Infection and response

Pathogens invade, defences respond, and medicine arms the fight. This reference covers AQA GCSE Biology 8461, topic 4.3 Infection and response.

How the exam treats this topic:

• Paper 1 carries this topic; RP2 (antibiotics/antiseptics on agar) links from topic 1.
• Monoclonal antibodies (4.3.2) and plant disease (4.3.3) are biology only, and monoclonals plus plant-disease detection are HT only.
• Recall exactly: each named disease → pathogen type, symptoms and spread; the defence systems; the drug-testing stages.

Pathogens are microorganisms 病原体 causing infectious disease: viruses, bacteria, protists, fungi. They infect plants or animals and spread by direct contact, water or air. Bacteria may produce toxins 毒素 that damage tissues; viruses live and reproduce inside cells.

Reducing spread: hygiene (hand washing), sterilising wounds, clean water, sewage treatment, using condoms, vector control (mosquito nets, draining water).

Non-specific defences: skin (barrier, scabbing); nose (hairs, mucus); trachea and bronchi (mucus, cilia); stomach (hydrochloric acid).

The immune system destroys pathogens that enter. White blood cells defend by:

1. phagocytosis — engulfing 吞噬 pathogens;
2. antibody production — proteins 抗体 that bind the specific antigen;
3. antitoxin production — neutralising 抗毒素 toxins.

Vaccination 疫苗: small quantities of dead or inactive pathogen stimulate white blood cells to make antibodies; on re-infection, memory cells respond quickly with the correct antibodies, preventing infection. Herd effect: immunising a large proportion reduces the spread for everyone. Evaluate: benefits (epidemic control, e.g. measles) vs rare side effects.

• Antibiotics 抗生素 (e.g. penicillin) cure bacterial disease by killing bacteria inside the body — specific antibiotics for specific bacteria. They cannot kill viruses (viruses live inside cells; killing them would damage body tissue). Resistance is a major concern — the race for new antibiotics.
• Painkillers treat symptoms but do not kill pathogens.

Drug development: traditionally extracted from plants/microorganisms — digitalis from foxgloves (heart), aspirin from willow, penicillin from Penicillium mould (Fleming). Most new drugs are chemically synthesised, but starting points may still be plant chemicals.

Testing stages: preclinical (cells, tissues, live animals) → clinical trials on healthy volunteers (low dose, safety) then patients (optimum dose, effectiveness) → double-blind trials with a placebo; results peer-reviewed.

Monoclonal antibodies 单克隆抗体 are produced from a single clone of cells, specific to one binding site on one protein antigen. Producing them: stimulate mouse lymphocytes to make the antibody → fuse with a tumour cell to make a hybridoma 杂交瘤 — it divides AND makes the antibody → clone the hybridoma → collect and purify large amounts of one specific antibody.

Uses: pregnancy tests; measuring hormone/chemical levels in blood; detecting pathogens; research — fluorescent antibodies locate molecules; cancer treatment — antibody bound to a radioactive/toxic substance delivers it only to cancer cells.

Detection (HT): stunted growth, spots on leaves, areas of decay, growths, malformed stems/leaves, discolouration, presence of pests.

Identification (HT): gardening manual/website; laboratory analysis; testing kits using monoclonal antibodies.

Named plant problems: TMV (viral), rose black spot (fungal), aphids 蚜虫 (insects).

Ion deficiencies: too little nitrate → stunted growth (nitrate needed for amino acids/protein); magnesium deficiency → chlorosis (yellow leaves 缺绿 — magnesium needed to make chlorophyll).

Defence responses: physical — cellulose cell walls, waxy cuticle, bark/dead layers; chemical — antibacterial chemicals, poisons; mechanical — thorns, hairs, drooping/curling leaves, mimicry.

• Four pathogen types; three routes of spread; two ways to reduce each named disease's spread.
• Recite the disease table: pathogen, spread, symptoms, control.
• Four non-specific defences; three white-blood-cell actions.
• Vaccination mechanism + population effect; antibiotic vs painkiller; why antibiotics fail on viruses.
• Drug sources (foxglove, willow, Penicillium) and the preclinical/clinical stages with placebo and double-blind.
• (HT, biology) Hybridoma production sequence and four monoclonal uses.
• (Biology) Plant disease signs, identification methods, nitrate/magnesium deficiencies, three defence classes.