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Barriers to Pathogens
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: August 4, 2012
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Barriers to Pathogens
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Barriers to Pathogens
• Describe the barriers that keep most pathogens out of the human body.
How does your body keep most enemies out?
Many would consider the moat around this castle, together with the thick stone castle walls, as the first line of
defense. Their role is to keep the enemy out, and protect what’s inside.
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The First Line of Defense
FIGURE 1.1
Does this organism look like a space alien? A scary creature from a nightmare? In fact, it’s a 1-cm long worm that
lives in the human body and causes serious harm. It enters the body through a hair follicle of the skin when it’s in
a much smaller stage of its life cycle. Like this worm, many other organisms can make us sick if they manage to
enter our body. Fortunately for us, our immune system is able to keep out most such invaders. The immune system
protects the body from worms, germs, and other agents of harm. The immune system is like a medieval castle. The
outside of the castle was protected by a moat and high stone walls. Inside the castle, soldiers were ready to fight
off any invaders that managed to get through the outer defenses. Like a medieval castle, the immune system has a
series of defenses. In fact, it has three lines of defense. Only pathogens that are able to get through all three lines of
defense can harm the body.
The body’s first line of defense consists of different types of barriers that keep most pathogens out of the body.
Pathogens are disease-causing agents, such as bacteria and viruses. These and other types of pathogens are described
in Figure 1.2. Regardless of the type of pathogen, however, the first line of defense is always the same.
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Concept 1. Barriers to Pathogens
FIGURE 1.2
Types of pathogens that commonly cause human diseases include bacteria, viruses, fungi, and protozoa. Which
type of pathogen causes the common cold? Which type causes athlete’s foot?
Mechanical Barriers
Mechanical barriers physically block pathogens from entering the body. The skin is the most important mechanical
barrier. In fact, it is the single most important defense the body has. The outer layer of the skin is tough and very
difficult for pathogens to penetrate. Mucous membranes provide a mechanical barrier at body openings. They
also line the respiratory, GI, urinary, and reproductive tracts. Mucous membranes secrete mucus, a slimy substance
that traps pathogens. The membranes also have hair-like cilia. The cilia sweep mucus and pathogens toward body
openings where they can be removed from the body. When you sneeze or cough, pathogens are removed from the
nose and throat (see Figure 1.3). Tears wash pathogens from the eyes, and urine flushes pathogens out of the urinary
tract. You can watch the sweeping action of cilia at the following link: http://mcdb.colorado.edu/courses/2115/units/
Other/mucus%20animation.swf.
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FIGURE 1.3
A sneeze can expel many pathogens
from the respiratory tract. That’s why
you should always cover your mouth
and nose and when you sneeze.
Chemical Barriers
Chemical barriers destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat,
mucus, tears, and saliva all contain enzymes that kill pathogens. Urine is too acidic for many pathogens, and semen
contains zinc, which most pathogens cannot tolerate. In addition, stomach acid kills pathogens that enter the GI tract
in food or water.
Biological Barriers
Biological barriers are living organisms that help protect the body. Millions of harmless bacteria live on the human
skin. Many more live in the GI tract. The harmless bacteria use up food and space so harmful bacteria cannot grow.
Vocabulary
• biological barrier: A living organism that helps protect the body.
• chemical barrier: A barrier that destroys pathogens on the outer body surface, at body openings, and on inner
body linings.
• cilium (plural, cilia): Short, hairlike projection, similar to flagellum, that allows some cells to move.
• immune system: Body system that consists of skin, mucous, membranes, and other tissues and organs that
defends the body from pathogens and cancer.
• mechanical barrier: Barrier that physically block pathogens from entering the body, such as the skin.
• mucous membrane: Epithelial tissue that lines inner body surfaces and body openings and produces mucus.
• mucus: Slimy substance produced by mucous membranes that traps pathogens, particles, and debris.
• pathogen: Disease causing agent such as a bacterium, virus, fungus, or protozoan.
Summary
• Barriers that keep out pathogens are the body’s first line of defense.
• The first line of defense includes mechanical, chemical, and biological barriers.
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Concept 1. Barriers to Pathogens
Practice
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Biology for AP* → Search: The Barriers of Infection
1.
2.
3.
4.
5.
What are external barriers? Give examples.
Describe examples of chemical protections.
What is the role of lysozyme?
How may pathogens enter the body?
How do mucous membranes and cilia help prevent pathogens from entering the body?
Review
1. Identify three types of barriers in the body’s first line of defense. Give an example of each type of barrier.
2. Which type of pathogen causes the common cold? Which type causes athlete’s foot?
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