barriers to pathogens

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. 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.


Most body fluids that you release from your body contain chemicals that kill pathogens. For example, mucus, sweat, tears, and saliva contain enzymes called lysozymes that kill pathogens. These enzymes can 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. 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?


skin and mucous membranes

The skin is a very important barrier to pathogens. The skin is the bodys largest organ. In adults, it covers an area of about 16 to 22 square feet! The skin is also the bodys most important defense against disease. It forms a physical barrier between the body and the outside world. The skin has several layers that stack on top of each other ( Figure 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 pictured below ( Figure 1.2). Cilia are 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.


the immune systems first line of defense

It is the immune systems job to protect the body. Your body has many ways to protect you from pathogens. Your bodys defenses are like a castle. 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. The first line of defence includes both physical and chemical barriers that are always ready and prepared to defend the body from infection. Pathogens must make it past this first line of defense to cause harm. If this defense is broken, the second line of defense within your body is activated. Your bodys first line of defense is like a castles moat and walls. It keeps most pathogens out of your body. This is a non-specific type of defense, in that it tries to keep all pathogens out. The first line of defense includes different types of barriers. Being the "first line", it starts with the skin. The first line also includes tears, mucus, cilia, stomach acid, urine flow, and friendly bacteria.

instructional diagrams

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the first line of defense keeps most pathogens out of the body.

-->  a. true

b. false

the skin is a mucous membrane.

a. true

-->  b. false

which of the following are covered with mucous membranes?

a) the lining of the mouth

b) the lining of the throat

c) the inside of the lungs

-->  d) all of the above

what is a characteristic of the stomach?

a) the stomach is lined with cilia, which help remove pathogens.

-->  b) the stomach releases acid, which kills pathogens.

c) the stomach is full of enzymes that kill bacteria.

d) all of the above

how do you think the flow of urine is involved in protection?

-->  a) your urine flow flushes out pathogens from the bladder area.

b) mucous in your urine removes pathogens.

c) cilia in urine help remove pathogens.

d) all of the above

what is a chemical defense associated with the first line?

-->  a) the enzyme lysozyme

b) bacteria on the skin

c) the skin itself

d) all of the above

what is a physical barrier associated with the first line?

a) tenzymes in sweat, tears, and saliva

b) hydrochloric acid

-->  c) the skin itself

d) all of the above

diagram questions

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