The Immune System and Disease Prevention
1. Physical and Chemical Defenses
Learning outcomes
- I can identify the body's first line of defense against pathogens.
- I can explain how skin protects against infection.
- I can describe chemical barriers such as stomach acid and mucus.
- I can explain how physical and chemical defenses work together.
- I can evaluate the importance of these defenses in disease prevention.
The Body Is Constantly Exposed to Pathogens
Every day, the human body encounters enormous numbers of microorganisms.
They may be present in:
- air
- food
- water
- soil
- surfaces
- other people
- animals
Some microorganisms are harmless or beneficial.
Others are pathogens capable of causing disease.
Before a pathogen can cause an infection, it usually has to:
enter the body and reach suitable tissues.
The body has several defenses designed to prevent this from happening.
The First Line of Defense
The body's first line of defense consists of barriers that prevent pathogens from entering the body.
These include:
- skin
- mucus
- cilia
- tears
- saliva
- stomach acid
- blood clotting
- normal microorganisms
These defenses are part of the:
innate immune system.
They are present before infection occurs and respond broadly rather than recognizing one specific pathogen.
Physical and Chemical Defenses
First-line defenses can be divided broadly into:
physical defenses
and
chemical defenses.
Physical defenses block, trap, or remove pathogens.
Chemical defenses create conditions that destroy pathogens or make it difficult for them to survive.
In many parts of the body:
physical and chemical defenses work together.
Physical Defenses
Important physical defenses include:
- skin
- mucus
- cilia
- hairs
- flushing by tears
- flushing by urine
- coughing and sneezing
- blood clotting
These defenses help prevent pathogens from reaching vulnerable:
body tissues.
Chemical Defenses
Chemical defenses include substances that inhibit or destroy microorganisms.
Examples include:
- stomach acid
- enzymes in tears
- enzymes in saliva
- acidic conditions on parts of the skin
- antimicrobial substances in body secretions
These create environments in which many pathogens cannot easily:
survive or reproduce.
The Skin
The skin is one of the body's most important physical barriers.
It covers the outside of the body and separates internal tissues from the:
external environment.
Healthy, intact skin is difficult for many pathogens to penetrate.
Structure of the Skin
The outermost major layer of the skin is the:
epidermis.
Its outer region contains layers of tightly packed, keratinized cells.
These form a strong physical:
barrier.
The skin also continually sheds cells from its surface.
Microorganisms attached to these cells may be removed as the cells are:
shed.
Keratin
Keratin is a tough structural protein found in the outer layers of the skin.
Keratin helps make the surface:
- strong
- resistant
- relatively waterproof
- difficult for microorganisms to penetrate
This strengthens the skin's role as a:
physical barrier.
Why Cuts Increase Infection Risk
If the skin is damaged by:
- cuts
- burns
- punctures
- scratches
the physical barrier is broken.
Pathogens may then gain access to:
internal tissues.
This is why wound cleaning and protection can be important for preventing infection.
Blood Clotting
When the skin is damaged, the body rapidly begins:
blood clotting.
A clot helps:
- reduce blood loss
- seal the wound
- reduce opportunities for microorganisms to enter
A scab can later provide a temporary physical barrier while the tissue underneath:
repairs itself.
The Skin Is Also a Chemical Barrier
Skin does more than physically block pathogens.
Its surface contains substances that can make conditions unfavorable for some microorganisms.
Sebaceous glands produce:
sebum.
Sweat glands produce:
sweat.
Together with other factors, these help create conditions that can inhibit the growth of some microorganisms.
Normal Skin Microbiota
The skin naturally contains populations of harmless or beneficial microorganisms called the:
microbiota.
These organisms can compete with potential pathogens for:
- space
- nutrients
This makes it more difficult for some pathogens to establish themselves.
The microbiota therefore contributes to the body's natural:
defenses.
The Problem of Body Openings
The body cannot be completely covered by skin.
We need openings for:
- breathing
- eating
- seeing
- reproduction
- removing waste
These openings create potential routes for pathogens to enter.
Therefore, they require additional:
defenses.
Mucous Membranes
Many internal passages that connect with the outside environment are lined by:
mucous membranes.
These occur in areas such as the:
- nose
- respiratory tract
- digestive tract
- reproductive tract
Mucous membranes produce a sticky substance called:
mucus.
Mucus
Mucus is a sticky substance that helps trap:
- microorganisms
- dust
- pollen
- other particles
Instead of allowing these particles to travel deeper into the body, mucus can hold them so that they can be:
removed.
Mucus Is Both Protective and Useful
Mucus helps:
- trap pathogens
- trap dust
- keep surfaces moist
- protect epithelial tissues
It is especially important in the:
respiratory system.
However, trapping pathogens is only useful if the body also has a way to remove the mucus.
This is where:
cilia
become important.
Cilia
Cilia are tiny hair-like structures found on some cells lining the respiratory tract.
The cilia move in coordinated waves.
Their movement helps transport mucus toward the:
throat.
The mucus can then be swallowed or removed.
The Mucociliary Escalator
The combination of mucus and cilia is sometimes called the:
mucociliary escalator.
The sequence is:
pathogen enters airway → mucus traps pathogen → cilia move mucus upward → mucus is swallowed or removed
This is an excellent example of different first-line defenses working:
together.
Nasal Hairs
The nose contains hairs that can trap some relatively large:
particles.
These help prevent dust and other material from travelling farther into the respiratory tract.
Smaller particles can then encounter:
mucus and cilia.
Coughing and Sneezing
Coughing and sneezing can physically remove material from the:
respiratory system.
A cough may help remove:
- mucus
- irritants
- microorganisms
A sneeze can remove irritants from the nasal passages.
These are examples of:
mechanical defenses.
Smoking and Cilia
Smoking can damage the cilia lining the:
respiratory tract.
If cilia do not function properly, mucus and trapped particles may not be removed as effectively.
This can weaken an important:
first-line defense.
Tears
The eyes are exposed directly to the environment.
Tears help protect them by:
washing the surface of the eye.
This flushing action can physically remove microorganisms and particles.
But tears also provide a:
chemical defense.
Lysozyme
Tears contain an enzyme called:
lysozyme.
Lysozyme can damage the cell walls of certain bacteria.
It therefore provides a:
chemical defense.
Tears demonstrate two defenses operating simultaneously:
flushing = physical defense
lysozyme = chemical defense
Saliva
Saliva helps protect the mouth.
It continuously washes surfaces within the mouth and helps remove:
- microorganisms
- food particles
- debris
Saliva also contains antimicrobial substances, including:
lysozyme.
Saliva therefore provides both physical and chemical protection.
The Digestive System
Food and drink provide another possible route for pathogens to enter the body.
The digestive system therefore contains several important:
defenses.
One of the most important is:
stomach acid.
Stomach Acid
The stomach contains:
hydrochloric acid.
This creates a strongly acidic environment.
The stomach commonly reaches approximately:
pH 1–3.
Many microorganisms cannot survive these conditions.
How Stomach Acid Protects Us
Imagine bacteria entering the body in contaminated food.
The bacteria travel through the:
mouth → esophagus → stomach.
Inside the stomach, they encounter:
strongly acidic conditions.
Many are destroyed or inhibited before they can travel farther through the digestive system.
Stomach acid is therefore an important:
chemical barrier.
Can All Pathogens Be Destroyed by Stomach Acid?
No.
Some pathogens are adapted to survive acidic environments.
Others may survive long enough to reach the:
intestines.
Therefore, stomach acid greatly reduces risk but does not provide perfect protection.
No single first-line defense is completely effective.
Vomiting
Vomiting can sometimes remove harmful substances or microorganisms from the:
digestive system.
This is primarily a physical removal mechanism.
However, vomiting is usually a response occurring after irritation or infection has already begun rather than a permanent barrier.
Diarrhea
Diarrhea can rapidly move material through the:
intestines.
This may help remove some pathogens and toxins.
However, severe diarrhea can cause dangerous losses of:
- water
- electrolytes
Therefore, a defensive response can sometimes also have harmful consequences.
Urine Flow
The urinary system also has physical defenses.
The flow of:
urine
helps flush microorganisms out of parts of the urinary tract.
This makes it more difficult for some microorganisms to remain attached and establish an infection.
Chemical Conditions in the Urinary and Reproductive Systems
Different parts of the urinary and reproductive systems have chemical environments that can limit microbial growth.
For example, the healthy vaginal environment is normally:
acidic.
Normal microorganisms help maintain this environment.
This can inhibit the growth of some pathogens.
Earwax
The ear canal produces:
cerumen, commonly called earwax.
Earwax can:
- trap particles
- trap microorganisms
- help maintain protective conditions in the ear canal
It therefore contributes to the body's first-line defenses.
Normal Microbiota
Large populations of microorganisms normally live:
- on the skin
- in the mouth
- in the intestines
- in parts of the reproductive system
Together these microbial communities are called the:
microbiota.
How Microbiota Protect Us
Normal microorganisms can compete with pathogens for:
space and nutrients.
They may also produce substances that inhibit other microorganisms.
This makes it more difficult for pathogens to:
colonize.
Healthy microbiota can therefore provide an important form of biological protection.
Antibiotics and the Microbiota
Antibiotics can kill susceptible bacteria.
Sometimes this includes beneficial bacteria in the normal:
microbiota.
If these populations are disrupted, other microorganisms may have more opportunities to grow.
This is another reason antibiotics should be used:
appropriately.
Physical and Chemical Defenses Work Together
The body's defenses are most effective because they do not operate:
independently.
Consider the respiratory system:
nasal hairs trap larger particles
↓
mucus traps smaller particles and microorganisms
↓
cilia move mucus
↓
coughing helps remove material
Multiple mechanisms work together to reduce the chance of infection.
Example: The Eye
The eye demonstrates another combination.
Tears wash the eye
This physically removes microorganisms.
At the same time:
lysozyme damages certain bacteria
This provides chemical protection.
Therefore:
physical removal + chemical destruction = stronger defense
Example: The Digestive System
Food may contain microorganisms.
Several defenses act before infection develops.
Saliva washes the mouth
↓
antimicrobial substances act in saliva
↓
food enters the stomach
↓
stomach acid destroys or inhibits many microorganisms
Multiple barriers make infection less likely.
Example: A Cut in the Skin
Imagine a person cuts their finger.
Normally:
intact skin blocks pathogens.
After the cut:
the barrier is broken.
The body responds through blood clotting.
The clot:
- reduces blood loss
- seals the damaged area
- helps reduce pathogen entry
Later immune responses may act against pathogens that successfully entered the tissue.
First Line vs Second Line of Defense
The first line of defense attempts to prevent pathogens from entering tissues.
Examples include:
- skin
- mucus
- cilia
- stomach acid
If a pathogen crosses these barriers, the body activates internal defenses.
These include:
- inflammation
- phagocytic cells
- other innate immune responses
These internal responses are often described as the:
second line of defense.
First Line vs Specific Immunity
The first-line defenses are:
non-specific.
Skin does not recognize influenza virus and respond differently from a bacterium.
It simply provides a broad:
barrier.
Specific immune responses, involving lymphocytes and antibodies, can recognize particular:
antigens.
This distinction becomes important when studying the immune system.
Why Are First-Line Defenses So Important?
Preventing pathogen entry is extremely efficient.
If a pathogen never reaches internal tissues:
- it cannot easily reproduce there
- it cannot damage those tissues
- a large immune response may not be required
- disease may never develop
Therefore:
preventing entry is better than fighting an established infection.
These Defenses Work Continuously
You do not have to consciously activate your:
skin.
You do not have to decide to produce:
stomach acid.
These defenses are operating continuously.
They provide protection even before the body knows that a pathogen is present.
Are First-Line Defenses Perfect?
No defense provides complete:
protection.
Pathogens may enter when:
- skin is damaged
- large numbers of pathogens are encountered
- a pathogen survives stomach acid
- respiratory defenses are damaged
- microorganisms reach vulnerable tissues
- pathogens have adaptations that overcome barriers
The body therefore needs several layers of defense.
Layered Defense
The immune system can be thought of as a series of:
layers.
Layer 1: Prevent entry
Skin, mucus, cilia, stomach acid.
↓
Layer 2: Attack invaders
Phagocytes, inflammation and other innate responses.
↓
Layer 3: Specific response
Lymphocytes and antibodies target particular pathogens.
This layered system makes the body much more difficult for pathogens to successfully:
infect.
Why Hygiene Helps First-Line Defenses
Good hygiene reduces the number of pathogens that reach the body's barriers.
Examples include:
- washing hands
- cleaning wounds
- safe food preparation
- clean drinking water
- dental hygiene
These practices support the body's natural defenses rather than replacing them.
Handwashing
The skin is a strong barrier, but microorganisms can remain on its:
surface.
Hands can transfer pathogens to:
- mouth
- nose
- eyes
- food
- other people
Handwashing removes many microorganisms before they reach a suitable:
entry point.
Wound Care
A wound represents a break in the body's:
physical barrier.
Cleaning and protecting a wound can reduce the number of microorganisms entering damaged tissue.
This gives the body's repair and immune mechanisms a better chance to prevent:
infection.
Worked Example 1
A bacterium lands on intact skin.
What is the first major barrier it encounters?
The skin.
The tightly packed outer layers help prevent the bacterium from reaching internal tissues.
Worked Example 2
A pathogen enters the nose.
What may happen?
Mucus traps it.
Cilia then move the mucus toward the throat.
The mucus can be swallowed or removed.
This demonstrates:
physical defenses working together.
Worked Example 3
Bacteria enter the body in food.
Which chemical defense do they encounter in the stomach?
Hydrochloric acid.
The low pH destroys or inhibits many microorganisms.
Worked Example 4
A bacterium reaches the surface of the eye.
Tears begin washing across the surface.
Identify two defenses.
Physical defense: flushing by tears.
Chemical defense: lysozyme in tears.
Worked Example 5
A person suffers a deep cut.
Why does infection risk increase?
The cut:
breaks the skin barrier.
Microorganisms now have a route into internal tissues.
Worked Example 6
A person regularly inhales cigarette smoke.
Their respiratory cilia become damaged.
How could this increase infection risk?
Damaged cilia are less effective at moving:
mucus and trapped microorganisms
out of the respiratory tract.
Worked Example 7
A pathogen survives stomach acid.
Does this mean the body has no remaining defenses?
No.
Other physical, chemical, cellular, and immune defenses can still act against the pathogen.
The body uses:
multiple layers of protection.
Worked Example 8
Why are mucus and cilia more effective together than either would be alone?
Mucus:
traps pathogens.
Cilia:
move the pathogen-containing mucus away.
Together they both trap and remove potential pathogens.
Comparing Physical and Chemical Defenses
Skin
Type:
mainly physical
Function:
blocks pathogen entry
Mucus
Type:
mainly physical
Function:
traps pathogens
Cilia
Type:
physical
Function:
move mucus and trapped particles
Tears
Type:
physical and chemical
Function:
wash surfaces and contain antimicrobial substances
Saliva
Type:
physical and chemical
Function:
washes the mouth and contains antimicrobial substances
Stomach acid
Type:
chemical
Function:
creates a low-pH environment that destroys or inhibits many microorganisms
Blood clotting
Type:
physical
Function:
seals damaged tissue
Normal microbiota
Type:
biological
Function:
compete with potential pathogens
Common Mistake: Mucus Causes Infection
Mucus is actually an important:
defense.
It traps microorganisms and particles so they can be removed.
During infection, increased mucus production can be noticeable, but mucus itself is part of the body's protective system.
Common Mistake: Stomach Acid Digests Only Food
Stomach acid contributes to digestion, but it also provides an important:
chemical defense.
Its low pH destroys or inhibits many microorganisms swallowed with food and water.
Common Mistake: Skin Is Just a Covering
Skin is an active protective organ.
It provides:
- a physical barrier
- chemical conditions that inhibit some microorganisms
- support for normal microbiota
Skin is therefore a major component of:
innate defense.
Common Mistake: First-Line Defenses Target Specific Pathogens
First-line defenses are generally:
non-specific.
They provide broad protection against many different microorganisms.
They do not need to identify a particular pathogen first.
Common Mistake: If a Pathogen Crosses One Barrier, Infection Is Guaranteed
The body has:
multiple defenses.
Crossing the skin or surviving stomach acid does not guarantee that a pathogen will successfully cause disease.
Other innate and adaptive immune defenses remain.
Common Mistake: Chemical Defenses Are Medicines
Chemical defenses such as:
- stomach acid
- lysozyme
- antimicrobial secretions
are produced naturally by the:
body.
They are not medicines taken from outside the body.
Check Your Understanding
1. What is the body's first line of defense?
2. What is meant by a physical defense?
3. What is meant by a chemical defense?
4. Give four examples of physical defenses.
5. Give three examples of chemical defenses.
6. Explain how intact skin prevents infection.
7. What is the epidermis?
8. How does keratin contribute to protection?
9. Why does a cut increase the risk of infection?
10. How does blood clotting help prevent pathogen entry?
11. What is mucus?
12. How does mucus protect the respiratory system?
13. What are cilia?
14. Explain how cilia and mucus work together.
15. What is the mucociliary escalator?
16. How can coughing help protect the respiratory system?
17. Why can smoking increase susceptibility to respiratory infection?
18. How do tears provide a physical defense?
19. What is lysozyme?
20. How do tears provide a chemical defense?
21. How does saliva protect the mouth?
22. What chemical is responsible for the strongly acidic environment of the stomach?
23. How does stomach acid protect against infection?
24. Why does stomach acid not prevent every digestive infection?
25. How does urine flow provide protection?
26. What is the microbiota?
27. How can normal microbiota reduce pathogen growth?
28. Explain why antibiotics can sometimes disturb normal microbiota.
29. Give an example of physical and chemical defenses working together.
30. Compare the roles of skin and stomach acid.
31. Why are first-line defenses described as non-specific?
32. What happens if a pathogen crosses the first line of defense?
33. Explain the difference between the first and second lines of defense.
34. Why are several layers of defense better than one?
35. How does handwashing support the body's first-line defenses?
36. Why is wound care important?
37. Explain why preventing pathogen entry is beneficial to the body.
38. Identify two first-line defenses in the respiratory system and explain how they work.
39. Identify two first-line defenses in the digestive system and explain how they work.
40. Explain how physical, chemical, and biological barriers combine to reduce the risk of infectious disease.
Key Terms
- First line of defense: Barriers that help prevent pathogens from entering body tissues.
- Innate defense: Non-specific protection present without previous exposure to a particular pathogen.
- Physical barrier: Structure or process that physically blocks, traps, or removes pathogens.
- Chemical barrier: Substance or chemical condition that destroys or inhibits pathogens.
- Skin: Protective organ forming a major physical barrier against pathogens.
- Epidermis: Outer major layer of the skin.
- Keratin: Tough structural protein that strengthens the outer skin.
- Mucous membrane: Tissue lining certain body passages and producing mucus.
- Mucus: Sticky substance that traps microorganisms and particles.
- Cilia: Microscopic hair-like structures that move material across cell surfaces.
- Mucociliary escalator: Combined action of mucus and cilia that removes particles from the respiratory tract.
- Lysozyme: Enzyme found in secretions such as tears that can damage certain bacterial cell walls.
- Hydrochloric acid: Strong acid in the stomach that contributes to digestion and defense.
- Microbiota: Communities of microorganisms normally living on or within the body.
- Blood clotting: Process that seals damaged blood vessels and helps close wounds.
- Pathogen: Disease-causing microorganism or infectious agent.
- Non-specific defense: Defense that acts broadly rather than targeting one particular pathogen.
Key Takeaways
- The body's first line of defense attempts to prevent pathogens from entering tissues.
- First-line defenses are part of the innate immune system.
- These defenses are generally non-specific.
- The skin provides one of the body's most important physical barriers.
- Keratin and tightly packed cells make the outer skin difficult for pathogens to penetrate.
- Damage to the skin creates possible entry points for microorganisms.
- Blood clotting helps seal wounds and restore a physical barrier.
- Mucus traps microorganisms and other particles.
- Cilia move mucus and trapped pathogens through the respiratory tract.
- Mucus and cilia work together as the mucociliary escalator.
- Coughing and sneezing can help physically remove material.
- Tears physically wash the eyes.
- Tears also contain antimicrobial substances such as lysozyme.
- Saliva provides both physical and chemical protection.
- Stomach acid creates a low-pH environment that destroys or inhibits many microorganisms.
- Urine flow can help flush microorganisms from the urinary tract.
- Normal microbiota can compete with pathogens for space and nutrients.
- Physical and chemical defenses often work together rather than independently.
- No first-line defense provides complete protection.
- If pathogens cross the first line, additional innate and adaptive immune defenses can respond.
- Maintaining healthy barriers and using good hygiene can reduce opportunities for pathogens to establish infections.
- Preventing pathogens from entering the body is one of the most efficient ways to prevent infectious disease.