What is the bodys first line of defense against pathogens

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Response against invaders
An evolutionary battle
What is the body’s first line of defence against pathogens?
What is the body’s second line of defence against pathogens?
What is the body’s third line of defence against pathogens?

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People constantly encounter viruses, bacteria or parasites. Fortunately, our skin, the specialized lining of our guts and other parts of our body that are exposed to the outside world prevent them from entering. When a pathogen breaches this barrier, our body’s defences come into play.

Those defences, which we also call the human immune system, have two branches — innate and adaptive. Our innate immune system is our first line of defence. It has special molecules that recognize “fingerprints” or patterns in proteins or genetic material that are only present in pathogens. These molecules signal the presence of a pathogen to the cellular genetic machinery, which then produces effector molecules called cytokines.

The effector molecules initiate a process to eliminate the pathogen. The adaptive immune response kicks in at a later stage. Its job is to mount a much more robust defence by destroying cells infected with a virus or by neutralizing the virus or toxins produced by bacterial pathogens.

The adaptive system also has a memory and activates very rapidly if it encounters the same pathogen again. The innate and adaptive immune systems are linked and the innate system primes the adaptive branch of immunity for a more robust immune response.

Response against invaders

The innate immune system is primitive and probably evolved earlier in invertebrates and lower vertebrates. Although innate immune response has systemic effects on the entire human body, every cell can be considered a factory housing some components of the innate immune response.

Innate immunity starts at the cellular level through conserved pathogen sensing proteins in the cell that sense a pathogen, such as a virus. During virus infection, sensors in human cells recognize viral proteins and nucleic acids to activate genes, such as interferons that inhibit viral replication.

Once infected cells have sensed an invading pathogen, they secrete molecules called cytokines and chemokines. Cytokines such as interferons are molecules that signal neighbouring cells and induce an antiviral state in them. These cells are then primed to resist an infection with the invading virus. Cytokines such as interferons activate anti-viral genes in the infected and neighbouring cells. These genes are called interferon stimulated genes (ISGs) since they are only made in the presence of interferons. ISGs impede virus replication in the cells.

Thermal scanner in use at Manila’s International Airport during a MERS outbreak in 2014. (AP Photo/Aaron Favila)

At the tissue level, an infection usually manifests itself in the form of an inflammation. Typical signs of inflammation include redness, swelling, pain, heat and loss of function. This is an area of intense battle between the invading microbe and the body’s immune system. Each cell within this area is like a soldier and several cell types together form an army against the microbes.

Chemokines are molecules that attract specialized immune cells to this site of infection. These include cells that “eat” pathogens and dead cells such as macrophages. In the battlefield, chemokines are like “radio calls.”

Macrophages respond to these calls and invade the site of infection, consuming any cell-free or cell-associated pathogen. These pathogens are then destroyed by internal mechanisms in the macrophages. Macrophages are also like the medics in a battle zone. They help remove damaged cells (“wounded soldiers”) and clear the area.

White blood cells such as neutrophils respond to chemokines by migrating to the site of infection. These cells secrete powerful inflammatory molecules and reactive oxygen species that aid in getting rid of the pathogen.

Neutrophils, just like macrophages, can also ingest microorganisms or particles. Neutrophils are cells that have internal granules that house powerful chemicals. When neutrophils accumulate at the site of infection, they release these chemicals that have bactericidal properties.

An evolutionary battle

Pathogens continue to evolve new mechanisms to evade the body’s immune response. Charles Darwin’s theory of evolution and survival of the fittest are relevant for microbes as well. Just like bacteria evolve to become resistant to antibiotics, viruses evolve ways to evade the immune responses.

Coronaviruses have evolved mechanisms to shut down cytokine production. In the absence of cytokines, the cell has no way to communicate with other cells about the viral infection. The virus can then multiply to higher levels, causing significant infection and even death in humans. This has been observed both in severe acute respiratory syndrome (SARS) and Middle-East respiratory syndrome (MERS).

It’s therefore very important to identify the mechanisms employed by these viruses to shut down host defences. Drugs can then be designed to inhibit these viral mechanisms and boost human innate immune responses.

The innate immune system has control mechanisms to inhibit damage to the human body by excessive secretion of cytokines. Cytokines are deemed necessary for controlling virus replication, but excessive production of cytokines leads to tissue damage. Excessive cytokine production, a phenomenon called “cytokine storm” in SARS patients has been associated with lung damage. So a timely balance between inflammatory and anti-inflammatory processes is ideal to tackle infections.

The innate immune response is really the sentinel of the human body; standing at the doors to detect invading pathogens. These sentinels fight off an invading organism while alerting the adaptive immune response (“the cannons”) about the threat.

Finger with a drop of blood (blueringmedia, iStockphoto)

When you cut your finger, your immune system kicks in to protect you from pathogens.

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!

If you cut yourself, what things can you do to help prevent an infection? What part of your immune system are you helping through these actions?
Which cells in the immune system can you name? In what context have you heard about them before?
Why does a doctor feel your lymph nodes during a physical examination? Have you ever felt that your lymph nodes were sore or swollen? When have you noticed this?

During surgery, how is a person’s immune system exposed to pathogens? What steps do surgical teams take to avoid spreading infections during surgery?

What physical barriers in the human body play an important role in the immune system?
What is phagocytosis? At which stage of the immune response does phagocytosis take place?
Where do dendritic cells originate? What is their role in the immune system?

What do you know about how T cells were discovered? (Note: This question will require additional research.)

Have you heard media stories about infection outbreaks at hospitals? (e.g., MRSA, Clostridium difficile, pneumonia, influenza.) What types of infections have been reported? Why are these instances usually reported as a serious problem?

This article supports teaching and learning in Biology and Human Health related to the immune system, structure and function of the immune system, and the immune response. Concepts introduced include pathogens, macrophages, phagocytosis, innate immunity, adaptive immunity, dendritic cells and T cells.
Before reading this article, teachers could provide students with a Vocabulary Preview to help engage prior knowledge and introduce new terms. Ready-to-use Vocabulary Preview learning strategy reproducibles are available in [Google doc] and [PDF] formats.
To help consolidate and extend learning, teachers could have students watch one or more of the videos from the Learn More section. To organize and compare the information from these resources, students could complete a Print-Video Venn Diagram learning strategy. Ready-to-use Print-Video Venn Diagram reproducibles for this article are available in [Google doc] and [PDF] formats.
In addition, after reading this article and/or viewing the videos, teachers could have students create a graphic organizer that clearly illustrates the levels of defence of the immune system and the structures and cellular processes involved at each level. Students could use an online infographic creator to convey this information in a creative and unique digital format.

If you cut yourself, what things can you do to help prevent an infection? What part of your immune system are you helping through these actions?
Which cells in the immune system can you name? In what context have you heard about them before?
Why does a doctor feel your lymph nodes during a physical examination? Have you ever felt that your lymph nodes were sore or swollen? When have you noticed this?

During surgery, how is a person’s immune system exposed to pathogens? What steps do surgical teams take to avoid spreading infections during surgery?

What physical barriers in the human body play an important role in the immune system?
What is phagocytosis? At which stage of the immune response does phagocytosis take place?
Where do dendritic cells originate? What is their role in the immune system?

What do you know about how T cells were discovered? (Note: This question will require additional research.)

Have you heard media stories about infection outbreaks at hospitals? (e.g., MRSA, Clostridium difficile, pneumonia, influenza.) What types of infections have been reported? Why are these instances usually reported as a serious problem?

This article supports teaching and learning in Biology and Human Health related to the immune system, structure and function of the immune system, and the immune response. Concepts introduced include pathogens, macrophages, phagocytosis, innate immunity, adaptive immunity, dendritic cells and T cells.
Before reading this article, teachers could provide students with a Vocabulary Preview to help engage prior knowledge and introduce new terms. Ready-to-use Vocabulary Preview learning strategy reproducibles are available in [Google doc] and [PDF] formats.
To help consolidate and extend learning, teachers could have students watch one or more of the videos from the Learn More section. To organize and compare the information from these resources, students could complete a Print-Video Venn Diagram learning strategy. Ready-to-use Print-Video Venn Diagram reproducibles for this article are available in [Google doc] and [PDF] formats.
In addition, after reading this article and/or viewing the videos, teachers could have students create a graphic organizer that clearly illustrates the levels of defence of the immune system and the structures and cellular processes involved at each level. Students could use an online infographic creator to convey this information in a creative and unique digital format.