OCR生物学 病原体 植物防御 免疫应答
OCR A Level Biology A 的 Module 4 把疾病与免疫放在同一章，编号 4.1.1：病原体类型、传播方式、植物防御、动物非特异防御、吞噬作用、特异性免疫、抗体、免疫类型、疫苗与抗生素。这一章的内容出现在 Paper 2 Biological diversity，满分 100 分、时长 2 小时 15 分、占整个 A Level 的 37%，试卷分 Section A 的 15 分选择题与 Section B 的 85 分结构化与论述题。下面按 OCR 官网 specification 的条目顺序拆开讲。
In the OCR A Level Biology A specification, section 4.1.1 bundles disease and immunity into one topic: pathogen types, transmission routes, plant defences, non-specific animal defences, phagocytosis, the specific response, antibodies, types of immunity, vaccination and antibiotics. The content is examined in Paper 2, Biological diversity, a 100-mark paper lasting 2 hours 15 minutes and worth 37% of the A Level, split into 15 marks of multiple choice in Section A and 85 marks of structured and extended response in Section B. The order below follows the specification rather than a textbook chapter, so revision maps onto the syllabus.
四类病原体与指定病例
OCR 的 specification 只要求四类病原体，每一类都配了指定病例。细菌对应结核、细菌性脑膜炎与马铃薯环腐病。病毒对应 HIV/AIDS、流感与烟草花叶病毒。原生生物对应疟疾与马铃薯晚疫病。真菌则是香蕉黑条叶斑病、牛的钱癣与人的脚气。答题时写「细菌产生毒素」并不够，题目往往要求说明该例子属于哪一类。很多学生以为病原体越大越危险，其实病毒没有细胞结构，只能借宿主细胞复制。
The specification names four pathogen groups and pairs each with a case. Bacteria cover tuberculosis, bacterial meningitis and ring rot in potatoes and tomatoes; viruses account for HIV/AIDS, influenza and tobacco mosaic virus; protoctista cause malaria and potato late blight; fungi produce black sigatoka in bananas, ringworm in cattle and athlete's foot in humans. Examiners mark the disease and the group as a pair, so a correct example filed under the wrong category earns nothing. Size is a poor guide to danger, since viruses carry no cellular machinery of their own and replicate only inside a host cell.
直接与间接传播 媒介与孢子
动物病原体的传播分直接与间接两条路。直接接触包括握手、体液与性接触。间接包括吸入飞沫、摄入污染的食物或水、经由媒介生物叮咬，以及接种式进入伤口。植物病原体同样走这两条路，土壤、雨水、风带孢子、昆虫与共用农具都能带走病原。Specification 明确提到气候与社会因素，拥挤的居住条件会让结核更易扩散。常见的错误是把媒介与病原体混为一谈，蚊子是媒介，疟原虫才是病原体。
Transmission is either direct or indirect, in animals as well as plants. Direct routes include physical contact and body fluids; indirect routes run through droplets, contaminated food or water, vectors that bite, and inoculation into a wound. Plants pick up pathogens from soil, rain splash, wind-borne spores, insects and shared tools. The specification also asks for living conditions, because overcrowding and poor ventilation raise the risk of tuberculosis spreading. A frequent slip is confusing vector with pathogen: the mosquito merely carries Plasmodium, and the protoctist causes the disease.
植物被动防御 角质层与木质素
植物的第一层防线是结构。叶片和茎表面的蜡质角质层让水难以停留，病原体失去萌发所需的水膜。纤维素细胞壁是第二道屏障，木质素加厚让壁更难被酶分解。老化的木质部导管被堵塞，病原体也难以沿维管束移动。气孔在检测到病原体时会关闭，减少进入叶片的机会。这些防御在感染发生前就已存在，属于被动防御。写答案时不要把它们与胼胝质沉积混在一起。
Structural barriers form the plant's first line. A waxy cuticle sheds water, depriving pathogens of the film they need to germinate, while cellulose walls and lignin thickening resist enzymatic breakdown. Older xylem vessels become blocked, which limits movement along the vascular tissue, and stomata close once a pathogen is detected. These defences exist before infection arrives, so they are classed as passive; mixing them with callose deposition loses the mark.
植物主动防御 胼胝质与坏死
感染发生后植物启动主动反应。胼胝质是一种多糖，沉积在细胞壁与细胞膜之间，也堆在筛板的孔上，把韧皮部的通道封住。酪丝从相邻细胞长入木质部导管，同样起到堵塞作用。局部细胞可以被主动杀死，形成坏死斑，病原体失去营养来源。部分植物还会释放酚类、生物碱与萜类等抗微生物化学物质。阅卷时常见到只写产生化学物质而不给类别的答案，这类答案通常只能拿到一半分。
Once infection begins, inducible responses take over. Callose, a polysaccharide, is laid down between the wall and the membrane and around sieve plates, sealing the phloem; tyloses grow from neighbouring cells into xylem vessels for the same purpose. A patch of tissue may be killed outright, producing a necrotic lesion that starves the pathogen. Many plants also release antimicrobial compounds such as phenols, alkaloids and terpenoids. Answers that mention chemicals without naming a class usually sit on half marks.
动物非特异防御 皮肤与黏膜
动物有若干不区分病原体种类的防线。完整皮肤是物理屏障，汗液中的溶菌酶能分解细菌细胞壁。呼吸道黏膜分泌黏液，纤毛把裹住病原的黏液向上扫出。胃酸降低吞入病原的存活率，眼泪中的溶菌酶同样起作用。咳嗽、打喷嚏与呕吐属于排出反射。这些反应不针对特定抗原，所以称为非特异防御。它们与后面 T 细胞、B 细胞主导的反应要分开描述。
Animals rely on barriers that do not distinguish one pathogen from another. Intact skin blocks entry, and lysozyme in sweat and tears breaks down bacterial cell walls; mucus in the airways traps particles that cilia sweep upwards, while stomach acid destroys most swallowed organisms. Coughing, sneezing and vomiting expel material physically. None of these responses recognises a particular antigen, which is why they are classed as non-specific and kept apart from T and B cell activity.
血凝与炎症 组胺与血管通透性
血管破损后胶原暴露，血小板黏附并触发凝血级联，纤维蛋白网把血细胞与病原一起拦住。炎症的四个表现是红、热、肿、痛。受损处的肥大细胞释放组胺，小动脉舒张带来更多血流，毛细血管通透性上升让组织液渗出。这些变化让吞噬细胞更容易到达现场。很多学生以为炎症是病原体直接造成的，其实是机体反应本身带来了红与肿。
When a vessel tears, exposed collagen triggers platelet adhesion and the clotting cascade, and a fibrin mesh traps blood cells along with any pathogen. Inflammation shows as redness, heat, swelling and pain: histamine from mast cells dilates arterioles, raising blood flow, and makes capillaries leakier so tissue fluid accumulates. The result is easier access for phagocytes. Redness and swelling come from the response itself rather than from the pathogen directly.
吞噬作用的五个步骤
吞噬细胞主要是中性粒细胞与巨噬细胞。病原体释放的化学物质吸引吞噬细胞靠近，这一步叫趋化。吞噬细胞表面的受体识别病原体抗原并附着。细胞膜内陷把病原包进吞噬体。溶酶体与吞噬体融合，水解酶把病原分解。巨噬细胞随后把抗原片段呈递到细胞膜上，成为抗原呈递细胞。调理素能先包裹病原，让识别与吞噬的效率明显提高。
Neutrophils and macrophages carry out phagocytosis. Chemical signals from the pathogen draw the cell along a gradient, a process called chemotaxis; membrane receptors then bind antigens on the surface. The membrane invaginates to form a phagosome, a lysosome fuses with it, and hydrolytic enzymes digest the contents. Macrophages go further and display antigen fragments on their membrane, becoming antigen-presenting cells. Opsonins coat the pathogen first and make recognition far quicker.
克隆选择与克隆扩增
抗原呈递细胞把抗原交给辅助 T 细胞，辅助 T 细胞再激活相应的 B 细胞克隆。克隆选择的意思是只有受体与抗原匹配的那一小群淋巴细胞被选中。被选中的细胞随后克隆扩增，产生大量同源细胞。B 细胞分化为浆细胞，浆细胞专门分泌抗体。部分细胞成为记忆细胞，留到下一次感染。这里最容易丢分的是把克隆选择与克隆扩增写成同一件事。
Antigen-presenting cells hand antigen to helper T cells, which in turn activate the matching B cell clone. Clonal selection picks out only the lymphocytes whose receptors fit the antigen; clonal expansion then multiplies them into a large population of identical cells. B cells differentiate into plasma cells that secrete antibody, and some descendants become memory cells. Treating selection and expansion as one step is a reliable way to lose marks.
抗体的结构与三项功能
抗体是球蛋白，由两条重链与两条轻链组成，链间靠二硫键连接。每条链有一个可变区，氨基酸序列在不同抗体之间变化，抗原结合位点就在这里。恒定区决定抗体的类别与后续作用。抗体本身不能杀死病原体，它做三件事。凝集素把病原聚成团便于吞噬，调理素标记病原使其更易被识别，抗毒素中和细菌毒素。答题要写抗体与抗原结合，不要写抗体吃掉病原。
An antibody is a globular protein built from two heavy and two light chains held by disulfide bridges. Each chain carries a variable region whose amino acid sequence differs between antibodies, and this is where antigen binds; the constant region determines class and downstream action. Antibodies do not kill pathogens themselves. They agglutinate them into clumps, act as opsonins that flag them for phagocytes, and neutralise toxins. Writing that antibodies engulf pathogens is wrong.
初次应答与再次应答的曲线差别
第一次接触抗原时，抗体浓度上升慢、峰值低、维持时间短，主要是 IgM。同一抗原再次进入时，记忆细胞迅速增殖分化，抗体上升快、峰值高、持续久，以 IgG 为主。这条差别正是疫苗加强针的依据。曲线题常要求标出潜伏期、上升段与平台段，横轴是时间，纵轴是血液抗体浓度。常见的错误是把再次应答的快解释成病原体变弱，其实变化发生在免疫系统的准备状态。
A first exposure produces a slow, low and short-lived antibody response, dominated by IgM. On second exposure memory cells divide quickly, so antibody rises faster, reaches a higher peak and persists longer, with IgG dominant. This difference underpins booster doses. Graph questions usually ask for the latent period, the rising phase and the plateau, with time on the x axis and antibody concentration on the y axis. The speed comes from prepared memory cells, not from a weaker pathogen.
自然与人工 主动与被动的四种组合
免疫按来源分自然与人工，按机制分主动与被动。自然主动是感染后自身产生抗体，自然被动是母体抗体经胎盘或母乳传给婴儿。人工主动是接种疫苗后自身产生抗体，人工被动是直接注射现成抗体，例如抗蛇毒血清。主动免疫建立慢但维持久，被动免疫立刻起效但很快消失。判断时先问抗体从哪里来，再问是否由本人免疫系统制造。
Immunity is classified by origin, natural or artificial, and by mechanism, active or passive. Natural active immunity follows infection; natural passive immunity is antibody passed from mother to infant across the placenta or in milk. Artificial active immunity comes from vaccination, while artificial passive immunity means injecting ready-made antibody such as antivenom. Active immunity takes time to build but lasts, passive protection works at once and fades. Ask where the antibody came from, then whether the person's own immune system made it.
疫苗 群体免疫与抗原变异
疫苗的作用是让免疫系统在安全条件下完成初次应答，形成记忆细胞。接种率足够高时，病原体难以找到易感宿主，群体免疫由此成立。抗原变异会破坏这层保护。流感病毒的基因突变让表面抗原逐年改变，疟原虫在宿主体内反复更换表面抗原。自身免疫病则是免疫系统把自身组织当成外来抗原攻击。OCR 的 specification 把疫苗原理与流行病预防写在同一项，答题时两条要一起出现。
Vaccination lets the immune system complete a primary response safely and leave memory cells behind. Once enough people are immune, a pathogen struggles to reach susceptible hosts, which is herd immunity. Antigenic variation undermines that protection: influenza accumulates mutations that alter surface antigens year by year, and Plasmodium switches surface antigens during infection. Autoimmune disease is the immune system attacking its own tissue as foreign. The specification pairs vaccination principles with epidemic prevention, so both belong in the answer.
抗生素的机制 益处与抗性风险
抗生素针对细菌，对病毒无效。β-内酰胺类抑制细胞壁合成，使细菌在低渗环境中破裂。氨基糖苷类与四环素类作用于核糖体，阻断蛋白质合成。抗生素滥用的后果是抗性菌株被筛选出来，例如耐甲氧西林金黄色葡萄球菌。OCR 要求讨论使用抗生素的益处与风险，益处是快速控制细菌感染，风险是耐药性与肠道菌群失衡。课程里还提到药物来源包括植物与微生物，青霉素来自青霉菌。
Antibiotics act on bacteria and do nothing against viruses. Beta-lactams block cell wall synthesis so the cell bursts in hypotonic surroundings, while aminoglycosides and tetracyclines bind ribosomes and stop protein synthesis. Overuse selects resistant strains, with MRSA the standard example. The specification asks for benefits and risks: rapid control of bacterial infection on one side, resistance and disruption of gut flora on the other. Medicines also come from plants and microorganisms, penicillin being the classic fungal source.
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