Year 9 Science Unit 1 · Disease Lesson 10 of 20 45 min SC5-DIS-10

When Immunity Fails

The immune system is extraordinarily powerful — but sometimes it makes mistakes. It can overreact to harmless substances, attack the body's own cells, or be too weak to fight infections. Understanding when immunity fails helps us develop better treatments and show compassion for those affected.

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Think First

Before You Begin

Think about someone you know who has hay fever, asthma, or a food allergy.

Write down your answers before reading on:

  • What happens when they are exposed to the thing they are allergic to?
  • Why do you think their body reacts this way?
  • How is this different from a normal immune response to a pathogen?
Write your thinking in your book before reading on.

Work mode: Digital — answers typed below

Learning Intentions

Know

  • What allergies are and how they occur
  • What autoimmune diseases are
  • What immunodeficiency is and how HIV/AIDS affects the immune system

Understand

  • Why the immune system sometimes overreacts or attacks the body
  • How immunodeficiency makes people vulnerable to infections
  • That these conditions are medical issues, not personal choices or character flaws

Can Do

  • Explain the difference between allergy, autoimmunity, and immunodeficiency
  • Describe how HIV affects the immune system
  • Discuss immune system disorders with factual accuracy and empathy
Key Terms
Allergy An overreaction of the immune system to a harmless substance (allergen), causing symptoms like sneezing, itching, or swelling.
Autoimmune disease A condition in which the immune system mistakenly attacks the body's own cells and tissues.
Immunodeficiency A state in which the immune system's ability to fight infection is compromised or absent.
HIV Human Immunodeficiency Virus — a virus that infects and destroys helper T cells, progressively weakening the immune system.
AIDS Acquired Immunodeficiency Syndrome — the most advanced stage of HIV infection, characterised by severe immune suppression and opportunistic infections.
Anaphylaxis A severe, potentially life-threatening allergic reaction that requires immediate medical treatment.
Core Content
1

Allergies

When the immune system overreacts

Vaccination

Vaccination

An allergy occurs when the immune system mistakes a harmless substance for a dangerous pathogen and mounts an excessive response. The harmless substance is called an allergen.

Common allergens include:

  • Pollen (hay fever)
  • Dust mites
  • Pet dander
  • Foods (peanuts, shellfish, eggs, milk)
  • Insect stings
  • Latex
  • Certain medications

When exposed to an allergen, the immune system produces IgE antibodies that trigger mast cells to release histamine. Histamine causes the familiar allergy symptoms: sneezing, itching, runny nose, watery eyes, and swelling.

In severe cases, allergies can cause anaphylaxis — a life-threatening reaction where airways narrow and blood pressure drops. Anaphylaxis requires immediate treatment with an adrenaline auto-injector (EpiPen).

2

Autoimmune Diseases

When the body attacks itself

In autoimmune diseases, the immune system loses the ability to distinguish between "self" and "non-self" and attacks the body's own tissues. The exact cause is not fully understood, but genetic and environmental factors play a role.

Examples of autoimmune diseases include:

  • Type 1 diabetes: Immune system destroys insulin-producing cells in the pancreas
  • Rheumatoid arthritis: Immune system attacks joints, causing inflammation and pain
  • Multiple sclerosis (MS): Immune system damages the protective covering of nerve fibres
  • Lupus: Immune system attacks multiple organs including skin, joints, and kidneys
  • Coeliac disease: Immune system reacts to gluten, damaging the small intestine

Autoimmune diseases cannot be cured, but they can be managed with medications that suppress the immune system. Australia has high rates of some autoimmune diseases, including MS and type 1 diabetes.

3

Immunodeficiency

When defences are too weak

Immunodeficiency occurs when the immune system is weakened and cannot effectively fight infections. It can be:

Primary (inherited): Genetic conditions present from birth that affect immune function. Examples include severe combined immunodeficiency (SCID), where children have virtually no immune protection.

Secondary (acquired): Caused by external factors such as:

  • Malnutrition
  • Certain medications (e.g., chemotherapy, immunosuppressants)
  • Chronic diseases (e.g., diabetes, kidney disease)
  • Infections that damage the immune system

People with immunodeficiency are vulnerable to infections that healthy people fight off easily. They may need protective isolation, prophylactic antibiotics, or immunoglobulin replacement therapy.

4

HIV and AIDS

A virus that destroys immunity

HIV (Human Immunodeficiency Virus) is a virus that specifically infects and destroys helper T cells — the coordinators of the immune response. Without helper T cells, the entire immune system gradually collapses.

HIV is transmitted through:

  • Unprotected sexual contact
  • Sharing needles
  • From mother to child during pregnancy, birth, or breastfeeding
  • Blood transfusions (extremely rare in countries with screening)

HIV is not transmitted through casual contact, hugging, sharing food, or insect bites.

When HIV has destroyed enough T cells, the person develops AIDS (Acquired Immunodeficiency Syndrome). At this stage, the immune system is so weak that opportunistic infections (infections that take advantage of weak immunity) become life-threatening.

Antiretroviral therapy (ART) can suppress HIV replication, allowing T cell counts to recover. People on effective ART can live long, healthy lives and cannot transmit HIV sexually. However, there is still no cure or vaccine.

Common Misconceptions

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"Allergies are not serious — they are just sneezing and itching." No — while many allergies are mild, some can be life-threatening. Anaphylaxis can kill within minutes without treatment. Allergies should always be taken seriously.

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"Autoimmune diseases are caused by poor lifestyle choices." No — autoimmune diseases result from complex interactions between genetics and environment. They are not caused by personal choices and cannot be prevented by healthy living alone.

✗trong>"You can catch HIV from hugging or sharing food." No — HIV is not transmitted through casual contact, saliva, tears, sweat, or insect bites. It requires specific routes: sexual contact, blood exposure, or mother-to-child transmission.

Australian Context

HIV in Australia

Australia's HIV response: Australia has been a world leader in HIV prevention and care. Since the 1980s, partnerships between government, healthcare providers, and affected communities have kept HIV rates relatively low. Needle exchange programs, accessible testing, and antiretroviral therapy have all contributed.

Pre-exposure prophylaxis (PrEP): Australia was one of the first countries to subsidise PrEP — a medication that prevents HIV infection. Since becoming widely available in 2018, new HIV infections in Australia have declined significantly among gay and bisexual men, the group most affected.

U=U (Undetectable = Untransmittable): People living with HIV who take antiretroviral therapy and maintain an undetectable viral load cannot sexually transmit HIV. This scientific fact, supported by Australian research, has transformed lives and reduced stigma. However, stigma and discrimination against people living with HIV remain significant issues in Australia.

✍ Copy Into Your Books

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Allergies

  • Immune system overreacts to harmless substance
  • IgE antibodies trigger histamine release
  • Can range from mild to life-threatening (anaphylaxis)

Autoimmune Diseases

  • Immune system attacks body's own cells
  • Examples: type 1 diabetes, MS, lupus, rheumatoid arthritis
  • Managed but not cured

Immunodeficiency

  • Immune system too weak to fight infections
  • Primary = genetic; Secondary = acquired
  • HIV destroys helper T cells, leading to AIDS
Activities
Activity 1

Compare Conditions

Distinguish between immune system disorders.

1 Complete a table comparing allergies, autoimmune diseases, and immunodeficiency in terms of cause, what goes wrong, and an example.
Answer in your book.
2 Explain why people with AIDS are vulnerable to opportunistic infections that rarely harm healthy people.
Answer in your book.
3 Describe one way that modern medicine helps people with each type of immune disorder: allergies, autoimmune disease, and HIV.
Answer in your book.
Activity 2

Stigma and Science

Discuss HIV/AIDS with factual accuracy and empathy.

1 Explain why HIV cannot be transmitted through casual contact like hugging or sharing food. Use your knowledge of how HIV spreads.
Answer in your book.
2 The U=U campaign states that people with an undetectable viral load cannot transmit HIV sexually. Explain the science behind this statement.
Answer in your book.
3 Why is it important to address stigma against people living with HIV, even when effective treatments exist?
Answer in your book.
Multiple Choice
Q

Test Your Understanding

RememberBand 3

1. What happens in an allergic reaction?

AThe immune system fails to respond
BThe immune system overreacts to a harmless substance
CThe immune system attacks the body's own cells
DThe immune system is destroyed by a virus
RememberBand 3

2. In an autoimmune disease, the immune system:

AIs too weak to fight infections
BOverreacts to allergens
CMistakenly attacks the body's own cells
DIs destroyed by bacteria
RememberBand 3

3. HIV specifically destroys which type of immune cell?

AB cells
BNeutrophils
CHelper T cells
DMacrophages
RememberBand 3

4. Which of the following is NOT a way HIV is transmitted?

AUnprotected sexual contact
BSharing needles
CHugging an infected person
DMother to child during birth
UnderstandBand 4

5. People on effective antiretroviral therapy for HIV:

AAre cured of HIV
BCannot transmit HIV sexually and can live long healthy lives
CNo longer need medication
DHave no T cells
Short Answer
✍

Short Answer Questions

UnderstandBand 4

1. Distinguish between allergy, autoimmune disease, and immunodeficiency. For each, describe what goes wrong in the immune system and give one example. 4 MARKS

Answer in your book.
UnderstandBand 4

2. Explain how HIV infection leads to AIDS. Include the role of helper T cells and opportunistic infections in your answer. 4 MARKS

Answer in your book.
EvaluateBand 5

3. Evaluate the statement: "Because there is no cure for HIV, people living with HIV cannot live normal lives." Use evidence about modern treatment. 4 MARKS

Answer in your book.

Revisit Your Thinking

Go back to your Think First answer. Has your understanding changed?

Update your thinking in your book.

Answers

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MCQ 1

B — In an allergic reaction, the immune system mistakenly identifies a harmless substance (allergen) as dangerous and mounts an excessive response.

MCQ 2

C — In autoimmune diseases, the immune system loses self-tolerance and attacks the body's own cells and tissues.

MCQ 3

C — HIV specifically infects and destroys helper T cells, which coordinate the immune response. Without helper T cells, the entire immune system collapses.

MCQ 4

C — HIV is not transmitted through casual contact like hugging, sharing food, or insect bites. It requires specific routes: sexual contact, blood exposure, or mother-to-child transmission.

MCQ 5

B — People on effective antiretroviral therapy (ART) can suppress HIV to undetectable levels, cannot transmit HIV sexually (U=U), and can live long, healthy lives. However, ART is not a cure and must be taken lifelong.

Short Answer 1

Model answer: Allergy: The immune system overreacts to a harmless substance (allergen), producing IgE antibodies that trigger histamine release. Example: hay fever (pollen allergy) causes sneezing and itchy eyes. Autoimmune disease: The immune system mistakenly attacks the body's own cells, unable to distinguish "self" from "non-self." Example: type 1 diabetes, where immune cells destroy insulin-producing beta cells in the pancreas. Immunodeficiency: The immune system is too weak to effectively fight infections. Example: HIV/AIDS, where the virus destroys helper T cells, progressively weakening immunity until opportunistic infections become life-threatening.

Short Answer 2

Model answer: HIV infects and destroys helper T cells, which are essential for coordinating the immune response. Helper T cells activate B cells to produce antibodies, activate cytotoxic T cells to kill infected cells, and release chemical signals that recruit other immune cells. As HIV gradually destroys helper T cells over months to years, the immune system loses its ability to coordinate effective responses. Without sufficient T cells, the body cannot fight infections that healthy people control easily. When T cell counts fall below a critical level (typically 200 cells per microlitre), the person develops AIDS. At this stage, opportunistic infections such as pneumonia, tuberculosis, and certain cancers take hold because the immune system is too weak to defend against them.

Short Answer 3

Model answer: This statement is incorrect. While there is currently no cure for HIV, modern antiretroviral therapy (ART) has transformed HIV from a fatal diagnosis to a manageable chronic condition. People on effective ART can suppress the virus to undetectable levels, live normal lifespans, have healthy children, and engage in relationships without transmitting HIV (U=U — Undetectable = Untransmittable). Australian research has contributed to this understanding. However, the statement contains a grain of truth in that people living with HIV must take medication daily, face potential side effects, and may experience stigma and discrimination. While medical treatment enables normal lives, social barriers remain. The key message is that with treatment, HIV is no longer the death sentence it once was, and people living with HIV can and do live full, productive lives.

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Lesson Game

Immune Balance

Restore immune system balance! Calm overactive responses, strengthen weak defences, and learn how treatments help in this biology puzzle game.

Mark lesson as complete

Tick when you have finished all activities and checked your answers.

Unit 1 overview · Science subject page