The Immune Response

Every day, you encounter things that can make you sick. From bacteria to viruses to fungi, the world around you is full of pathogens. Pathogens are organisms (usually microorganisms) that can cause disease. But in spite of all these pathogens, 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. The immune system includes specialized cells, proteins such as enzymes, and antibodies. It also includes parts of your body you might not have thought of as part of your immune system - such as your skin! 

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.

What is the body’s first line of defence against pathogens?

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

The first line of defence is your innate immune system. Level one of this system consists of physical barriers like your skin and the mucosal lining in your respiratory tract. The tears, sweat, saliva and mucous produced by the skin and mucosal lining are part of that physical barrier, too. These quick and simple responses 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 a finger, bacteria would have a way to get into your body. At that point, the next level of your innate immune system would respond. 

Did you know?

There’s a difference between an infection and a 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.

What is the body’s second line of defence against pathogens?

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 response happens and the same cells and proteins are at work.

Cells called phagocytes live in your tissue and blood stream. Macrophages and neutrophils are two types of phagocytes. Phagocytes recognize when something enters your body that doesn’t belong there and jump to work. They destroy the invaders using a process called phagocytosis. 

First, a macrophage identifies and binds to the invader. It then engulfs it and breaks it down with the help of lysosomes. This destroys the invader. Macrophages also sound an alarm by producing proteins called cytokines to recruit other types of white blood cells to help. These other types of white blood cells are called neutrophils, eosinophils and basophils. 

Neutrophils make up 40-70% of the white blood cells (WBC). Their key job is to engulf and destroy (phagocytose) invading bacteria and fungi. Eosinophils make up only 5% or less of the WBCs. They contain toxins that can kill pathogens too large to be engulfed. They also release protein substances involved in producing inflammation.  

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

But there are some situations that 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.

What is the body’s third line of defence against pathogens?

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

Special cells called dendritic cells are the liaison (point of communication) 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 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 adaptive immune response involves two main types of specialized white blood cells called lymphocytes - B cells and T cells. B cells are found in the blood. Their main function is to mature into cells that produce antibodies to counteract the antigens (foreign invaders) that get into the body. To do this, they work with the T cells. 

In the lymph nodes, the dendritic cells search for T cells. Your body makes millions of different T cells. Each type of T cell can recognize a different type of 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 fully mature, but have never encountered the pathogen they’re supposed to fight. These cells are essentially asleep. The dendritic cells’ job is to wake them up and bring them to the pathogens. 

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 defence 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. 
• T regulatory cells are the police of the adaptive immune system. They shut down the attack launched by other immune cells once the pathogen has been cleared. This stops the immune response from getting out of control. 

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.

Did you know?

The 2018 Nobel Prize in Physiology and Medicine was won for discoveries of how the immune system is regulated. These discoveries led the way to immunotherapy drugs to treat skin cancer!

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!