bloodstream throughout the body. Because of their shape, the antibody can bind to intruders – like a virus – by attaching to receptors on the surface of viruses, bacteria or infected or dangerous looking cells. By attaching to a bacteria or a virus, antibodies make it difficult for them to make us sick. But this also makes it easier for immune cells to discover and eat the intruders.

An illustration of an antibody attaching to the surface of the coronavirus, from a Lifeology course on what the coronavirus does in our bodies. Illustrated by Elfy Chiang.

B cells are pretty amazing, they can make antibodies for practically any pathogen you will ever encounter.

Does that mean we have genes for every different kind of antibody?

Not really. B cells have many strategies for rearranging the parts of their DNA that hold the antibody genes. In this way they can use very few genes to create incredible variations of antibodies that keep us safe from pathogens.

7. Your immune system knows self from stranger!

How do T cells know the difference between your own cells and an intruder?

They go to school to learn!

Immune system school! by Signe Aasberg.

T cells begin their life in the bone marrow. As immature cells they move from the bone marrow to the thymus, which sits just below your throat. The thymus acts much like a school for T cells and here they learn what they should and should not recognize and respond to.

T cells learn to recognize your own cells by the proteins on their surfaces. They also learn that they shouldn’t set off an immune response when meeting these cells. The body has a very clever way of teaching your immune cells that is called positive and negative selection. The cells that recognize the right proteins receive a signal for survival. The T cells that fail to recognize the right proteins or that initiate an immune response to your own healthy cells are signaled to undergo controlled cell death. When they are done with school, T cells graduate from the thymus and move to lymphoid organs throughout your body, ready to take on the important job of protecting you from pathogens.

8. Your immune system can learn from vaccines!

Measles, polio, smallpox, rabies, even cervical cancer caused by Human papillomavirus (HPV) and several other diseases have become preventable through vaccines! The global reach of vaccination programs ensures that 2-3 million deaths are prevented each year. Recent outbreaks of diseases such as ebola and zika virus and the current Covid-19 pandemic reminds us of how important vaccines are.

But how do they work?

Vaccines teach the immune system to recognize a pathogen. Depending on the vaccine, it will contain parts of a pathogen or a dead or dysfunctional pathogen. This allows your B cells and T cells to learn to recognize and respond to the pathogen without having to meet the real thing. The B cells can then develop antibodies and both cell types are involved in “immunological memory”.

Immunological memory is created by long-lived B cells, T cells and Natural Killer cells. These specialized cells are able to respond to a certain pathogen many years after you were vaccinated. In some cases, the memory lasts throughout your life, while in other cases you might have to be re-vaccinated after a while.

9. Your immune system can remember a lot for a very long time!

Immunological memory ensures immunity or partial immunity against many diseases. We don’t yet fully understand how immunological memory is maintained. This is a very complex process that involves different subgroups of B cells, T cells and Natural Killer cells and many different signaling molecules (Ratajczak et al., 2018).

Research on immunological memory has taught us a lot about different aspects of the immune system. The more we know about how immunity is achieved and maintained, the more likely it is that we can produce new vaccines.

10. Your immune system plays an extremely well coordinated team sport!

The immune system involves many different organs, cell types, proteins and signaling molecules that are spread throughout your body. Some of the cells sit around in tissue or a lymph node, waiting to be called into action. Others patrol your tissue, travel from one organ to another or ride around in your blood. They keep an overview of your cells and your microbiome and are always ready to rapidly and aggressively respond to intruders.

All of this is tightly controlled to keep us as safe as possible while causing as little damage as possible. It only becomes clear how well coordinated the immune system is when something goes wrong. Autoimmune diseases cause the immune system to target the body’s own cells. This happens in diabetes, rheumatoid arthritis, multiple sclerosis, Crohn’s disease and inflammatory bowel disease, to name a few. Autoimmune diseases cause serious damage and suffering to those affected by them.

The importance of keeping a tight control on the immune system also becomes clear when it overreacts to an infection. In some cases an infection causes the immune system to respond too aggressively, causing serious damage to the body. When this happens, for instance during sepsis, the inflammatory response causes blood pressure to drop and organs might be severely damaged. If not treated quickly it can lead to organ failure and death.

You might have heard that Covid-19 can cause something called Acute Respiratory Distress Syndrome (ARDS). When the lungs become severely inflamed, which can happen due to Covid-19, the inflammation can cause liquid from the blood vessels to leak into the small air sacs in the lungs. This makes breathing difficult and it’s the reason why many Covid-19 patients are admitted to intensive care units. 

When an immune system overreacts to the coronavirus, lots of inflammation and damage to lung tissue can happen. Illustration from a Lifeology course on what the coronavirus does in the body, by Elfy Chiang.

Learn about how the body can overreact to COVID-19 here, and how you can prevent your immune system from overreacting here.

How can you celebrate your immune system?

In these days of social distancing and hand-washing, how can we celebrate the immune system?

First, you can appreciate the role that your immune system plays in your health by learning more about how it works! This article is a good first step! The next could be finding visual and fun online resources that teach your about the immune system, such as these art-drive COVID and immune system Lifeology courses, Youtube channels Kurzgesagt & Crash Course, or play the immune system-inspired game ImmuneQuest!

The other way that you can celebrate your immune system is by helping it to protect your body from pathogens! This may include social distancing and hand-washing behaviors to prevent your immune system from encountering COVID-19, keeping yourself and your kids up to date with vaccinations, and knowing the warning signs for when your immune system needs help from medical professionals!

Illustration from a Lifeology course on how to strengthen your immune system, by Gaius Augustus.
Illustration from a Lifeology course on how to strengthen your immune system, by Gaius Augustus.
Learn to strengthen your immune system!

*PSA about sepsis warning signs*

Sepsis happens when the immune system overreacts to infection. Any infection, such as COVID-19, can cause sepsis, and the warning signs for sepsis are the same as the warning signs for COVID-19. 

If you have an infection and you experience any of these symptoms, you should call for emergency health care. Don’t be afraid to let them know that you might have sepsis. 

S – Slurred speech or confusion

E – extreme shivering or muscle pain, fever

P – passing no urine all day

S – severe breathlessness

I – it feels like you’re going to die

S – skin mottled or discolored