Human defence systems

The body's two-tier defence — four non-specific barriers and three white blood cell mechanisms. Every part is spec-named, and every part can score marks.

SPEC 8461 • MODULE 3 • LESSON 3.04

Two Layers — Seven Named Components

AQA's "human defence" question is one of Paper 1's most predictable. The mark scheme is built from a fixed list of spec-named items.

The spec gives four non-specific defences (skin · nose · trachea & bronchi · stomach) and three white blood cell mechanisms (phagocytosis · antibody production · antitoxin production). Students who write "white blood cells fight germs" miss easy marks — examiners want each mechanism named individually. — Recurring AQA Mark-Scheme Pattern

Why This Lesson

The body defends in two stages: (1) stop pathogens entering (non-specific barriers), and (2) destroy them if they get in (white blood cells / immune system). Both stages are testable, and the spec names every component you need to know.

Learning Objectives

By the end of this lesson, you'll be able to:

Objective 1

Describe the four non-specific defence systems of the human body — skin, nose, trachea & bronchi, stomach.

Objective 2

Explain the role of the immune system in destroying pathogens, including the three ways white blood cells defend the body.

Objective 3

Name and apply the three white blood cell mechanisms — phagocytosis, antibody production, antitoxin production.

Spec ref: 4.3.1.6 · Paper: 1 · Tier: Higher

The Two-Tier Defence Framework

The body uses two distinct types of defence — and they work in sequence.

Tier 1 — Non-Specific

Physical and chemical barriers that stop pathogens entering the body in the first place.

Spec sites (4):

→ Skin

→ Nose

→ Trachea & bronchi

→ Stomach

Called "non-specific" because they work against any pathogen, not a particular one.

Tier 2 — Immune System

White blood cells (WBCs) destroy pathogens that get past tier 1.

Spec mechanisms (3):

→ Phagocytosis

→ Antibody production

→ Antitoxin production

Spec wording: "If a pathogen enters the body the immune system tries to destroy the pathogen."

Exam Tip

For 6-mark "describe how the body defends against disease" questions, you typically need to mention both tiers — at least 2 non-specific defences AND at least 2 WBC mechanisms. Don't only write about one tier.

Non-Specific Defences — Skin & Nose

The four spec-named non-specific defence sites

Skin

A physical barrier that pathogens can't pass through. If skin is cut, a scab forms to seal the wound and prevent pathogen entry.

Nose

Hairs and mucus in the nose trap pathogens in the air you breathe — stopping them reaching the lungs.

Non-Specific Defences — Trachea/Bronchi & Stomach

If pathogens make it past the nose, two further defence sites stand in their way.

Trachea & Bronchi

The airways leading to the lungs are lined with ciliated cells and produce mucus.

Mechanism:

→ Mucus traps pathogens

→ Cilia waft mucus (with trapped pathogens) up to the throat

→ Mucus is then swallowed and destroyed by stomach acid

Stomach

The stomach produces hydrochloric acid (HCl).

Mechanism:

→ Acid creates a very low pH

→ Kills pathogens swallowed in food, drink, or mucus

This is a chemical defence — the body's strongest non-specific barrier against ingested pathogens.

The Chain

Notice the link between trachea and stomach: cilia push trapped pathogens UP to the throat → swallowed → stomach acid kills them. Examiners love to test this chain because it shows the body uses defences in combination, not in isolation.

The Immune System — Three WBC Mechanisms

If a pathogen gets past the non-specific defences and enters the body, the immune system steps in. Spec language: "If a pathogen enters the body the immune system tries to destroy the pathogen."

WBC #1

Phagocytosis

WBC engulfs and digests pathogens.

Non-specific — works against any pathogen.

WBC #2

Antibody Production

WBCs make antibodies — proteins that bind to specific antigens on a pathogen.

Specific — each antibody type targets one particular pathogen; causes pathogens to clump together / be destroyed.

WBC #3

Antitoxin Production

WBCs make antitoxins — proteins that neutralise toxins released by bacteria.

Stops the bacterial poisons from damaging cells.

Don't Mix Them Up

Antibodies attack the pathogen itself. Antitoxins attack the toxins the pathogen releases. They're different products doing different jobs — both are made by WBCs.

Phagocytosis — The Engulf-and-Destroy Method

The classic mechanism — a WBC literally swallows and digests the pathogen.

Phagocytosis — works against any pathogen (non-specific WBC response)

Mark-Scheme Phrasing

"White blood cells engulf the pathogen and digest it" — both verbs together earn the mark. Just writing "white blood cells eat germs" is too vague.

Antibody Production — The Specific Attack

Antibodies are proteins made by WBCs. They lock onto specific antigens on pathogens.

Antibodies are specific — each type only binds one type of antigen

How They Work

Each antibody has a binding site that's complementary in shape to one specific antigen — like a lock and key. Each antibody is therefore specific to a particular pathogen. Once bound, antibodies cause pathogens to clump together and be destroyed.

Antitoxin Production — Neutralising the Poisons

Bacteria don't just damage cells directly — they also release toxins (covered in Lesson 3.02). Antitoxins are the body's response to these toxins.

Antitoxin

A protein made by white blood cells that neutralises toxins released by bacteria — stopping the toxin from damaging cells.

The Problem

Bacteria like Salmonella secrete toxins that damage gut cells and cause symptoms (vomiting, diarrhoea). The bacteria themselves might be cleared — but the toxins keep doing harm.

The Solution

WBCs produce antitoxins that bind to and neutralise the toxins. This stops the toxins damaging cells while phagocytes / antibodies deal with the bacteria themselves.

Antitoxin vs Antibody

Both are proteins made by white blood cells, but they target different things:

→ Antibody = binds the pathogen (the cell itself, via its antigens)

→ Antitoxin = binds the toxin (the chemical poison the pathogen releases)

Mixing these up loses easy marks.

Worked Example — Steps 1 & 2

"Describe three ways in which white blood cells help to defend the body against pathogens." — Exam-style question · [3 marks]

Step 1 — Read the question

Command word: "Describe" — listing facts. Mark allocation: 3 marks, the question explicitly says "three ways". Topic: white blood cells.

Step 2 — Identify the pattern

"Three ways" + 3 marks = one mark for each spec-named mechanism:

→ Phagocytosis (1 mark)

→ Antibody production (1 mark)

→ Antitoxin production (1 mark)

Each mechanism needs to be named and briefly described.

Mark Allocation Tip

3 marks = 3 mechanisms. Don't describe phagocytosis in detail and forget the other two — examiners want all three named. Even one-line descriptions of each get full marks.

Worked Example — Steps 3, 4 & 5

Step 3 — Plan

One sentence per mechanism. Each sentence must include the named mechanism + a brief description of what it does.

✗ 1 mark

"White blood cells engulf and destroy pathogens by surrounding them with their membrane and digesting them with enzymes."

Only describes one mechanism (phagocytosis) in detail — misses 2 marks.

✓ 3 marks

"(1) White blood cells engulf and digest pathogens by phagocytosis. (2) They produce antibodies that bind to specific antigens on pathogens. (3) They produce antitoxins that neutralise toxins released by bacteria."

All three spec mechanisms named. Brief description for each.

Step 5 — Match to mark scheme

Mark scheme accepts: (1) "phagocytosis" / "engulf and digest pathogen", (2) "antibody production" / "antibodies destroy pathogens", (3) "antitoxin production" / "antitoxins neutralise toxins". ✓ 3 marks awarded.

Three Examiner Alerts

Alert 1 — Name the mechanism

For WBC questions, name the spec mechanism: "phagocytosis", "antibody production", "antitoxin production". Generic phrases like "WBCs fight germs" or "WBCs attack pathogens" don't score on "describe how" questions.

Alert 2 — Antibody ≠ Antitoxin

Mark schemes consistently distinguish these. Antibody targets the pathogen; antitoxin targets the toxin. Writing "antibody neutralises toxins" loses the mark.

Alert 3 — Trachea/Bronchi use BOTH cilia AND mucus

"Cilia" alone scores partial credit; "mucus" alone scores partial credit. The full mechanism is mucus traps pathogens, cilia waft mucus up. Pair them.

Common Errors — Wrong vs Right

✗ Wrong (loses marks)✓ Right (mark-scheme version)

"white blood cells fight germs""WBCs defend the body by phagocytosis · antibody production · antitoxin production "

"antibodies kill toxins"" antibodies bind pathogens ; antitoxins neutralise toxins "

"mucus protects the lungs"" mucus traps pathogens and cilia waft mucus up the trachea/bronchi"

"stomach kills germs"" hydrochloric acid in the stomach kills pathogens "

"skin is a barrier""skin is a physical barrier against pathogens; forms a scab if cut"

"phagocytes eat pathogens""WBCs engulf and digest pathogens by phagocytosis "

Recap — Three Takeaways

1

Four non-specific defences:skin (barrier · scabs) · nose (hairs + mucus trap) · trachea/bronchi (cilia waft mucus up) · stomach (HCl kills pathogens).

2

Three WBC mechanisms:phagocytosis (engulf + digest) · antibody production (bind to antigens on pathogens) · antitoxin production (neutralise bacterial toxins).

3

Name everything. Mark schemes for "how does the body defend itself" want specific named mechanisms, not "WBCs fight germs". One mark per named mechanism — easy points if you list them.

Exam Tip

Three things. You're set for any defence-system question on Paper 1. For 6-mark synoptic questions, link non-specific (tier 1) with immune system (tier 2) — show how they work together.

Summary

Key Points

Two tiers of defence: non-specific barriers (stop pathogens entering) + immune system (destroy pathogens that get in)

4 spec-named non-specific sites: skin · nose · trachea/bronchi · stomach

3 spec-named WBC mechanisms: phagocytosis · antibody production · antitoxin production

Antibody targets the pathogen ; antitoxin targets the toxin

Spec wording: " If a pathogen enters the body the immune system tries to destroy the pathogen "

Tier 1 — Quick Recall

Skin: barrier · scab if cut

Nose: hairs + mucus trap

Trachea/bronchi: cilia waft mucus up

Stomach: HCl kills pathogens

Tier 2 — Quick Recall

Phagocytosis: engulf + digest

Antibody production: bind antigens

Antitoxin production: neutralise toxins

All three by white blood cells.

Exam Tip

The biggest mark loss on this topic is generic language: "WBCs fight germs", "skin protects you". Every defence has a named mechanism — use it. Next: Lesson 3.05 — Vaccination, Antibiotics, and Antibiotic Resistance.