Above: Image © blueringmedia, iStockphoto.com

Every day, you encounter things that can make you sick. From bacteria to viruses to fungi, the world around you is full of pathogens, organisms (usually microorganisms) that can cause disease. And yet, you might be in pretty good health.

This is thanks to your immune system, a series of defense mechanisms in your body that work 24/7 to keep you healthy. It includes specialized cells, proteins such as enzymes and antibodies, and things you might not have thought of as part of your immune system - such as your skin!

Your immune system has three levels. If a pathogen passes through one level, the next level takes over.

Imagine you cut your finger and bacteria infected the wound. Let’s look at the war that would wage inside your body to keep you healthy.

Physical Barriers: Innate immunity, part one

The first line of defense against pathogens is your innate immune system. Level one of this system consists of physical barriers like your skin or the mucosal lining in your respiratory tract. These are quick, simple responses that can eliminate some pathogens before they have a chance to reach your tissue or blood.

For example, your skin is a physical barrier that prevents pathogens from entering the body. But if you cut the skin on your finger, bacteria would have a portal of entry into your body. At that point, the next level of your innate immune system would respond.

Did you know? There’s a difference between infection and disease. An infection occurs when a microorganism invades and multiplies in your body. A disease occurs when the infection damages your cells and causes symptoms of illness.

First responders: Innate immunity, part two

The second level of the innate immune system consists of cells and proteins that attack invaders. Innate defences are non-specific. In other words, no matter what pathogen your body is fighting, the same events happen and the same cells and proteins are at work.

Cells called macrophages live in your tissue. They recognize when something enters your body that doesn’t belong there, and jump to work. They phagocytose the invader, which means they engulf, destroy, and literally eat it! They also sound the alarm to recruit other cells to help. More on that in a minute.

Often, this line of defense is enough to resolve the infection. At the very least, it can limit the spread of infection. For example, those bacteria that entered through the cut on your finger might not make it any further into your body.

But there are some situations the innate immune system can’t handle. For example, there might be too many bacteria, or the bacteria might multiply too quickly. That’s when your adaptive immune response kicks in.

Alert the authorities: Adaptive immunity

The third level of your immune system consists of cells tailor-made to get rid of the specific microorganism that has invaded your tissue.

Special cells called dendritic cells are the liaison between innate and adaptive immunity. Remember macrophages? When they sound that alarm, dendritic cells are part of the crew that responds. They travel to the site of the infection, where they phagocytose and break off small parts of the pathogen. They carry these parts to your lymph nodes, where adaptive immunity begins.

The Crew: T cells

In the lymph node, the dendritic cell searches for a type of white blood cell called a T cell. Your body makes millions of different T cells. Each one can recognize a different pathogen. This means your body can combat almost every invader, even the ones it’s never seen before!

In the lymph nodes, the T cells are naïve: they’re fully mature, but have never encountered the pathogen they’re supposed to fight. Naïve cells are essentially asleep. The dendritic cell’s job is to wake them up and bring them this pathogen.

Different kinds of T cells have different jobs:

  • Memory T cells remember pathogens you’ve seen before. They help your body launch a quicker, more effective defense the next time around.
  • Cytotoxic (“cell-killing”) T cells destroy any of your own cells that have been infected with a virus.
  • T helper cells help other cells, such as B cells, often by releasing proteins called cytokines. These proteins bind to other cells in your body and tell them how to strengthen the immune response. For example, a cytokine might activate a B cell, which would make antibodies against the invading pathogen. When you’re dealing with a bacterial infection from a cut finger, a T helper cell is one of the more useful kinds of T cells.

Usually, T cells can eliminate a bacterial infection just days after they’ve been activated. At this point, your body can stop fighting, and you’ll start to feel better.

You and your cells

As you can see, your immune system is a complex system working around the clock to keep you healthy. So the next time you’re feeling down, just remember: there are billions of cells in your body, and all they care about is you!

Learn more!

Immunology Chapter 1 - Innate (Non-Specific) Immunity (2016)
G. Meyer, Microbiology and Immunology Online (University of South Carolina School of Medicine)

The innate and Adaptive Immune Systems (2016)
PubMed Health

Helper T Cells and Lymphocyte Activation (2002)
B. Alberts et al., Molecular Biology of the Cell, 4th Edition

Viral Attack - T cells
Ask a Biologist, Arizona State University School of Life Sciences

Kate MacDonald

I am a third year student at the University of British Columbia (UBC) in Vancouver, working toward a degree in cell and developmental biology. Early in my second year, volunteering in a genetics lab sparked my passion for research, and I was driven to explore a career in cellular biology. I am beginning work on my undergraduate thesis, and I plan to pursue an MSc after I graduate. Outside of the lab, I am enthusiastic about science outreach, writing and volunteering for Let’s Talk Science. I also host a series of biodiversity-themed YouTube videos for the Beaty Biodiversity Museum at UBC.