Infection & Response

• A pathogen is a microorganism that causes an infectious disease; AQA groups examples as viruses, bacteria, protists and fungi.
• Pathogens infect animals or plants and can spread by direct contact, water or air; some use a vector such as a mosquito.
• Reducing spread means interrupting the actual route through hygiene, isolation, vector control, barrier methods or vaccination.

The punchline: In a case-study answer, write pathogen type → route → control.

• Viruses reproduce inside living cells, and their replication can damage those cells.
• Bacteria reproduce inside the body and some release toxins that damage tissues and make us feel ill.
• Antibiotics target susceptible bacteria, not viruses living inside host cells.

The punchline: Name the damage mechanism; do not say only that the pathogen ‘attacks’.

• Measles causes fever and a red skin rash, can be serious if complications arise, and spreads in inhaled cough or sneeze droplets; vaccination reduces risk.
• HIV may begin with a flu-like illness; without successful antiretroviral control it damages immune cells and can progress to late-stage HIV infection, or AIDS.
• HIV can pass during sexual contact or when infected blood enters another person's body, including through shared needles; antiretroviral drugs control rather than cure it.
• TMV causes mosaic leaf discolouration, reducing photosynthesis and therefore plant growth.

The punchline: For each named virus, keep symptom, transmission and control separate.

• Salmonella bacteria are ingested in contaminated food or food prepared unhygienically; bacteria and their toxins cause fever, cramps, vomiting and diarrhoea.
• UK poultry are vaccinated against Salmonella, while hygienic food preparation helps interrupt transmission.
• Gonorrhoea can cause a thick yellow or green discharge and pain when urinating; antibiotics can treat it, but many penicillin-resistant strains have appeared.
• A barrier method such as a condom reduces gonorrhoea transmission.

The punchline: Match the control to the route: food hygiene is not a substitute for a barrier method.

• Rose black spot produces purple or black spots; leaves can yellow and fall early, reducing photosynthesis and growth.
• Wind and water spread rose black spot; fungicides or removing and destroying affected leaves can control it.
• The malaria protist has a life cycle involving mosquitoes and causes recurrent fever; the disease can be fatal.
• Preventing mosquito breeding and using nets reduce malaria transmission by targeting the vector.

The punchline: A mosquito transmits the malaria protist; it does not cause malaria by itself.

• Skin forms a physical barrier and blood clotting helps seal cuts before microorganisms enter.
• Nasal hairs and mucus trap particles; cilia in the trachea and bronchi move mucus towards the throat to be swallowed.
• Hydrochloric acid in the stomach kills many swallowed pathogens.

The punchline: Non-specific means the same barrier acts against many pathogens.

• During phagocytosis, a white blood cell engulfs and digests a pathogen.
• Other white blood cells produce antibodies that bind to specific antigens on a pathogen.
• Antitoxins neutralise toxins released by bacteria.

The punchline: Phagocytosis targets the pathogen; antitoxins target its toxins.

• A vaccine introduces a small quantity of dead or inactive pathogen material, stimulating white blood cells to make matching antibodies.
• If the same pathogen enters later, white blood cells produce the correct antibodies rapidly, usually preventing illness.
• Immunising a large proportion of a population reduces opportunities for the pathogen to spread, helping protect people who remain susceptible.
• No vaccine is perfectly effective, so describe reduced risk rather than guaranteed protection.

The punchline: Explain both scales: a faster individual response and less population spread.

• A suitable antibiotic kills infective bacteria inside the body, but different bacteria may need different antibiotics.
• Antibiotics do not kill viral pathogens; drugs that harm viruses can also damage the host cells in which viruses reproduce.
• Painkillers relieve symptoms but do not kill the pathogen, while antiretroviral drugs can control HIV without curing the infection.

The punchline: State whether the medicine acts on the pathogen, the infection or only the symptom.

• Random mutation can produce a resistant bacterium before treatment begins.
• The antibiotic kills susceptible bacteria; resistant bacteria survive, reproduce and pass on resistance genes.
• Overuse increases selection for resistant strains, so antibiotics should be used only when needed and as prescribed.
• A resistant strain may require a different antibiotic and can spread between hosts.

The punchline: Write variation → selection → survival → reproduction → resistant population.

• Traditional sources include digitalis from foxgloves, aspirin from willow and Fleming's discovery of penicillin from Penicillium mould.
• Preclinical testing uses cells, tissues and live animals to investigate toxicity and efficacy before trials in people.
• Clinical trials begin with very low doses in healthy volunteers, then use patients to test efficacy and find an optimum dose.
• Double-blind placebo trials reduce bias, and published findings face peer review.

The punchline: The three testing targets are toxicity, efficacy and dose.

• A mouse lymphocyte is stimulated to produce an antibody specific to one binding site on one protein antigen.
• The lymphocyte fuses with a tumour cell to make a hybridoma that both divides and produces the antibody.
• One hybridoma is cloned; the identical cells make one monoclonal antibody, which is collected and purified.

The punchline: Explain why both parent cells are needed: specificity plus repeated division.

• Uses include pregnancy tests, measuring hormones, detecting pathogens and locating molecules with a fluorescent dye.
• For cancer treatment, a monoclonal antibody can carry a radioactive substance, toxic drug or growth-stopping chemical towards target cells.
• Targeting aims to reduce damage to other cells, but side effects can occur and monoclonal antibodies have not become as widely used as first hoped.
• AQA expects you to explain an unfamiliar example from supplied information, not memorise named tests or treatments.

The punchline: Follow the binding site: specificity explains both detection and targeted delivery.

• Signs include stunted growth, leaf spots, rot, growths, malformed stems or leaves, discolouration and visible pests.
• Nitrate deficiency causes stunted growth because nitrate ions are needed to make proteins; magnesium deficiency causes chlorosis because magnesium is needed for chlorophyll.
• Aphids are insect pests, whereas TMV and rose black spot are viral and fungal pathogens.
• Higher Tier: identify disease using a guide, laboratory testing or a kit containing monoclonal antibodies.

The punchline: Do not call an ion deficiency infectious; identify the cause from the evidence.

• Cellulose cell walls, a tough waxy cuticle and layers of dead cells around stems resist microorganism entry.
• Plants can produce antibacterial chemicals and poisons that deter herbivores.
• Thorns and hairs deter animals; some leaves droop or curl when touched, and some plants use mimicry.

The punchline: Classify a defence as physical, chemical or mechanical, then state what it prevents.