Health, disease and the immune system

What you'll learn

This topic covers the fundamental concepts of health and disease that you need for AQA GCSE Biology. You'll explore how different types of diseases affect the body, how pathogens cause infection, and the sophisticated defence mechanisms your immune system uses to protect you. Understanding these concepts is essential for both Paper 1 and Paper 2, particularly for 4-6 mark extended response questions.

Key terms and definitions

Health — a state of physical and mental well-being, not simply the absence of disease

Communicable disease — an infectious disease caused by a pathogen that can be transmitted from one organism to another

Non-communicable disease — a disease that cannot be spread between individuals and is typically long-lasting with slow progression

Pathogen — a microorganism that causes disease, including bacteria, viruses, fungi and protists

Phagocytosis — the process by which white blood cells engulf and digest pathogens

Antibody — a protein produced by lymphocytes that binds to specific antigens on pathogens

Antigen — a unique protein marker on the surface of a pathogen that triggers an immune response

Vaccination — the administration of antigens from a weakened or dead pathogen to stimulate immunity without causing disease

Core concepts

Defining health and disease

Health is more than just the absence of disease. The World Health Organization defines it as complete physical, mental and social well-being. Several factors affect health:

• Diet and exercise levels
• Life situations such as access to clean water and healthcare
• The number and type of diseases present
• Genetic inheritance

Diseases interact with each other and can affect health in different ways. For example, immune system diseases such as HIV/AIDS make individuals more susceptible to infectious diseases like tuberculosis. Mental health conditions can trigger physical symptoms, whilst chronic physical diseases often lead to depression or anxiety.

Communicable diseases and pathogens

Communicable diseases are caused by four main types of pathogen:

Bacteria are single-celled prokaryotic organisms (0.2-2 μm). They reproduce rapidly inside the body and produce toxins that damage tissues. Examples include:

• Salmonella causing food poisoning
• Gonorrhoea causing a sexually transmitted infection

Viruses are much smaller than bacteria (0.02-0.3 μm) and are not considered living organisms. They cannot reproduce independently; instead they invade host cells and use the cell's machinery to produce copies of themselves, often destroying the cell. Examples include:

• Measles virus causing fever and skin rash
• HIV (Human Immunodeficiency Virus) attacking immune system cells
• Tobacco Mosaic Virus (TMV) causing discolouration in plant leaves

Protists are single-celled eukaryotic organisms. Many are parasites requiring a vector (carrier organism) for transmission. Example:

• Plasmodium causing malaria, transmitted by mosquitoes

Fungi can be single-celled (yeast) or have a mycelium structure made of hyphae. Example:

• Rose black spot affecting plant leaves

How pathogens spread

Understanding transmission routes is crucial for disease prevention:

Direct contact:

• Skin-to-skin contact (athlete's foot fungal infection)
• Bodily fluids (HIV, gonorrhoea)
• Droplet infection from coughs and sneezes (influenza, measles)

Indirect contact:

• Contaminated water (cholera bacteria)
• Contaminated food (Salmonella bacteria)
• Vectors such as mosquitoes (malaria)
• Airborne particles (tuberculosis)

The human defence system

Your body has multiple defence layers to prevent pathogen entry and destroy those that breach these barriers.

Non-specific defences (first and second line):

The skin forms a physical barrier. It produces antimicrobial secretions and has a community of harmless microorganisms that outcompete pathogens. If cut, blood clotting seals the wound rapidly.

The respiratory system has several defences:

• Nose hairs trap particles
• Mucus in airways traps pathogens
• Cilia (tiny hair-like structures) waft mucus to the throat where it's swallowed
• Stomach acid destroys most swallowed pathogens

Phagocytosis occurs when white blood cells (phagocytes) detect chemicals released by pathogens:

1. The phagocyte moves towards the pathogen
2. It engulfs the pathogen, enclosing it in a vesicle
3. Enzymes inside the phagocyte digest and destroy the pathogen

Specific immune response (third line):

When a pathogen enters the body, lymphocytes (a type of white blood cell) produce specific responses:

Antibody production:

1. Lymphocytes detect antigens on the pathogen surface
2. Specific lymphocytes with complementary antibodies reproduce rapidly
3. These lymphocytes secrete large quantities of antibodies
4. Antibodies bind to antigens, causing pathogens to clump together
5. This marks pathogens for destruction by phagocytes or neutralises toxins
6. Some lymphocytes remain as memory cells

Antitoxin production:

• Certain lymphocytes produce antitoxins that neutralise toxins released by bacteria

Immunity and vaccination

After fighting an infection, some lymphocytes remain as memory cells. If the same pathogen enters again, memory cells rapidly produce antibodies, destroying the pathogen before symptoms develop. This is immunity.

Vaccination exploits this mechanism:

• Small quantities of dead or weakened pathogens are injected
• These carry antigens but cannot cause disease
• The immune system produces antibodies and memory cells
• If the live pathogen later infects the body, memory cells trigger rapid antibody production
• The pathogen is destroyed before illness develops

Vaccination programmes have successfully controlled many diseases:

• Measles, mumps and rubella (MMR vaccine) in the UK
• Polio has been eradicated in many countries
• COVID-19 vaccines reduced severe illness globally

Herd immunity occurs when a high percentage of a population is vaccinated. This protects vulnerable individuals who cannot be vaccinated (newborns, immunocompromised people) because the pathogen cannot spread easily.

Non-communicable diseases

These diseases cannot spread between individuals and include:

Cardiovascular diseases:

• Coronary heart disease caused by fatty deposits (atheroma) in coronary arteries
• Risk factors include high-fat diet, lack of exercise, smoking, genetic predisposition

Cancer:

• Uncontrolled cell division forming tumours
• Benign tumours remain in one place; malignant tumours invade tissues (cancer)
• Risk factors include smoking, UV exposure, viral infections, genetic mutations

Type 2 diabetes:

• The body no longer responds properly to insulin or doesn't produce enough
• Linked to obesity and diet high in simple sugars
• More prevalent in Caribbean and South Asian populations

Lung diseases:

• Chronic obstructive pulmonary disease (COPD) from smoking
• Asthma triggered by allergens or pollution

Risk factors for disease

A risk factor increases the likelihood of developing a disease but doesn't guarantee it will occur. Risk factors can be:

Lifestyle factors:

• Smoking (lung cancer, COPD, cardiovascular disease)
• Excessive alcohol consumption (liver disease, brain damage)
• Poor diet (obesity, Type 2 diabetes, some cancers)
• Lack of exercise (cardiovascular disease, obesity)

Environmental factors:

• Air pollution (asthma, lung disease)
• UV radiation exposure (skin cancer)
• Ionising radiation (various cancers)

Genetic factors:

• Inherited mutations increasing cancer risk (BRCA genes for breast cancer)
• Sickle cell anaemia in populations from malarial regions

Many diseases have multiple interacting risk factors. For example, cardiovascular disease links to diet, exercise, smoking, stress and genetics. This makes it difficult to prove causation rather than correlation.

Worked examples

Question 1: A student investigates the effect of antibiotics on bacterial growth. They place paper discs soaked in different antibiotics on agar plates covered with bacteria. After 24 hours, they measure the clear zones around each disc.

Explain why clear zones form around the antibiotic discs. (3 marks)

Mark scheme answer:

• The antibiotic kills the bacteria (1 mark)
• This prevents bacterial reproduction/growth in that area (1 mark)
• Creating a clear zone where no bacteria are present (1 mark)

Question 2: Describe how vaccination prevents illness. (4 marks)

Mark scheme answer:

• Dead or inactive pathogens are injected containing antigens (1 mark)
• White blood cells/lymphocytes produce antibodies specific to the antigen (1 mark)
• Memory cells remain in the blood (1 mark)
• If the live pathogen enters the body later, memory cells rapidly produce antibodies that destroy the pathogen before symptoms develop (1 mark)

Question 3: The graph shows the number of measles cases in the UK from 1950 to 2010. The MMR vaccine was introduced in 1988. In 1998, a discredited study suggested the vaccine caused autism, leading some parents to refuse vaccination.

Analyse the data and explain the trends shown. (6 marks)

Mark scheme answer:

• Cases were high before 1968 (approximately 400,000+ per year) (1 mark)
• Cases decreased after vaccination programmes began, showing vaccination effectiveness (1 mark)
• Cases dropped to very low levels by 1990s (under 10,000 per year) due to widespread vaccination (1 mark)
• Cases increased after 1998 when vaccination rates fell (1 mark)
• This occurred because herd immunity was compromised when fewer children were vaccinated (1 mark)
• The unvaccinated population allowed the virus to spread, affecting both unvaccinated children and those too young to be vaccinated (1 mark)

Common mistakes and how to avoid them

• Confusing bacteria with viruses — Remember that bacteria are cells that reproduce independently and produce toxins, whilst viruses invade host cells and reproduce inside them. Antibiotics only kill bacteria, never viruses.

• Stating that antibodies kill pathogens directly — Antibodies bind to antigens, marking pathogens for destruction or causing them to clump together. Phagocytes or other mechanisms actually destroy the pathogens.

• Thinking vaccination gives you the disease — Vaccines contain dead or weakened pathogens that cannot cause disease but still carry antigens to trigger antibody production.

• Confusing correlation with causation in risk factors — Just because two factors occur together doesn't mean one causes the other. Multiple risk factors often interact, making it difficult to identify single causes.

• Forgetting that health is more than physical — The WHO definition includes mental and social well-being, not just absence of disease.

• Mixing up memory cells and antibodies — Memory cells remain in the body long-term and produce antibodies when needed. Antibodies are proteins that bind to specific antigens.

Exam technique for "Health, disease and the immune system"

• Command word precision: "Describe" requires you to state features or processes without explanation. "Explain" requires reasons or mechanisms (use "because," "this causes," "leading to"). "Suggest" means apply knowledge to unfamiliar contexts.

• Extended response questions (4-6 marks): Structure answers logically, often chronologically. Include 4-6 distinct points. For "explain" questions, ensure each statement includes cause and effect. Use scientific terminology precisely.

• Data analysis questions: Quote figures from graphs or tables. Describe trends (increases, decreases, plateaus). Explain trends using biological knowledge. Compare different data sets when asked.

• Calculation and method questions: Show working clearly. Include units. For investigating antibiotics or antiseptics, mention controlling variables (temperature, volume of bacteria, time) and measuring clear zones.

Quick revision summary

Health is physical, mental and social well-being. Communicable diseases spread via pathogens (bacteria, viruses, protists, fungi) through direct contact, water, food or vectors. The body defends itself through skin barriers, stomach acid, cilia-mucus systems and phagocytosis. Lymphocytes produce specific antibodies against antigens, creating immunity via memory cells. Vaccination uses dead pathogens to trigger immunity without disease. Non-communicable diseases like cardiovascular disease, cancer and diabetes have risk factors including lifestyle, environment and genetics. Understanding disease mechanisms and prevention strategies is essential for individual and public health.