10.1Diseases and Immunity
Diseases and immunity — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions
5 min read 7 parts Lesson 1 of 3 3 quizzes
Lesson summary
Diseases, transmission and body defences — IGCSE BIOLOGY notes: Define pathogen and transmissible disease, describe how pathogens spread, how the body defends itself and how the spread of disease is controlled.Pathogen — a disease-causing organism, e.g.
- Not all diseases are transmissible
- Antibiotics
- Direct contact
- Phagocytosis
- Antibodies
- Pathogen — a disease-causing organism, e.g. bacteria, viruses, fungi and protoctists
Section 1 of 7Pathogens and transmissible disease
1Pathogen
- Pathogen a disease-causing organism, e.g. bacteria, viruses, fungi and protoctists.
2Transmissible disease
- Transmissible disease a disease in which the pathogen can be passed from one host to another.
3Not all diseases are transmissible
(Supplement) — some are caused by the immune system attacking the body's own cells.
- Exampletype 1 diabetes, from destruction of insulin-secreting cells in the pancreas.
4Antibiotics
Antibiotics kill bacteria but do not affect viruses.
Section 2 of 7How pathogens are transmitted
Direct contact
- Direct contact the pathogen passes straight from one host to another through blood or other body fluids.
- ExamplesHIV, gonorrhoea, hepatitis B and C.
Indirect contact
- Indirect contact the pathogen leaves the host and is carried by an intermediate medium or vector.
- Media include contaminated surfaces, air (droplets or aerosols), food, water, and animals such as insects.
| Method of transmission | Examples |
|---|---|
| Droplets in the air | Common cold, influenza |
| Food or water | Cholera, typhoid, dysentery, salmonella |
| Touching contaminated surfaces | Athlete's foot |
| Insect bites | Malaria, dengue fever |
Section 3 of 7Body defences
| Type | Examples | How it protects |
|---|---|---|
| Mechanical barriers | Skin; hairs in the nose | Physically stop pathogens entering; skin heals cuts by forming a scab |
| Chemical barriers | Mucus; stomach acid (HCl) | Mucus traps pathogens so they can be removed; acid kills pathogens in food, water or swallowed mucus |
| Cells | White blood cells: phagocytes and lymphocytes | Phagocytes engulf and digest pathogens; lymphocytes make antibodies that target them |
Phagocytosis
- Phagocytosis the engulfing and digestion of pathogens by phagocytes.
Antibodies
- Antibodies proteins made by lymphocytes that clump (agglutinate) pathogens and signal phagocytes to destroy them.
- Each antibody is specific, with a shape complementary to one antigen.
- The details of active and passive immunity and vaccination are covered in the immunity lesson.
Section 4 of 7Controlling the spread of disease
| Measure | Why it works |
|---|---|
| Clean water supply | Water free of pathogens prevents waterborne disease such as cholera |
| Hygienic food preparation | Washing hands, cooking correctly and clean preparation stop contamination with bacteria and fungi |
| Personal hygiene | Washing with soap and using tissues for coughs and sneezes reduces colds and flu |
| Waste disposal | Reduces pests such as flies that act as vectors |
| Sewage treatment | Removes pathogens before sewage reaches the environment and water |
- Cholera (Supplement) — a bacterium spread in contaminated water.
- Its toxin makes chloride ions pass into the small intestine and water follows by osmosis, causing diarrhoea, dehydration and loss of ions.
- Treatment is oral rehydration therapy, which replaces water and salts.
Section 5 of 7More from the specification
1Cholera Pathogenesis and Osmotic Diarrhoea
- Ingestion of Vibrio cholerae bacteria in contaminated food or water leads to toxin secretion in the small intestine; the cholera toxin stimulates active secretion of chloride ions into the intestinal lumen,
- causing water to follow by osmosis and triggering severe dehydration and watery diarrhoea.
2Active Immunity and Memory Cells
- Following exposure to a pathogen or vaccine, lymphocytes undergo clonal selection to produce antibodies and long-lived memory cells, enabling a rapid and robust secondary immune response upon subsequent re-infection.
3Passive Immunity via Breastfeeding
- Maternal antibodies (secretory IgA) transferred to the newborn through breast milk and colostrum provide immediate, temporary passive protection against infections while the infant's own immune system matures.
4Herd Immunity in Populations
- Vaccinating a large proportion of a population interrupts pathogen transmission chains, providing indirect protection to vulnerable, unvaccinated individuals who cannot be immunised.
5Population
- A group of organisms of one species in the same area at the same time.
6Memory cell
- A lymphocyte that keeps instructions for making a specific antibody.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Pathogen | A disease-causing organism. |
| Transmissible disease | A disease in which the pathogen can be passed from one host to another. |
| Direct contact | Spread of pathogens by blood or other body fluids from one person to another. |
| Indirect contact | Spread of pathogens by contaminated surfaces, food, water, air or animals. |
| Phagocytosis | The engulfing and digestion of pathogens by white blood cells. |
| Antibody | A protein made by lymphocytes that binds to a specific antigen. |
| Vector | An organism, such as a mosquito, that carries a pathogen from one host to another. |
Section 7 of 7Exam technique
1
Antibiotics treat viral diseases — they kill bacteria only.
2
All diseases are transmissible — some, e.g. type 1 diabetes, are not caused by a pathogen.
3
Stomach acid is a mechanical barrier — it is a chemical barrier.
4
Mucus kills pathogens — it traps them so they can be removed.
5
Pathogens are only bacteria — viruses, fungi and protoctists can also be pathogens.
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