10.1.1Diseases and Immunity
Pathogens and Barriers — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 2 of 7
Section 1 of 6Pathogens and transmission
Pathogen
- Pathogen A pathogen is an organism that causes disease.
- Bacteria, fungi, protoctists and viruses can all be pathogens.
Transmissible disease
- Transmissible disease Because pathogens pass from an infected host to another host, the diseases they cause are called transmissible diseases.
Direct contact
- Direct contact The pathogen passes straight from one person to another, for example through blood or other body fluids.
- HIV and hepatitis B are examples.
Indirect contact
- Indirect contact The pathogen leaves one host and is carried to another through contaminated food or water, through the air in droplets,
- or by a vector such as a mosquito.
Section 2 of 6Virus structure (HT only)
A virus particle
Genetic material
- Genetic material A virus contains DNA or RNA at its centre.
Protein coat
- Protein coat The genetic material is surrounded by a protein coat.
Not a living organism
- Not a living organism Viruses do not carry out the life processes by themselves, so they are not placed in any classification system.
- They take over a host cell and use it to make many copies of the virus.
Section 3 of 6Body defences
| Type of defence | Example | How it works |
|---|---|---|
| Mechanical | Skin | Covers the body surface as a barrier; if cut, it heals and a scab forms. |
| Mechanical | Hairs in the nose | Make it harder for pathogens to travel up the nose and into the lungs. |
| Chemical | Mucus | Traps pathogens so they can be removed by coughing, sneezing or swallowing. |
| Chemical | Stomach acid | Hydrochloric acid kills pathogens swallowed in food, water or mucus. |
| Cellular | White blood cells | Destroy pathogens by phagocytosis and by producing antibodies. |
Section 4 of 6How white blood cells defend
1
Phagocytes detect a pathogen that has got past the barriers, engulf it and digest it with enzymes.
2
Lymphocytes produce antibodies that attach to the pathogen.
3
The antibodies cause pathogens to clump together, which limits their movement.
4
Chemical signals are released that attract phagocytes to the infection site, so the pathogens are destroyed.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Pathogen | An organism, or virus, that causes disease. |
| Transmissible disease | A disease whose pathogen can be passed from one host to another. |
| Vector | An organism that carries a pathogen from one host to another. |
| Phagocytosis | Engulfing and digesting a pathogen. |
| Antibody | A protein made by lymphocytes that binds to a specific antigen. |
| Mucus | A sticky fluid that traps microorganisms and dust in the airways. |
Section 6 of 6Exam technique
1
Describing viruses as living cells. They have no cell structure and cannot reproduce outside a host cell.
2
Saying stomach acid kills pathogens in the lungs. It acts in the stomach, on what has been swallowed.
3
Confusing mechanical barriers with chemical ones. Skin and nasal hairs are physical; mucus and acid are chemicals.
4
Writing that all microorganisms are pathogens. Many are harmless or useful.
5
Saying pathogens are always passed on by touch. Many spread through air, food, water or vectors.
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