Human Defence Systems and the Immune System

Your body stops most pathogens before they ever cause illness, using defences that fall into two clear groups.

A pathogen is a microorganism — a bacterium, virus, protist or fungus — that causes disease. To make you ill, a pathogen must first get into the body and then survive and reproduce there. The body fights back at both stages.

• Non-specific defences act against every pathogen in the same way. They are the body's first line of defence — physical and chemical barriers that try to keep pathogens out, regardless of what kind they are.
• The immune system is the second line. If a pathogen gets past the barriers and enters the body, white blood cells attempt to destroy it. Some of this response is targeted at one specific pathogen.

Defence layer	Targets...	Examples
Non-specific	All pathogens, the same way	Skin, nose, trachea, bronchi, stomach acid
Immune system	Pathogens that get inside	White blood cells (phagocytosis, antibodies, antitoxins)

Key distinction: non-specific defences treat all pathogens alike; the immune system can mount a specific response against a particular pathogen. Keep these two ideas separate — it is the backbone of this whole topic.

The body's first job is to stop pathogens entering. Several non-specific defences act as physical barriers, and they work against any pathogen that arrives.

• Skin covers the body as a tough physical barrier, keeping pathogens out. If skin is cut, it scabs over quickly — the scab seals the break before pathogens can get into the blood.
• The nose is lined with hairs and mucus. These trap pathogens and particles in the air before they travel deeper into the airways.
• The trachea and bronchi (the tubes carrying air to the lungs) are lined with cilia and mucus. The sticky mucus traps pathogens; the cilia are tiny hair-like structures that waft the mucus up and away from the lungs, towards the throat where it is swallowed.

Remember: the cilia do not trap the pathogens — the mucus traps them, and the cilia move the mucus. Mixing these two up is a common slip.

These defences are all non-specific: they block bacteria, viruses, protists and fungi alike, without "recognising" any of them.

Not every pathogen is breathed in — many are swallowed in food and drink, or arrive in the mucus that the cilia sweep up from the airways. The stomach deals with these.

The stomach produces hydrochloric acid, giving the stomach contents a very low (strongly acidic) pH. This acid kills pathogens that are swallowed before they can reach and infect the rest of the digestive system.

Key idea: stomach acid is a chemical non-specific defence. It kills swallowed pathogens, including those carried down in mucus from the nose, trachea and bronchi.

This is why the cilia–mucus system and the stomach work together: pathogens trapped in mucus in the airways are wafted up, swallowed, and then destroyed by the acid rather than reaching the lungs.

(Extra context — not required by AQA 8461.) The hydrochloric acid in the stomach has a pH of roughly 1.5–2. AQA only asks you to know that stomach acid is acidic and kills pathogens, not the exact pH value.

So far, none of these defences has needed to identify the pathogen. The next slides cover what happens when a pathogen gets past these barriers.

If a pathogen breaks through the barriers and enters the body, the immune system tries to destroy it. The work is done by white blood cells, and the first method is phagocytosis.

In phagocytosis, a white blood cell engulfs (surrounds and takes in) the pathogen, then digests it using enzymes. This destroys the pathogen completely.

Worked walk-through 1 — from a cut to a destroyed pathogen. You cut your finger and a bacterium enters the blood:

1. The bacterium has slipped past the skin barrier through the cut.
2. A white blood cell detects the bacterium in the blood.
3. The white blood cell moves to it and engulfs it, surrounding it entirely.
4. Enzymes inside the white blood cell digest the bacterium.
5. The bacterium is destroyed before it can reproduce and cause illness.

Key idea: phagocytosis is non-specific within the immune system — a phagocyte will engulf many different pathogens. The cell engulfs the pathogen and then digests it.

The second white-blood-cell method is antibody production, and this part of the response is specific — it is aimed at one particular pathogen.

Every pathogen carries antigens on its surface — unique marker molecules. A type of white blood cell (a lymphocyte) recognises a pathogen's antigens as foreign and produces antibodies: proteins that lock onto those specific antigens.

Because an antibody's shape matches one specific antigen, antibodies are specific: an antibody made against one pathogen will not lock onto a different pathogen with different antigens.

Once antibodies lock onto the antigens, they cause the pathogens to clump together and mark them out, so they are destroyed more easily.

Worked walk-through 2 — why antibodies are specific. Suppose a virus enters the body:

1. The virus carries antigens on its surface with a particular shape.
2. A lymphocyte recognises these antigens as foreign and makes antibodies with a complementary shape.
3. The antibodies lock onto the virus's antigens — like a key fitting one lock.
4. A different pathogen has differently shaped antigens, so these antibodies will not fit it. A new, specific antibody is needed for each new pathogen.

Antibody: a protein made by white blood cells that locks onto the specific antigens of a particular pathogen. One antibody type matches one antigen type.

The third white-blood-cell method tackles a different problem. Many bacteria do their damage by releasing toxins — poisons that harm the body's tissues and make you feel ill. Killing the bacteria does not remove toxins that have already been released.

White blood cells produce antitoxins: molecules that neutralise (counteract) the toxins produced by pathogens. A neutralised toxin can no longer damage the body's tissues.

Key idea: an antitoxin neutralises a toxin — it does not destroy the pathogen itself. Phagocytosis destroys the pathogen; antitoxins deal with the poison the pathogen has released.

White blood cell method	What it acts on	What it does	Specific or non-specific?
Phagocytosis	The pathogen	Engulfs and digests it	Non-specific
Antibody production	The pathogen's antigens	Antibodies lock onto specific antigens	Specific
Antitoxin production	The toxins	Neutralises toxins so they cannot harm cells	Targeted at the toxin

Putting the whole topic together:

Defence	How it works	Type
Skin (and scabs)	Physical barrier; scabs seal cuts	Non-specific
Nose	Hairs and mucus trap pathogens	Non-specific
Trachea and bronchi	Mucus traps pathogens; cilia waft mucus away	Non-specific
Stomach acid	Acid kills swallowed pathogens	Non-specific
Phagocytosis	White blood cell engulfs and digests the pathogen	Immune system
Antibody production	Antibodies lock onto a pathogen's specific antigens	Immune system
Antitoxin production	Neutralises toxins released by pathogens	Immune system

(Extra context — not required by AQA 8461.) Once the immune system has made antibodies against a pathogen, it can respond faster if that pathogen returns — the principle behind vaccination, which is covered in a separate lesson.

1. Calling antibodies "non-specific"

Antibodies are specific — each type locks onto one particular antigen. The non-specific defences are the barriers (skin, nose, trachea, bronchi, stomach acid) and, within the immune system, phagocytosis engulfs many different pathogens.

2. Saying antitoxins destroy or kill the pathogen

Antitoxins neutralise toxins — the poisons a pathogen releases. They do not destroy the pathogen itself. Phagocytosis is what engulfs and digests the pathogen.

3. Confusing antigens with antibodies

An antigen is the marker molecule on the pathogen's surface. An antibody is the protein the white blood cell makes to lock onto that antigen. Antigen = on the pathogen; antibody = made by you.

4. Saying the cilia trap pathogens

The mucus traps pathogens. The cilia are hair-like structures that sweep the mucus (with the trapped pathogens) up and out of the airways towards the throat.

5. Describing phagocytosis as just "the cell eats the pathogen"

State both steps for full marks: the white blood cell engulfs the pathogen and then digests it. Leaving out the digesting step loses detail the mark scheme looks for.

6. Treating all white blood cell jobs as the same thing

Phagocytosis, antibody production and antitoxin production are three distinct methods. A question on "how white blood cells defend the body" usually expects more than one of them.