The immune system is one of the most complicated but important systems in the human body. Without it, we wouldn’t be able to stay healthy!

The immune system is made up of many different organs and specialized cells all working together to fight off infections from things like bacteria, viruses, parasites and other germs (or “pathogens“). Our immune systems also protect us against any of our own cells that are growing too much or aren’t working properly, otherwise they can turn into serious diseases like cancer.

All of the specialized immune cells are produced in our bone marrow – the squishy sponge-like centres of large bones like the femur. The lymphatic system makes up the organs involved in the immune system: tonsils (in our throats), thymus (in our chests), lymph nodes (in our armpits, necks and groins), and spleen (in our guts). Immune cells move around the lymphatic system via lymphatic vessels, always on the lookout for infection.

There are two main parts of the immune system: the innate immune system and the acquired immune system.

The Innate Immune System

The innate immune system is designed to fight off anything that isn’t supposed to be in our bodies – it isn’t picky about what it targets. When a pathogen enters our body, a type of white blood cell called a neutrophil will notice the invader right away and gobble it up, a process called “phagocytosis.” Monocytes and eosinophils are other immune cell types which also eat up germs that find their way inside our bodies. While monocytes target anything, eosinophils particularly like eating parasites like worms.

The neutrophils, monocytes and eosinophils are our first line of defence, but there are many other cells that also get involved in launching an immune response against a pathogen.

As soon as the neutrophils and monocytes start eating the invading germ, another type of white blood cell called a basophil will take notice and start sounding the alarm to other immune cells. Now, more immune cells are becoming involved and chemicals called cytokines are being released by the cells.

Cytokines are important because they will start a process called the “inflammatory response.” If you’ve ever gotten an infection, you will recognize the inflammatory response by its symptoms: fever, swelling, redness, heat, and pain. Even though it’s uncomfortable, the inflammatory response is an important step for our bodies to contain the infection.

In order to increase temperature, our bodies have to increase blood flow to the site of infection. This causes swelling, pain and heat. Increasing body temperature will increase metabolism to get as many helpful nutrients to the scene of infection as possible. Sometimes, pathogens have a harder time living in higher body temperatures which makes them easier for our bodies to fight.

At this point, after the inflammatory response to an invading pathogen has happened, the innate immune system will hand over the reigns to the adaptive immune system to continue fighting the invader.

The Acquired Immune System

The acquired immune system is a bit different – instead of killing the pathogen right away, it will teach our bodies to remember the pathogen first, in case it ever enters our bodies again. We humans have actually learned to enhance our acquired immune systems with vaccines!

Vaccines are a way of teaching the immune system to fight off a specific germ without having to be infected with the real germ. Instead, scientists have found ways to take small bits of a particular germ that won’t make us sick but will trick our immune systems into remembering the germ. This means we are protected, or “immune” to the germ we were vaccinated for. But how does this work? First we have to learn a few things about the acquired immune system.

Once the front-line defence cells have digested the germ, they present pieces of the pathogen on their cell surface. These pieces are known as “antigens,” and are unique to the pathogen that’s been digested. Next, a group of cells called lymphocytes spring into action to learn the characteristics of the antigen and create a way of recognizing the antigen in the future. First, T-helper cells recognize the antigen and help B-cells recognize it too. B-cells then produce a protein called an “antibody” which will attach itself to the antigen on the pathogen when the pathogen is in our bodies.

Antibodies are really important for the adaptive immune system because they are the components that allow the system to learn how to fight recurring infections. Once a pathogen is identified and an antibody that can attach to the pathogen’s antigen is made, our bodies will never have to mount an inflammatory response against that pathogen, again! If the same pathogen ever re-enters our bodies, an antibody will recognize it and alert the neutrophils and monocytes to come and gobble it up. Since it was an antibody that alerted the neutrophils and monocytes to the invader, they know not to trigger an inflammatory response which means we won’t feel sick!

But usually, a pathogen manages to get into some of our own body cells before it can be digested by immune cells. Luckily when this infection happens, our body cells know what to do to signal for help. The infected cells present the pathogen’s antigen on their surface which attracts another lymphocyte called a T-cell. T-cells recognize the antigen and send chemicals and other killer T-cells to destroy the infected cells. While this may sound like a bad thing, sometimes we have to make small sacrifices to make sure the rest of our bodies stay healthy!

Usually, our immune systems work hard to keep us healthy, but sometimes they get confused and think that something harmless is an invading pathogen. For some people, something like a peanut or a kiwi could trigger an inflammatory response by their immune systems, even though peanuts and kiwis shouldn’t be dangerous! This is known as an allergic reaction – head over to the STEM Up Holiday Issue to learn more!

This article was originally published in STEM Up on December 22, 2020.