Infection and response — GCSE Biology Revision
Everything you need to revise infection and response for GCSE Biology: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Pathogens: The Causes of Disease
Pathogens are microorganisms that cause infectious diseases. The four main types are bacteria, viruses, fungi, and protists. Bacteria are single-celled organisms that can reproduce rapidly inside the body, producing toxins that damage cells and tissues. Viruses are much smaller, reproduce inside host cells, and cause cell damage. Fungi can be single-celled or multicellular. Protists are single-celled eukaryotes, often transferred by a vector like a mosquito.
Spread of Pathogens and Prevention
Pathogens can spread via air (e.g., influenza, common cold), direct contact (e.g., athlete's foot, STIs), or water/food (e.g., cholera, salmonella). Good hygiene practices, such as hand washing, using disinfectants, and cooking food thoroughly, help prevent transmission. Isolation of infected individuals and vaccination programmes also play crucial roles in controlling the spread of infectious diseases within populations.
The Body's Defence Systems
The body has several non-specific defence systems to prevent pathogen entry. These include the skin, which acts as a physical barrier; mucus in the respiratory tract, trapping pathogens; cilia, sweeping mucus away; stomach acid, killing most ingested pathogens; and tears/saliva, containing enzymes that destroy bacteria. If pathogens breach these defences, the immune system mounts a specific response.
The Immune System: Antibodies and Memory Cells
If pathogens enter the body, the immune system, particularly white blood cells, takes action. Phagocytes engulf and digest pathogens. Lymphocytes produce specific antibodies that target and destroy pathogens. Some lymphocytes also produce antitoxins to neutralise bacterial toxins. After an infection, some lymphocytes remain as memory cells, providing long-term immunity against that specific pathogen, allowing for a faster, stronger response upon re-exposure.
Key facts
- Pathogens cause infectious diseases.|Bacteria, viruses, fungi, and protists are pathogen types.|The body has non-specific and specific defence systems.|White blood cells are crucial for immunity.|Vaccines create immunity without causing disease.|Antibiotics treat bacterial infections, not viral ones.
Key terms
- Pathogen::A microorganism that causes infectious disease.|Antibody::Proteins produced by lymphocytes that specifically target and destroy pathogens or toxins.|Phagocytosis::The process by which phagocytes engulf and digest pathogens.|Vaccination::The introduction of a weakened or inactive form of a pathogen to stimulate an immune response and provide immunity.|Antitoxin::A substance produced by lymphocytes that neutralises toxins produced by bacteria.|Immunity::The ability of an organism to resist a particular infection or toxin.
Common mistakes
- Thinking antibiotics can kill viruses; they only work on bacteria.|Confusing antigens (molecules on pathogen surfaces) with antibodies (immune proteins).|Believing that all bacteria are harmful; many are beneficial.|Not understanding that vaccinations prevent disease, they don't cure existing ones.
Exam tips
- Distinguish clearly between bacteria and viruses: structure, reproduction, and how they cause harm.|Remember the difference between non-specific (general barriers) and specific (immune response) defences.|When explaining antibody function, mention they are specific and how they 'lock on' to pathogens.|Practise explaining how vaccination works, including the role of memory cells.
Quick quiz
1. Which of the following is a non-specific defence mechanism of the human body?
- Production of antibodies
- Memory cell formation
- Stomach acid
- Phagocytosis by lymphocytes
Show answer
Stomach acid — Stomach acid is a non-specific chemical barrier that kills most ingested pathogens. Antibody production, memory cell formation, and phagocytosis by lymphocytes are all parts of the specific immune response.
2. Which type of pathogen reproduces by inserting its genetic material into a host cell?
- Bacteria
- Fungi
- Protists
- Viruses
Show answer
Viruses — Viruses are obligate intracellular parasites, meaning they can only reproduce by infecting and using the machinery of host cells. Bacteria, fungi, and protists can reproduce independently.
3. What is the primary role of memory cells in the immune system?
- To engulf and digest pathogens
- To produce antitoxins against bacterial poisons
- To provide long-term immunity by allowing a rapid secondary response
- To prevent pathogens from entering the body
Show answer
To provide long-term immunity by allowing a rapid secondary response — Memory cells remain in the body after an infection or vaccination and allow the immune system to respond much faster and more effectively if the same pathogen is encountered again, providing long-term immunity.
4. Which method of pathogen transmission is primarily associated with diseases like cholera and salmonella?
- Airborne droplets
- Direct physical contact
- Contaminated water or food
- Insect vectors
Show answer
Contaminated water or food — Cholera and salmonella are typically spread through the ingestion of food or water contaminated with the bacteria. Airborne droplets transmit diseases like influenza, direct contact transmits STIs, and insect vectors transmit diseases like malaria.
5. Antibiotics are effective against which type of pathogen?
- Viruses only
- Bacteria only
- Fungi and viruses
- Bacteria and some fungi
Show answer
Bacteria only — Antibiotics work by targeting bacterial cell structures or processes, such as cell walls or protein synthesis. They are not effective against viruses, which have different structures and replication mechanisms.
Exam-style questions
Explain how antibodies protect the body against pathogens. [3 marks]
Show mark scheme
- Antibodies are specific to particular pathogens/antigens.
- They 'lock on' to the pathogen/antigen.
- This can destroy the pathogen directly, or 'label' it for destruction by phagocytes / prevent it from entering cells / neutralise toxins.
Describe two ways in which the body's non-specific defence systems prevent pathogens from entering. [2 marks]
Show mark scheme
- Skin acts as a physical barrier (prevents entry).
- Stomach acid kills most pathogens (chemical barrier).
- Mucus traps pathogens in the respiratory tract (physical/chemical barrier).
- Cilia sweep mucus and trapped pathogens out of the respiratory tract (physical barrier).
- Tears/saliva contain enzymes that destroy pathogens (chemical barrier).
Explain why it is not advisable to use antibiotics to treat a viral infection like the common cold. [3 marks]
Show mark scheme
- Antibiotics only kill bacteria / are ineffective against viruses.
- Viruses live inside host cells, making them hard to target without harming host cells.
- Using antibiotics for viral infections can lead to antibiotic resistance in bacteria.
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