Defences Against Pathogens

Section 1 of 8Barriers that keep pathogens out

Skin

• Skin A physical barrier that covers the body. If cut, blood clots to seal the wound and form a scab.

Nose

• Nose Hairs and sticky mucus trap dust and microbes before they reach the lungs.

Trachea and bronchi

• Trachea and bronchi Goblet cells make mucus, and cilia on lining cells sweep the mucus upwards to be swallowed or coughed out.

Stomach

• Stomach Hydrochloric acid gives a very low pH that kills most pathogens swallowed with food or mucus.

Section 2 of 8Physical or chemical?

Defence	Type	How it works
Skin	Physical	Forms a tough barrier; scabs close breaks
Mucus and cilia	Physical	Trap and move pathogens out of airways
Stomach acid	Chemical	Kills microbes by acidity
Enzymes in tears and saliva	Chemical	Break down bacterial cell walls

Section 3 of 8If a pathogen gets through

1Pathogen enters

• Pathogen enters It passes the first barriers, for example through a wound.

2Detection

• Detection White blood cells recognise foreign antigens on the pathogen.

3Attack

• Attack Phagocytes engulf and digest pathogens; lymphocytes release antibodies and, in some cases, antitoxins.

4Memory

• Memory Memory cells remain, allowing a faster response if the same pathogen returns.

Section 4 of 8Why defences are called non-specific

1

They act in the same way against any pathogen.

2

They work immediately and do not need to learn the invader first.

3

They are the first line of defence, stopping most microbes from entering at all.

4

Only the immune response that follows is specific to one type of pathogen.

Section 5 of 8Worked example: tracing a cut

Stage	What happens	Type of defence
Skin is cut	Platelets clot the blood and a scab forms	Physical barrier, non-specific
Bacteria enter the wound	Phagocytes engulf and digest them	Non-specific white blood cell response
Bacteria multiply	Lymphocytes make specific antibodies	Specific immune response
Infection is cleared	Memory cells stay in the blood	Immunity to the same pathogen

Section 6 of 8Applications and exam context

Smoking

• Smoking Smoke damages cilia and paralyses them,
  • so mucus containing microbes is not swept out of the airways,
  • which makes chest infections more likely.

Stomach acid and medicines

• Stomach acid and medicines Some medicines reduce stomach acid, so more swallowed pathogens may survive and cause infection.

Wound care

• Wound care Cleaning and covering a cut protects the broken barrier while the clot and scab form.

Section 7 of 8Key terms

Term	Meaning
Non-specific defence	A barrier or response that acts against all pathogens in the same way.
Mucus	A sticky fluid that traps microbes and particles.
Cilia	Tiny hair-like projections that beat to move mucus.
Phagocyte	A white blood cell that engulfs and digests pathogens.
Antibody	A protein made by lymphocytes that binds to a particular antigen.

Section 8 of 8Exam technique

1

Saying that cilia trap pathogens. Mucus traps them; cilia move the mucus along.

2

Forgetting that stomach acid is a chemical defence, not a physical one.

3

Describing the skin as a barrier only when intact. Blood clotting also seals breaks.

4

Mixing up non-specific defences with antibody production, which is specific.

5

Calling the whole immune response non-specific; antibodies are specific to one antigen.