# The Immune Response

> How plants and animals defend themselves, from barriers and inflammation to B cells, T cells, antibodies and immune memory.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/immune-response/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Why you only get chickenpox once
Ravi caught chickenpox when he was six. For a week he was itchy, spotty and miserable. Years later his little cousin had chickenpox at a family party, and Ravi played with her all afternoon. He never got sick.

His body remembered. The first time, it took days to work out how to beat the virus. While doing that, it made **memory cells** that still circulate in his blood. When the virus turned up again, those cells recognised it at once and wiped it out before it could make him ill. This memory is also how vaccines work.

**In short:** The immune system defends the body against pathogens in three lines. The first line is barriers like skin and mucus, the second is non-specific responses like phagocytes, inflammation and fever, and the third is the specific response of B cells, T cells and antibodies, which leaves memory cells for immunity.

## Starting point: what you already know

- Pathogens include viruses, bacteria, fungi and protists (see the previous chapter).
- Blood contains white blood cells (see Transport in Animals).
- Cells have proteins on their membranes that act like ID tags (see The Cell Membrane and Transport).

## Antigens: how the body tells self from non-self

An **antigen** is a molecule (usually a protein) that the immune system recognises. Your own cells carry "self" markers called **MHC** proteins. Pathogens carry foreign antigens, so the body can tell they are not you.

## Three lines of defence

### Key idea: Three lines of defence
1. **First line**: barriers that stop pathogens getting in. **Innate** (you are born with it) and **non-specific** (works on any pathogen).
2. **Second line**: attacks pathogens that get in. Innate and non-specific.
3. **Third line**: targets one particular pathogen and remembers it. **Adaptive** and **specific**.

**First line.**

- **Skin**: a tough, dry barrier. Sweat and oils contain chemicals that kill microbes.
- **Mucous membranes** and **mucus** trap pathogens. **Cilia** sweep mucus out of the airways.
- **Chemicals**: stomach acid, and lysozyme (an enzyme) in tears and saliva.
- **Reflexes**: coughing, sneezing, vomiting.
- **Good bacteria** on the skin and in the gut outcompete pathogens.

**Second line.**

- **Phagocytes** (neutrophils and macrophages) engulf and digest pathogens by phagocytosis.
- **Inflammation**: damaged tissue releases **histamine**. Blood vessels widen and leak, so the area becomes red, warm, swollen and sore, and more white blood cells arrive.
- **Fever**: a higher body temperature slows many pathogens and speeds up immune cells.
- **Natural killer cells** destroy body cells infected with viruses.
- **Complement proteins** in the blood punch holes in bacteria.

**Third line (adaptive immunity).** Done by **lymphocytes**, white blood cells made in the bone marrow:

| | B cells (humoral immunity) | T cells (cell-mediated immunity) |
|---|---|---|
| Mature in | Bone marrow | Thymus |
| Main job | Make **antibodies** that float in blood and lymph | **Helper T cells** coordinate. **Cytotoxic (killer) T cells** destroy infected cells |
| Best against | Pathogens in body fluids (bacteria, viruses outside cells) | Pathogens hiding inside cells, and cancer cells |

## Antibodies

An **antibody** is a Y-shaped protein made by B cells (as **plasma cells**). The tips of the Y fit one specific antigen, like a lock and key.

*Diagram: A Y-shaped antibody whose two tips fit the matching antigens on the surface of a pathogen*
*Each tip of the antibody has a binding site that fits one antigen (red) like a key in a lock. With two tips, one antibody can join two pathogens and clump them together.*

Antibodies

- clump pathogens together (**agglutination**) so phagocytes can eat them,
- cover viruses so they cannot enter cells (**neutralisation**), and
- mark pathogens for destruction.

## Immune memory

After an infection, some B and T cells become long-lived **memory cells**.

- **First exposure** (primary response): slow, about 1 to 2 weeks, small antibody response, so you get sick.
- **Second exposure** (secondary response): fast, strong and long. Usually no symptoms. You are **immune**.

## Plant defences

Plants have no immune cells, but they still fight back.

- **Physical barriers**: waxy cuticle, bark, thick cell walls, thorns and hairs.
- **Chemical defences**: toxins, oils (like eucalyptus oil) and antimicrobial chemicals.
- **Responses to attack**: plants seal off infected areas, kill their own infected cells to stop the spread, close stomata, and send chemical alarm signals to the rest of the plant.

**Mind map**

```text
Immune response
  Antigens
    Self: MHC markers
    Non-self: foreign
  First line
    Skin and mucus
    Cilia, acid, lysozyme
  Second line
    Phagocytes
    Inflammation
    Fever
  Third line
    B cells: antibodies
    Helper T cells
    Killer T cells
    Memory cells
  Plants
    Cuticle, bark
    Toxic chemicals
    Kill infected cells
```

### Year 11: The adaptive response, step by step
1. A **macrophage** engulfs a pathogen and displays pieces of its antigen on its surface (it is an **antigen-presenting cell**).
2. A **helper T cell** with a matching receptor binds to it and is activated.
3. The helper T cell releases **cytokines** (chemical signals) that activate:
   - the **B cell** with matching receptors, which divides rapidly (**clonal selection** and expansion) into **plasma cells** that make antibodies and **memory B cells**, and
   - **cytotoxic T cells** that find and kill infected body cells.
4. When the infection is beaten, **suppressor (regulatory) T cells** calm the response down.
5. Memory cells remain for years.

HIV destroys **helper T cells**, so the whole adaptive response fails. That is why people with untreated AIDS die from infections that are usually harmless.

**MHC and T cells.** Almost all body cells show pieces of their own proteins on **MHC class I** markers. If a cell is infected, it shows pieces of the virus there instead, and **cytotoxic T cells** recognise and kill it. Phagocytes show antigens on **MHC class II**, which helper T cells recognise.

**Hiding from the immune system.** Devil facial tumour disease cells show very little MHC, so the new devil's immune system does not see them as foreign and does not reject them.

### Year 12: Active and passive immunity
| | Active immunity | Passive immunity |
|---|---|---|
| How | Your own body makes antibodies and memory cells | You receive ready-made antibodies |
| Natural example | Getting sick and recovering | Antibodies passing from mother to baby across the placenta and in breast milk |
| Artificial example | **Vaccination** | Antivenom for snake bites, antibody injections |
| Speed | Slow to start | Immediate |
| How long | Long term (memory cells) | Short term (weeks), no memory |

Australian antivenoms for snakes, spiders and box jellyfish are made by injecting small doses of venom into horses or sheep and collecting the antibodies they make.

**Why increasing doses?** A small first dose of venom will not kill the horse. Each larger dose triggers a stronger secondary response, so the horse makes more antibodies.

**Catching flu again.** Flu viruses mutate, so this year's strain has different antigens. Last year's memory cells do not recognise it.

**Transplants.** Immunosuppressant drugs stop T cells attacking the donor organ, whose MHC markers are foreign.

## Quick summary

- Antigens are molecules the immune system recognises as self or non-self.
- First line: barriers like skin, mucus and acid. Second line: phagocytes, inflammation and fever. Both are innate and non-specific.
- Third line: B cells make specific antibodies. T cells coordinate and kill infected cells.
- Memory cells give a faster, stronger secondary response: immunity.
- Plants defend themselves with barriers, chemicals and by killing infected cells.

## Quiz

Try these questions from the chapter quiz.

1. A molecule that the immune system recognises as foreign is an
   - A. antibody
   - B. antigen
   - C. enzyme
   - D. hormone
   - Answer: B. antigen. Antigens trigger the immune response.

2. Which is part of the first line of defence?
   - A. Skin
   - B. Antibodies
   - C. Killer T cells
   - D. Memory cells
   - Answer: A. Skin. Skin is a physical barrier.

3. Phagocytes destroy pathogens by
   - A. making antibodies
   - B. engulfing and digesting them
   - C. releasing histamine only
   - D. copying their DNA
   - Answer: B. engulfing and digesting them. Phagocytosis means "cell eating".

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/immune-response/#quiz