SB5 · Health, disease and the development of medicinesTopic 5 — Health, disease and the development of medicines
Pathogens, immunity and medicines
Revise the key ideas
Health, disease and interpreting evidence
Health includes physical, mental and social well-being, not only the absence of disease. Problems in one area can affect the others.
Communicable diseases are caused by pathogens and can spread between organisms. Non-communicable diseases are not passed from one person to another by infection.
A pathogen is a disease-causing agent, such as a bacterium, virus, fungus or protist. Not all microorganisms cause disease.
One disease can increase vulnerability to another. For example, HIV damages immune defences, so other infections can become harder to control.
A correlation is a pattern linking two variables. It does not prove that one causes the other (causation). For example, income and life expectancy may be linked, but healthcare, nutrition and housing can also affect the pattern.
Compare health data using suitable samples, rates or percentages. Look for other factors that could explain the pattern (confounding factors). An increased risk means something is more likely, not certain to happen.
Lifestyle, nutrition and risk calculations
Many non-communicable diseases result from interacting genetic, environmental and lifestyle factors. Examples include cardiovascular disease, some cancers and some lung or liver diseases.
Malnutrition means an unbalanced intake of nutrients or energy, including deficiencies and excesses. Lack of vitamin C can cause scurvy; excess energy intake can contribute to increased body fat.
Diet and physical activity affect energy balance. Long-term energy intake exceeding energy expenditure can contribute to obesity, which is associated with increased risk of several diseases.
Body mass index (BMI) = mass in kilograms ÷ (height in metres)². Square the height before dividing; convert centimetres to metres if needed.
A hypothetical adult of mass 72 kg and height 1.8 m has BMI = 72 ÷ 1.8² = 22.2 to one decimal place.
In the conventional adult BMI classification commonly used in GCSE examples, 30 or above indicates obesity. BMI is a screening measure, not a diagnosis; it cannot distinguish muscle from fat, and children require age-related interpretation.
Waist : hip ratio = waist circumference ÷ hip circumference. Use the same units: a waist of 80 cm and hips of 100 cm give a ratio of 0.80.
BMI and waist : hip ratio can contribute to risk assessment, but do not capture every influence on health. Use the information supplied in an exam question rather than diagnosing an individual from one value.
Alcohol misuse can damage the liver and contribute to cirrhosis, where healthy tissue is replaced by scar tissue. Alcohol is not the only possible cause of liver disease.
Smoking increases cardiovascular risk by damaging blood vessels and contributing to plaque formation and clotting. Nicotine can raise heart rate and blood pressure; carbon monoxide reduces the blood’s oxygen-carrying capacity.
Lifestyle-related disease affects individuals and families, healthcare services and wider society. Population effects depend on exposure, access to care and other factors; avoid assuming that a disease is solely a person’s fault.
Cardiovascular disease and treatments
Cardiovascular disease affects the heart or blood vessels. In coronary heart disease, fatty deposits can narrow the coronary arteries supplying heart muscle.
Reduced blood flow limits the supply of oxygen for aerobic respiration in heart muscle. A clot blocking a coronary artery can cause a heart attack.
Lifestyle changes can reduce cardiovascular risk. Their benefits usually develop over time; they do not instantly remove an established arterial blockage.
Medicines such as statins lower blood cholesterol and can reduce risk. They may require long-term use and can have side effects, so evaluate benefits and limitations.
A stent is a small mesh tube inserted into a narrowed artery to help keep it open. Angioplasty widens the artery, and the stent supports the opening.A stent supports a widened passage; it does not remove every underlying risk factor.
Coronary bypass surgery uses a blood vessel graft to make an alternative route around a narrowed or blocked artery, improving blood supply to heart muscle.
Stents and bypass surgery can improve blood flow relatively quickly, but involve procedural risks and do not remove all underlying risk factors.
Compare treatments using effectiveness, side effects, surgical risk, recovery, long-term care and cost. The most suitable treatment depends on the condition and the person.
Pathogens and named infections
Bacteria are cells that can reproduce in suitable conditions. Some cause disease by damaging tissues or producing toxins; viruses reproduce inside host cells and can damage them.
Cholera is caused by bacteria and can cause severe diarrhoea. It is commonly spread by water contaminated with infected faeces.
Tuberculosis (TB) is caused by bacteria and can damage the lungs. Infectious material from a person with respiratory TB can spread through the air.
Chalara ash dieback is caused by a fungus, Hymenoscyphus fraxineus. It causes leaf loss and bark lesions in ash trees; airborne fungal spores can spread infection.
Athlete’s foot is a fungal infection that affects human skin, especially between the toes.
Malaria is caused by Plasmodium protists and can damage blood and the liver. Mosquitoes act as vectors that carry the pathogen between people.
HIV is a virus that damages immune-system cells. Untreated infection can lead to AIDS, leaving the body much less able to control other infections; effective treatment can prevent this progression.
Chlamydia is a bacterial sexually transmitted infection (STI). HIV can also be sexually transmitted, but it is caused by a virus, so the two pathogens are different types.
A vector transports a pathogen between hosts. In malaria, the mosquito is the vector and the Plasmodium protist is the pathogen; they are not the same organism.
Reducing transmission
Clean water supplies, sewage treatment and hygiene reduce faecal contamination and help prevent cholera transmission. Handwashing also reduces transfer of some pathogens to the mouth.
Ventilation and appropriate infection-control measures reduce airborne transmission, including the spread of respiratory TB. Different transmission routes require different controls.
For malaria, insecticide-treated mosquito nets, reducing breeding sites and controlling mosquitoes can reduce contact with the vector and interrupt transmission.
Control measures for ash dieback aim to limit movement of infected material and follow plant-health guidance. Airborne spores make complete prevention difficult.
STIs such as chlamydia and HIV can spread through sexual contact involving infected fluids or tissues. Condoms reduce the risk of transmission; they do not guarantee protection against every STI.
Screening can identify infections, including those without obvious symptoms. Detection enables treatment and measures to reduce onward transmission; avoiding sexual contact prevents exposure by that route.
Physical barriers and chemical defences
Skin is a physical barrier that reduces entry of pathogens. Breaks in the skin can provide an entry route.
Mucus traps particles and pathogens in the airways. Cilia move the mucus towards the throat, helping clear trapped material.
Tears contain lysozyme, an enzyme that breaks down cell walls of some bacteria. This is a chemical defence, rather than a physical wall.
Hydrochloric acid in the stomach creates acidic conditions that help destroy many swallowed pathogens. It is a chemical defence.Physical barriers and clearance work alongside chemical defences.
Physical barriers reduce entry or remove pathogens; chemical defences use substances that damage them. These general defences do not target one particular antigen.
The specific immune response
Antigens are molecules, often on a pathogen’s surface, that can trigger a specific immune response. Different pathogens have different antigens.
Lymphocytes are white blood cells involved in specific immune responses. Those that recognise a particular antigen multiply and produce antibodies with binding sites that match (are complementary to) that antigen.
Antibodies are proteins that bind specifically to matching antigens. They can neutralise pathogens or toxins and help other immune cells remove pathogens; binding does not mean every pathogen is instantly destroyed.
Other white blood cells, including phagocytes, can engulf and digest pathogens. Antibodies can help mark a pathogen for removal.
The first exposure produces a primary immune response. There is a delay while the appropriate lymphocytes are activated and multiply.
Some lymphocytes become memory cells and remain after the initial response. They enable a faster response if the same antigen is encountered again.
The secondary response usually produces antibodies more rapidly and in greater quantities. Specificity means memory for one antigen does not automatically protect against every other pathogen.
Vaccination and immunity
Vaccination introduces a safe form of an antigen, for example from an inactive pathogen or a harmless part of it. This stimulates an immune response without causing the disease the vaccine aims to prevent.
The immune system produces specific antibodies and memory lymphocytes. Vaccination prepares the body for a later encounter; it does not simply supply a permanent stock of antibodies.
On later exposure to the matching pathogen, memory cells enable a faster secondary response. The pathogen may be controlled before serious symptoms develop.
Vaccine protection varies between pathogens, vaccines and people. Some vaccines require additional doses to build or maintain protection.
A graph of antibody level against time can compare responses: the secondary response rises sooner and often reaches a higher level than the primary response. Interpret the axes and exposure times.Responses to the same antigen: schematic trend, not measured data.
Antibiotics and resistance
Antibiotics treat bacterial infections by killing bacteria or stopping their growth (inhibiting it). They target bacterial processes, such as making cell walls, that differ from processes in human cells.
Antibiotics do not treat viral infections. Viruses use host-cell machinery and lack the bacterial structures or processes targeted by these medicines.
Antibiotic-resistant bacteria survive a treatment that kills susceptible bacteria. Selection can make resistance more common as the survivors reproduce.
Appropriate antibiotic use reduces unnecessary selection pressure. Pain-relieving medicines can relieve symptoms, but this does not mean they remove the pathogen.
Developing and testing medicines
Medicine development starts by finding a substance that might have a useful effect. Researchers develop it into a form that can be given to a patient (a formulation) and investigate how it works.
Before testing on humans, preclinical studies test a possible medicine on cells or tissues and, where appropriate, whole organisms. Researchers check its effects and toxicity (how harmful it is) and find out how it behaves in the body.
Early clinical trials often use small numbers of healthy volunteers to study safety, side effects and suitable doses. Some medicines, such as certain cancer treatments, are first tested in patients instead.
Later clinical trials include people with the condition and assess efficacy: how well the treatment works. Larger trials also give more information about side effects and comparisons with existing treatments.Studies move from preclinical tests to human trials, followed by review and monitoring.
Random allocation helps make treatment and comparison groups similar. A placebo is a treatment without the tested active ingredient; some trials instead compare with an existing medicine.
In a double-blind trial, participants and the relevant researchers assessing outcomes do not know which treatment each person receives. This helps reduce expectation and observer bias.
Trials require informed consent, ethical review and monitoring. A placebo is not suitable if it would mean withholding essential effective treatment.
Evaluate a medicine using efficacy, safety, appropriate dose and the quality of the evidence. A larger trial can detect effects missed by a small trial, but cannot prove a medicine has no risks.
After approval, continued monitoring can identify rare or long-term side effects. Passing early tests is not a guarantee that every later study will be successful.
Viruses and plant protection
In a lytic viral cycle, a virus infects a host cell, uses its machinery to copy viral genetic material and make proteins, assembles new viruses, then releases them as the cell bursts.The two pathways have different timing of host-cell destruction.
In a lysogenic pathway, viral genetic material becomes part of the host’s DNA and is copied when the host divides. It may later become active and enter a lytic cycle; the host is not immediately destroyed.
Plant cell walls and the waxy leaf cuticle form physical barriers to pathogens and pests. Plants also make chemicals that kill or slow microorganisms (antimicrobials), and chemicals that discourage plant-eating animals (herbivores).
Some plant chemicals are useful medicines: salicylic-acid-related compounds led to aspirin, while other plant compounds can relieve symptoms or treat disease. A natural origin alone does not guarantee safety.
(Higher tier) Identify plant disease using visible symptoms, the distribution of affected plants and laboratory or diagnostic tests. First consider environmental causes such as mineral deficiency, water shortage or temperature damage; a yellow leaf alone does not identify a pathogen.
Microbial cultures: core practical
Aseptic techniques keep unwanted microorganisms out and protect people. Sterilise equipment and nutrient media in an autoclave, use sterile inoculating loops, disinfect the bench and keep culture containers covered. School work uses approved organisms that do not cause disease (non-pathogenic organisms) under teacher supervision.
An autoclave uses steam under pressure to sterilise equipment and medium. Briefly lift lids only as needed; flame a suitable reusable loop and allow it to cool before inoculating, following the laboratory’s safe procedure.
Spread bacteria evenly over sterile agar and place discs containing measured antiseptic, antibiotic or plant-extract solutions onto it. Include a solvent-only control disc and keep concentration, disc size, bacterial culture and incubation conditions consistent.
Incubate school cultures at a suitable low temperature, commonly 25 °C, rather than body temperature; this reduces the chance of cultivating human pathogens. Secure plates as instructed, usually with a few tape strips rather than an airtight seal; do not open cultures after incubation.
Measure the diameter of the clear area around a disc (the inhibition zone). Calculate its area using πr², where radius = diameter ÷ 2. A larger zone suggests the substance inhibited bacterial growth more in this test. How easily it diffuses through agar also affects zone size, so the test alone does not show how well it would work in a patient.
If comparing only the clear agar around a disc, subtract the area of the disc from the total circular zone area. Repeat trials, calculate a mean and explain anomalies instead of choosing only the largest result.
Immunisation and monoclonal antibodies
Immunisation reduces the risk of disease and can limit transmission. When a sufficiently large proportion is immune, herd immunity helps protect vulnerable people; it depends on the pathogen, vaccine effectiveness and uptake. Consider rare adverse effects alongside the much larger benefits of preventing serious disease.
(Higher tier) To make monoclonal antibodies, join (fuse) antibody-producing lymphocytes with tumour cells that can keep dividing. The new hybrid cell is a hybridoma: it produces antibodies and divides repeatedly.Fusion combines antibody production with the capacity to divide repeatedly.
(Higher tier) Test the hybridomas to find one making the required antibody. Clone that cell and grow the clone. All the antibodies it produces are identical and bind to one particular antigen, so they can target it specifically.
(Higher tier) Pregnancy tests use antibodies that recognise the hormone hCG in urine. A visible test line indicates binding when the hormone is present; a control line checks that the test has operated properly.
(Higher tier) Labelled antibodies can locate cancer cells or clots in diagnostic imaging. Antibodies can also carry a drug or radioactive substance to target cells, reducing some damage to other tissues compared with less targeted treatment.
(Higher tier) Monoclonal antibodies can still cause side effects and do not work for every patient or tumour. Evaluate their specificity, effectiveness, possible immune reactions and cost rather than assuming targeting removes all risk.
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