DSE Revision and Exam Preparation
FoundationInfectious diseases arise from viruses, bacteria, fungi and protists spread by water, droplets, food and body fluids; the body defends in layers — skin and mucus, phagocytes and lymphocytes — vaccination builds active immunity, and antibiotics work only on bacteria; Paper 1 carries sixty per cent, the main battlefield of revision.
- Health spans physical, mental and social well-being; infectious diseases arise from transmissible pathogens, while non-infectious diseases track genes and lifestyle.
- Four kinds of pathogen — viruses, bacteria, fungi and protists; transmission runs by water, food, droplets, body fluids, vectors and direct contact, each route with its own countermeasure.
- Defence has two layers: skin, mucus, phagocytes and inflammation are non-specific; B cells making antibodies and T cells attacking infected cells are specific.
文體小知識：科學知識說明文——整合健康與疾病課題的病原體、傳播途徑、防禦機制與免疫原理，並拆解文憑試考評結構與各課題易錯位，是中六生物的收官課。
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1Health is more than the absence of disease: it covers physical, mental and social well-being. Diseases fall into two classes: infectious diseases are caused by pathogens and can spread from person to person, while non-infectious diseases such as cancer, coronary heart disease and diabetes relate to genes, age and lifestyle and never pass between people. Telling them apart is the first step of public health: the former calls for cutting transmission routes, the latter for improving habits.
2Common pathogens come in four kinds: viruses (such as those causing influenza and dengue fever), bacteria (causing cholera and tuberculosis), fungi (causing ringworm) and protists (causing malaria). Transmission routes include water, air and droplets, food, body fluids, vectors and direct contact; cutting the right route — purifying water, wearing masks, treating wounds properly — sharply lowers the chance of spread.
3The body's defence has two layers, non-specific and specific. Non-specific defence covers the physical barrier of skin and mucus, germ-killing secretions, cilia sweeping away debris, phagocytes engulfing pathogens, blood clotting to seal wounds and the inflammatory response — all of them indiscriminate. Specific defence belongs to lymphocytes: B cells recognise antigens and produce antibodies, while T cells attack infected cells, both striking precisely at particular pathogens.
4An antigen is a substance on a pathogen that the immune system can recognise; antibodies bind to it and disable the pathogen. The primary response on first meeting an antigen is slow and weak; on re-encounter, memory cells make the secondary response faster and stronger, often clearing the pathogen before symptoms appear. Vaccination exploits exactly this: weakened or killed pathogens, or their antigens, are injected to induce active immunity — the body making its own antibodies and memory cells. Injecting ready-made antibodies is passive immunity, fast but short-lived.
5Antibiotics work only on bacteria, never on viruses; their misuse breeds resistant bacteria and makes infections harder to treat. Non-infectious diseases are prevented by healthy living: a balanced diet, regular exercise, no smoking and little alcohol. As for the Diploma exam: Paper 1 tests the compulsory part, sixty per cent, over two and a half hours; Paper 2 tests the elective part, twenty per cent, with two of four electives answered within one hour; school-based assessment carries twenty per cent; and grades are standards-referenced. Knowing the structure lets revision aim true.
1Summary 重點摘要
Infectious diseases arise from viruses, bacteria, fungi and protists spread by water, droplets, food and body fluids; the body defends in layers — skin and mucus, phagocytes and lymphocytes — vaccination builds active immunity, and antibiotics work only on bacteria; Paper 1 carries sixty per cent, the main battlefield of revision.
2Vocabulary cards Vocabulary
An organism that causes infectious disease — viruses, bacteria, fungi and protists.
A disease caused by pathogens and transmissible between people through water, air, droplets, food, body fluids or vectors.
A substance on a pathogen that the immune system recognises, triggering an immune response.
A protein made by B cells against a specific antigen, binding it and disabling the pathogen.
The white blood cells of specific immunity — B cells that make antibodies and T cells that attack infected cells.
Immunity in which the body makes its own antibodies and memory cells after meeting the antigen, as after vaccination — lasting but slower to arise.
Immunity gained by injecting ready-made antibodies, as with tetanus antitoxin or antibodies in breast milk — immediate but short-lived.
Injecting weakened or killed pathogens, or their antigens, to induce active immunity and prevent infectious disease.
The survival and spread of bacteria selected by antibiotic overuse, making the drugs ineffective.
A disease that never passes between people — cancer, coronary heart disease and diabetes — linked to genes and lifestyle.
3Step by step Steps
- Step 1: Health and the classification of diseaseHealth means more than not being ill: it spans physical, mental and social well-being. Diseases fall into two classes. Infectious diseases are caused by pathogens and pass from person to person; non-infectious diseases — cancer, coronary heart disease and diabetes — stem from genes, age and lifestyle and never spread between people. Classification questions apply a single criterion: is it caused by a transmissible pathogen? For example, influenza is a viral, droplet-borne infectious disease, while coronary heart disease results from fatty deposits in vessels and is non-infectious — their prevention strategies differ completely.
- Step 2: Four kinds of pathogen and the routes of spreadPathogens come in four kinds: viruses, bacteria, fungi and protists, each with signature diseases — viruses cause influenza, dengue fever and hepatitis B; bacteria cause cholera and tuberculosis; fungi cause ringworm; protists cause malaria. Transmission routes include water, air and droplets, food, body fluids, vectors and direct contact, and each route has a matching way to be cut. For example, cholera spreads by water and food, so purifying water and cooking food thoroughly are decisive; malaria rides on mosquito vectors, so draining stagnant water and using nets work best; tuberculosis travels in droplets, making masks and ventilation essential.
- Step 3: Non-specific defence mechanismsWhen pathogens break in, non-specific defence answers first: skin and mucus form the physical barrier, secretions kill germs, and airway cilia sweep debris away. Should pathogens get through, phagocytes engulf and digest them, blood clotting seals the wound against further entry, and the inflammatory response reddens and warms the site while concentrating the defence. These mechanisms respond instantly and indiscriminately to every pathogen. For example, the redness and warmth around a cut from a nail are the inflammatory response widening vessels and summoning phagocytes — a defence, not merely damage.
- Step 4: Specific defence and the immune responseSpecific defence is the work of lymphocytes, aimed at particular pathogens. Once an antigen on the pathogen's surface is recognised, B cells multiply and differentiate, producing antibodies that bind the antigen and disable the pathogen; T cells attack cells already infected. The primary response on first contact is slow and weak while enough antibody is made; on re-encounter, the memory cells left behind drive a secondary response that is faster and stronger. For example, after recovering from chickenpox you normally never catch it again — memory cells launch a powerful secondary response the moment the virus returns, clearing it before symptoms appear.
- Step 5: Active versus passive immunityActive immunity is the body making its own antibodies and memory cells after meeting an antigen: vaccination injects weakened or killed pathogens, or their antigens, to induce it — long-lasting and dependable. Passive immunity means injecting ready-made antibodies: it acts at once but fades within weeks and leaves no memory. Exam questions often ask you to classify a scenario and justify it. For example, an antitoxin injection after stepping on a rusty nail is passive immunity, protection at once; a vaccine given on schedule is active immunity, protection for the long run; and breast milk passing the mother's antibodies to her baby is passive too.
- Step 6: Antibiotics, resistance and non-infectious diseaseAntibiotics work only on bacteria and are useless against viral diseases such as influenza; a course must be completed, for stopping halfway lets surviving bacteria multiply. Overuse selects resistant bacteria, rendering once-effective drugs useless — a major public health threat. Prevention of non-infectious diseases rests on lifestyle: a balanced diet and regular exercise lower the risk of coronary heart disease and some cancers, while moderating sugar intake and keeping a healthy weight help prevent diabetes. For example, taking antibiotics for a cold — a viral illness — does nothing against the virus yet encourages bacteria in your body to develop resistance: all cost, no benefit.
- Step 7: The exam structure and revision planThe Diploma exam structure in Biology: Paper 1 tests the compulsory part, sixty per cent, in two and a half hours, with multiple-choice, short, structured and essay questions; Paper 2 tests the elective part, twenty per cent, answering two of four electives within one hour; school-based assessment carries twenty per cent; grading is standards-referenced, comparing performance against set standards. Paper 1 is the main battlefield: sweep the concepts topic by topic, drill by question type under time limits, then sit full mock papers. For example, the six lessons of this course, added to the S4 and S5 topics, cover the whole compulsory part — redoing the worked example of each lesson is a rehearsal of the data-question technique.
4Diagrams Diagrams
5Worked example Example
A study measured volunteers' blood antibody levels against a pathogen on the day of vaccination (day 0) and over the following weeks, then re-exposed them to the same antigen on day 240 and measured again. Results: after the first exposure, antibodies began rising after a few days, reached a rather low peak around day 10 and slowly fell; after the second exposure, antibodies surged within two days and peaked around day 7 far above the first peak. Explain why the two responses differ, and identify the kind of immunity the vaccine established; also explain why a doctor would not prescribe antibiotics for a patient with influenza.
- The first response is the primary response: meeting the antigen for the first time, B cells need time to recognise it, multiply and differentiate into antibody-producing cells, so the antibody rise is slow and the peak low.
- The second response is the secondary response: the memory cells left from the first response recognise the same antigen at once and divide rapidly, producing abundant antibodies, so the rise is faster and the peak far higher.
- The vaccine induced the body to make its own antibodies and memory cells — active immunity, the opposite of passive immunity, where ready-made antibodies are injected for immediate but short-lived protection.
- Influenza is a viral disease, and antibiotics act only on bacteria, never on viruses; prescribing them would not help the patient and would hasten the emergence of resistant bacteria.
- Answer: The first response was slow and low (primary: B cells taking time to mobilise); the second was fast and high (secondary: memory cells producing abundant antibodies at once). The vaccine established active immunity. Influenza is viral, antibiotics only work on bacteria, so the prescription would be useless and would speed the rise of resistant bacteria.
6Common mistakes Mistakes
7Quick check Quick check
Q1Name the four kinds of common pathogen, each with one disease it causes.
Q2Which mechanisms make up the body's non-specific defence?
Q3How do the primary and secondary immune responses differ?
Q4Distinguish active from passive immunity, with one example of each.
Q5How is the HKDSE Biology assessment structured?
8Recap Recap
- Health spans physical, mental and social well-being; infectious diseases arise from transmissible pathogens, while non-infectious diseases track genes and lifestyle.
- Four kinds of pathogen — viruses, bacteria, fungi and protists; transmission runs by water, food, droplets, body fluids, vectors and direct contact, each route with its own countermeasure.
- Defence has two layers: skin, mucus, phagocytes and inflammation are non-specific; B cells making antibodies and T cells attacking infected cells are specific.
- Vaccination builds active immunity (antibodies plus memory cells, lasting); injected antibodies give passive immunity (immediate, short-lived); antibiotics act only on bacteria, and misuse breeds resistance.
- Paper 1 on the compulsory part (sixty per cent, two and a half hours) is the main battlefield; Paper 2 is two of four electives (twenty per cent); SBA carries twenty per cent; grading is standards-referenced.