Diseases and Immunity
Summary: Pathogens (bacteria, viruses, fungi, protists) cause infectious diseases and are transmitted through direct contact, airborne droplets, contaminated food/water, and vectors. The body has physical barriers (skin, mucus/cilia, stomach acid) and an immune system involving phagocytosis (non-specific) and antibody production by lymphocytes (specific). Memory cells provide long-term immunity. Vaccination introduces dead or weakened pathogens to trigger antibody and memory cell production. Herd immunity protects unvaccinated individuals when a high proportion of the population is immunised. Tags: igcse biology diseases immunity pathogens vaccination antibodies Created: 2026-07-16 Last Updated: 2026-07-16
1. Pathogens and Types
A pathogen is a microorganism that causes disease. There are four main types:
| Pathogen Type | Description | Examples of Diseases |
|---|---|---|
| Bacteria | Single-celled prokaryotic organisms. Reproduce rapidly by binary fission. Some produce toxins that damage host cells | Cholera (contaminated water), tuberculosis (airborne), food poisoning (Salmonella), tetanus |
| Viruses | Much smaller than bacteria. Not cells — a protein coat surrounding genetic material (DNA or RNA). Hijack host cells to replicate, destroying cells in the process | Influenza (flu), HIV/AIDS, common cold, measles, COVID-19, tobacco mosaic virus (plants) |
| Fungi | Eukaryotic organisms, unicellular (yeast) or multicellular (moulds). Some are parasitic or produce toxins | Athlete’s foot, ringworm, thrush (Candida) |
| Protists | Single-celled eukaryotic organisms. Some are parasitic | Malaria (caused by Plasmodium, transmitted by female Anopheles mosquitoes as a vector) |
2. Transmission of Disease
| Transmission Route | Description | Examples |
|---|---|---|
| Direct contact | Touching an infected person or their body fluids | HIV (sexual contact, blood), skin infections, ringworm |
| Airborne droplets | Tiny droplets containing pathogens released when an infected person coughs or sneezes, inhaled by others | Influenza, tuberculosis, common cold, COVID-19 |
| Contaminated food and water | Ingesting pathogens through food or water contaminated with faeces or untreated sewage | Cholera, Salmonella (food poisoning), typhoid |
| Vectors | An organism (usually an arthropod) that carries a pathogen from one host to another without getting the disease itself | Malaria: Female Anopheles mosquito bites infected person → picks up Plasmodium protist → bites healthy person → transmits pathogen |
Malaria transmission in detail:
- Female Anopheles mosquito (vector) bites an infected human, sucking up blood containing Plasmodium gametes
- Plasmodium undergoes sexual reproduction inside the mosquito
- The mosquito bites a healthy human, injecting Plasmodium sporozoites into the blood
- Plasmodium invades liver cells, multiplies, then infects red blood cells, causing them to burst → fever, chills, anaemia
Malaria control: mosquito nets, insecticides, draining stagnant water (breeding sites), prophylactic (preventative) drugs.
3. Body Defences Against Infection
Mechanical and Chemical Barriers (First Line of Defence)
| Defence | How It Works |
|---|---|
| Skin | Physical barrier — outer layer of dead, keratinised cells forms a tough, impermeable barrier. Sebum (oil) on skin contains antimicrobial substances |
| Mucus and cilia | Goblet cells in the trachea/bronchi secrete sticky mucus that traps pathogens and particles. Cilia (tiny hair-like projections) on epithelial cells sweep the mucus upward to the throat where it is swallowed or coughed out |
| Stomach acid | The stomach produces hydrochloric acid (HCl, pH ~2), which kills most ingested bacteria and pathogens before they reach the intestines |
| Blood clotting | When a blood vessel is damaged, platelets trigger a cascade: fibrinogen (soluble plasma protein) → fibrin (insoluble mesh). This traps red blood cells, forming a clot that seals the wound, preventing pathogen entry and further blood loss |
| Tears | Contain lysozyme, an enzyme that breaks down bacterial cell walls |
The Immune System (Second Line of Defence)
Phagocytosis (non-specific response):
- Phagocytes (a type of white blood cell) detect pathogens or foreign particles
- The phagocyte engulfs the pathogen, enclosing it in a vesicle (phagosome)
- Enzymes from lysosomes digest and destroy the pathogen
- Phagocytosis is non-specific — phagocytes attack any foreign organism, they do not distinguish between different pathogens
Antibody production by lymphocytes (specific response):
- Each pathogen has unique molecules on its surface called antigens (usually proteins)
- Lymphocytes (B-lymphocytes) have receptors that recognise specific antigens
- When a lymphocyte binds to its matching antigen, it is activated and divides rapidly (clonal expansion)
- The activated lymphocytes produce large quantities of antibodies — Y-shaped proteins that are specific to that antigen
- Antibodies bind to antigens on the pathogen, either neutralising the pathogen directly or marking it for destruction by phagocytes
- Some lymphocytes become memory cells — they remain in the blood for years (or decades), providing long-term immunity. If the same pathogen re-enters the body, memory cells recognise the antigen immediately and produce antibodies very rapidly → the secondary response is much faster and stronger than the primary response
Primary vs Secondary Immune Response:
| Feature | Primary Response | Secondary Response |
|---|---|---|
| When? | First exposure to a pathogen | Subsequent exposure to the same pathogen |
| Speed | Slow — takes days to produce enough antibodies | Very fast — hours to 1-2 days |
| Antibody level | Lower peak concentration | Much higher peak concentration |
| Symptoms? | Person becomes ill | Person usually does not become ill — pathogen eliminated before it can cause disease |
| Mechanism | Lymphocytes activated for the first time | Memory cells recognise the antigen immediately |
4. Vaccination
Vaccination is the deliberate introduction of a dead, weakened (attenuated), or inactivated form of a pathogen (or just its antigens) into the body to stimulate an immune response without causing the disease.
How vaccination works:
- The vaccine (dead/weakened pathogen or antigen) is injected or given orally
- The immune system responds as if it were a real infection — lymphocytes produce specific antibodies against the pathogen’s antigens
- Memory cells are produced and remain in the blood
- If the person is later exposed to the actual (live) pathogen, memory cells recognise it immediately → rapid, high-level antibody production (secondary response) → pathogen is destroyed before the person becomes ill
Importance of vaccination:
- Prevents serious and potentially fatal diseases (e.g. polio, measles, tetanus, diphtheria)
- Protects individuals from disease
- When a large proportion of the population is vaccinated, herd immunity develops
Herd immunity: When a high percentage of a population is immunised against a disease, the pathogen cannot spread easily because there are too few susceptible hosts. This protects those who cannot be vaccinated (e.g. newborn babies, people with weakened immune systems). Typically requires >90-95% vaccination coverage.
5. Autoimmune Diseases
In an autoimmune disease, the immune system mistakenly attacks the body’s own cells, recognising self-antigens as foreign.
Example — Type 1 diabetes:
- The immune system destroys the insulin-producing beta cells in the pancreas (islets of Langerhans)
- Without beta cells, the body cannot produce insulin
- This results in high blood glucose levels that must be managed with regular insulin injections
- Unlike type 2 diabetes (linked to diet/lifestyle), type 1 is an autoimmune condition and is not preventable by lifestyle changes
Sources
- BBC Bitesize GCSE Biology — Communicable diseases / Disease and defence, BBC (free educational resource)
- OpenStax Biology 2e — Ch. 42 The Immune System, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Syllabus 10: Diseases and immunity, Cambridge Assessment International Education
- CK-12 Biology for High School — Immune System, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Transport in Animals — White blood cells (phagocytes and lymphocytes) are blood components; blood clotting by platelets
- Human Nutrition — Stomach acid as a chemical barrier in the digestive system
- Gas Exchange in Humans — Mucus and cilia in the trachea trap pathogens
- Variation and Selection — Antibiotic resistance as an example of natural selection
- IGCSE-Bio-Index — Full IGCSE Biology index
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Antibiotics kill viruses” | Antibiotics kill bacteria only. They have no effect on viruses. Antibiotic resistance is a separate issue about bacteria |
| ”Vaccines contain the live, dangerous pathogen” | Vaccines contain dead, weakened (attenuated), or inactivated pathogens, or just their antigens — they do not cause the disease in healthy people |
| ”Memory cells only last a few weeks” | Memory cells can last for years or decades, sometimes for life (e.g. measles vaccination) |
| “All white blood cells produce antibodies” | Only lymphocytes (specifically B-lymphocytes) produce antibodies. Phagocytes engulf and digest pathogens — they do not produce antibodies |
| ”Stomach acid digests pathogens like it digests food” | Stomach acid kills bacteria/pathogens (chemical barrier). Digestion of food is a separate process carried out by enzymes |
| ”Once vaccinated, you will never get the disease” | Some vaccines require booster shots (e.g. tetanus every 10 years) because memory cells may decline over time. Also, some pathogens mutate (e.g. influenza) so new vaccines are needed |