8.3 First Two Lines of Defense

Learning Objectives

• Describe your body’s first line of defense against pathogens.
• Explain how inflammation helps protect you from pathogens.

Introduction

What do you think your body's first line of defense against disease might be?

Your body’s first line of defense is like a castle’s moat and walls. It keeps most pathogens out of your body. The first line of defense includes different types of barriers.

Guided Learning

First Line of Defense

Your body has many ways to protect you from pathogens. Your body’s defenses are like a castle of old. The outside of a castle was protected by a moat and high walls. Inside the castle, soldiers were ready to fight off any enemies that made it across the moat and over the walls. Like a castle, your body has a series of defenses. Only pathogens that get through all the defenses can harm you.

Skin and Mucous Membranes

The skin is a very important barrier to pathogens. The skin is the body’s largest organ. In adults, it covers an area of about 16 to 22 square feet!

The skin is also the body’s most important defense against disease. It forms a physical barrier between the body and the outside world. As shown in Figure below, the skin has several layers. The outer layer is tough and waterproof. It is very difficult for pathogens to get through this layer of skin.

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This drawing shows that the skin has many layers. The outer layer is so tough that it keeps out most pathogens.

The mouth and nose are not lined with skin. Instead, they are lined with mucous membranes. Other organs that are exposed to the outside world, including the lungs and stomach, are also lined with mucous membranes. Mucous membranes are not tough like skin, but they have other defenses.

One defense of mucous membranes is the mucus they release. Mucus is a sticky, moist substance that covers mucous membranes. Most pathogens get stuck in the mucus before they can do harm to the body. Many mucous membranes also have cilia. Cilia in the lungs are shown in Figure below. Cilia are like tiny finger-like projections. They move in waves and sweep mucus and trapped pathogens toward body openings. When you clear your throat or blow your nose, you remove mucus and pathogens from your body.

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This is what the cilia lining the lungs look like when they are magnified. Their movements constantly sweep mucus and pathogens out of the lungs. Do they remind you of brushes?

Chemicals

Most body fluids that you release from your body contain chemicals that kill pathogens. For example, mucus, sweat, tears, and saliva contain enzymes that kill pathogens. The enzymes are called lysozymes. They break down the cell walls of bacteria to kill them.

The stomach also releases a very strong acid, called hydrochloric acid. This acid kills most pathogens that enter the stomach in food or water. Urine is also acidic, so few pathogens can grow in it.

Helpful Bacteria

You are not aware of them, but your skin is covered by millions (or more!) of bacteria. Millions more live inside your body. Many bacteria live inside your large intestine. Most of these bacteria help defend your body from pathogens. How do they do it? They compete with harmful bacteria for food and space. This prevents the harmful bacteria from multiplying and making you sick.

Second Line of Defense

The little boy in Figure below has a scraped knee. A scrape is a break in the skin that may let pathogens enter the body. If bacteria enter through the scrape, they could cause an infection. These bacteria would then face the body’s second line of defense.

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This little boy just got his first scraped knee. It doesn’t seem to hurt, but the break in his skin could let pathogens enter his body. That’s why scrapes should be kept clean and protected until they heal.

Inflammation

If bacteria enter the skin through a scrape, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. Inflammation is caused by chemicals that are released when skin or other tissues are damaged. The chemicals cause nearby blood vessels to dilate, or expand. This increases blood flow to the damaged area. The chemicals also attract white blood cells to the wound and cause them to leak out of blood vessels into the damaged tissue. To help review, watch a video animation of this process.

White Blood Cells

After white blood cells leave a blood vessel at the site of inflammation, they start “eating” pathogens. White blood cells are one type of cell that makes up the blood.

The main role of white blood cells is to fight pathogens in the body. There are actually several different kinds of white blood cells. Some white blood cells have very specific functions. They attack only certain pathogens.

Other white blood cells attack any pathogens they find. These white blood cells travel to areas of the body that are inflamed. They are called phagocytes, which means “eating cells.” In addition to pathogens, phagocytes “eat” dead cells. They surround the pathogens and destroy them. This process is called phagocytosis. How phagocytosis occurs is shown in Figure below. To further help your understanding, watch a video of a phagocyte gobbling up and destroying a pathogen.

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This drawing show phagocytosis. In this process, a phagocyte surrounds a pathogen and breaks it down as shown by the white blood cell attacking the green bacteria.

White blood cells also make chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 98.6° F (37° C). Most bacteria and viruses that infect people reproduce fastest at this temperature.

When the temperature is higher, the pathogens cannot reproduce as fast, so the body raises the temperature to kill them. A fever also causes the immune system to make more white blood cells. In these ways, a fever helps the body fight infection.

Summary

• Your body’s first line of defense includes the skin and other barriers that keep pathogens out of your body.
• If pathogens enter your body, inflammation occurs, and phagocytes come to the body’s defense.

Vocabulary

fever

A higher-than-normal body temperature which causes pathogens to reproduce more slowly; also caused the immune system to make more white blood cells, helping the body fight the infection.

inflammation

One way the body reacts to infections or injuries, causing the area to become red, warm and painful; caused by chemicals that are released when skin or other tissues are damaged, the chemicals cause nearby blood vessels to dilate, thereby increasing blood flow to the damaged area.

mucus

A sticky, moist substance that covers mucous membranes; causes pathogens to get stuck before they can do harm to the body.

phagocytes

A type of white blood cells that attack any pathogen they find by surrounding the pathogen and destroying them; means "eating cells."

phagocytosis

The process of a phagocyte engulfing and consuming a pathogen.