S6 Biology | Immunity prep guide (for tutors) | three lines of defence, antibodies, vaccines

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can say that white blood cells eat bacteria but cannot separate B cells from T cells. This lesson lines the chapter up as three verbs: recognise, attack, remember.

Objectives — Students can: (1) describe what each of the three lines of defence does and give an example of failure; (2) distinguish B cells, which make antibodies, from killer T cells, which destroy infected cells; (3) use primary and secondary responses to explain vaccination, and compute herd immunity thresholds and efficacy.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Cell structure	membranes and antigens	recognise something foreign
Proteins	antibodies are proteins	denatured by heat
Blood composition	plasma and blood cells	antibodies travel in plasma

02 Lesson flow (50 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three lines of defence	skin, phagocytes, lymphocytes	disease needs a breach
8-20 min	B cells and T cells	T kills infected cells, B makes antibodies	the division of labour
20-32 min	Primary and secondary	titre 1000 to 100000	100-fold difference
32-42 min	Vaccines and herd immunity	R0 = 3	threshold 66.7 per cent
42-48 min	Computing efficacy	20 per cent to 4 per cent	80 per cent
48-50 min	Exit check	three questions collected	independent work

03 Teaching script (read it out)

Scene 1: Three lines: name the layer

The first line is physical and chemical: keratinised skin, mucous membranes, stomach acid, lysozyme in tears. Its job is to keep pathogens out. The second line is innate: phagocytes such as neutrophils and macrophages engulf pathogens, and inflammation widens vessels and raises local temperature, which is why the area becomes red, hot, swollen and painful.
The third line is adaptive: it targets specific antigens and it remembers. Give students one test question: does this response remember a particular pathogen? Yes means line three, no means line two.
In answers, always name the line and the cell type. Writing that white blood cells fight germs earns nothing.

Student task: sort six situations (broken skin, stomach acid, inflammation, antibodies, memory cells, fever) into the three lines.

Scene 2: B and T: one builds the weapon, one demolishes the factory

A B cell activated by antigen, with help from a helper T cell, divides into plasma cells that secrete antibodies. Antibodies bind antigen so pathogens clump, are neutralised, or become easier to phagocytose. Some B cells become memory B cells, which respond fast and hard on the next exposure.
The primary response peaks after about 12 days at a titre near 1:1000; the secondary response peaks in 3 days at up to 1:100000, a 100-fold difference that arrives 9 days sooner. T cells make no antibodies: a killer T cell recognises viral antigen displayed on the surface of an infected cell and triggers its death, because antibodies cannot reach a virus once it is inside.
This division of labour is where long questions award marks: antibodies handle extracellular pathogens, T cells handle the intracellular stage.

Student task: in two sentences explain why antibodies cannot deal with a virus inside a cell, mentioning antigen display.

Scene 3: Vaccines: manufacturing memory in advance

A vaccine delivers harmless or weakened antigen, such as killed bacteria, an attenuated strain, a subunit protein or an mRNA instruction. It triggers a primary response and leaves memory cells without causing disease, so a real infection meets a secondary response.
The herd immunity threshold is 1 – 1/R0. With R0 = 3 it is 1 – 1/3 = 66.7 per cent: once that share is immune, each case infects fewer than one other person and chains break.
Efficacy is (attack rate in the unvaccinated minus attack rate in the vaccinated) divided by the unvaccinated rate. With 20 in 100 and 4 in 100 that is (0.20 – 0.04) / 0.20 = 80 per cent. Stress that 80 per cent efficacy does not mean 80 per cent of recipients are immune; it means their risk of disease falls by 80 per cent.

Student task: recompute the threshold for R0 = 5 (answer 80 per cent) and explain why a higher R0 is harder to beat.

04 Question types and worked steps

Type	Example	Steps	Answer
Classify a defence	six situations	specificity and memory	line two or line three
Compare cell types	B cell versus killer T	antibodies versus killing	complementary roles
Explain the secondary response	antibody titre curve	12 days versus 3 days	100-fold
Compute herd immunity	R0 = 3	1 – 1/R0	66.7 per cent

Sanity check (teach this): The titre rises from 1:1000 to 1:100000, which is 100 times; an answer of 10 times means a power of ten was miscounted. The threshold 66.7 per cent must lie between 0 and 100, and a larger R0 approaches but never passes 100. Efficacy 80 per cent must also lie between 0 and 100; a negative value means the vaccinated group did worse, so check the denominators.

05 Independent practice (3 questions)

Question	Answer	What to watch
R0 = 4: herd immunity threshold?	75 per cent	use 1 – 1/R0
Unvaccinated attack rate 10 per cent, vaccinated 2 per cent: efficacy?	80 per cent	denominator is the unvaccinated rate
Which cell clears a virus once it is inside a cell?	killer T cell	antibodies cannot reach inside

06 Common mistakes and what to say

Mistake	Student writes	Say this
Antibodies kill pathogens	writes that antibodies destroy them directly	“Antibodies tag and neutralise; phagocytes do the eating.”
T cells making antibodies	confuses plasma cells with T cells	“Plasma cells make antibodies; T cells kill infected cells.”
Vaccines contain antibodies	writes that a vaccine supplies antibodies	“A vaccine supplies antigen so you make your own antibodies and memory cells.”
Herd immunity as 100 per cent	says everyone must be immune	“The threshold is 1 – 1/R0, not 100 per cent.”

07 Homework (paper, section and question numbers)

Secondary biology chapter index (cells, enzymes, genetics, plant transport, ecology) — immunity before ecology
Secondary prep guide hub — all subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
How many days sooner is the secondary response?	9 days	wrong: redo act two
R0 = 3: herd immunity threshold?	66.7 per cent	wrong: redo the formula in act three
Why can antibodies not clear an intracellular virus?	they act outside; killer T cells are needed	two wrong: back to act one

09 Tutor FAQ

Which year is this for?

S6; it covers the HKDSE immunity topic and the disease and defence topic.

How many cell types must be recalled?

Five: phagocyte, helper T, killer T, B cell and plasma cell, plus memory cells.

Is there practice attached?

The homework list links to the biology chapter index, pairing with genetics and ecology.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.