A-Level生物 免疫系统 细胞免疫 体液免疫

A-Level Biology: The Immune System ： Cellular and Humoral Immunity

1. Introduction to the Immune System

The immune system is the body’s defence network against pathogens ： organisms or particles that cause disease, including bacteria, viruses, fungi, and parasites. It consists of a complex array of cells, tissues, and molecules that work together to recognise and eliminate foreign invaders while distinguishing them from the body’s own healthy cells. A-Level biology focuses on the distinction between non-specific (innate) and specific (adaptive) immunity, and within adaptive immunity, the complementary roles of cell-mediated and humoral responses. 免疫系统是人体对抗病原体的防御网络。病原体是引起疾病的生物或颗粒，包括细菌、病毒、真菌和寄生虫。免疫系统由一系列复杂的细胞、组织和分子组成，它们协同工作，识别并清除外来入侵者，同时将其与人体自身的健康细胞区分开来。A-Level生物学重点关注非特异性（先天）免疫和特异性（适应性）免疫的区别，以及在适应性免疫中，细胞免疫和体液免疫的互补作用。

2. Non-Specific (Innate) Defences

Innate immunity provides the first line of defence and responds to all pathogens in the same way without prior exposure. Physical barriers include the skin ： a tough, impermeable keratinised layer ： and mucous membranes lining the respiratory, digestive, and urogenital tracts. Chemical barriers include lysozyme (an enzyme in tears and saliva that breaks down bacterial cell walls), stomach acid (HCl at pH 1-2, which denatures proteins and kills most ingested microorganisms), and sebum (an oily secretion from sebaceous glands that inhibits microbial growth). The inflammatory response is also non-specific: damaged cells release histamine, causing vasodilation and increased capillary permeability, which brings more phagocytes to the site of infection. 先天免疫提供第一道防线，对所有病原体以相同方式作出反应，无需预先接触。物理屏障包括皮肤：一层坚韧、不透水的角质化层：以及内衬于呼吸道、消化道和泌尿生殖道的粘膜。化学屏障包括溶菌酶（眼泪和唾液中的一种酶，可分解细菌细胞壁）、胃酸（pH 1-2的盐酸，可使蛋白质变性并杀死大多数摄入的微生物）以及皮脂（皮脂腺分泌的油性物质，可抑制微生物生长）。炎症反应也是非特异性的：受损细胞释放组胺，引起血管舒张和毛细血管通透性增加，从而将更多吞噬细胞带到感染部位。

3. Phagocytosis

Phagocytosis is the process by which phagocytes (primarily neutrophils and macrophages) engulf and destroy pathogens. It begins with chemotaxis ： the phagocyte is attracted to the pathogen by chemical signals from damaged cells or the pathogen itself. The phagocyte attaches to the pathogen via surface receptors, then extends pseudopodia around it, engulfing the microorganism into a membrane-bound vesicle called a phagosome. Lysosomes within the phagocyte fuse with the phagosome, releasing hydrolytic enzymes and reactive oxygen species that digest the pathogen. The harmless breakdown products are then expelled by exocytosis, or in the case of macrophages, fragments of the pathogen’s antigens are presented on the cell surface via MHC class II molecules ： a critical bridge to adaptive immunity. 吞噬作用是吞噬细胞（主要是中性粒细胞和巨噬细胞）吞噬并消灭病原体的过程。它始于趋化作用：吞噬细胞被受损细胞或病原体本身释放的化学信号吸引到病原体处。吞噬细胞通过表面受体附着到病原体上，然后伸出伪足将其包围，将微生物吞入一个称为吞噬体的膜结合囊泡中。吞噬细胞内的溶酶体与吞噬体融合，释放水解酶和活性氧物质来消化病原体。无害的分解产物随后通过胞吐作用排出，或者在巨噬细胞的情况下，病原体抗原的片段通过MHC II类分子呈递在细胞表面：这是通向适应性免疫的关键桥梁。

4. Antigens and Self/Non-Self Recognition

An antigen is any molecule (usually a protein or polysaccharide) that can trigger an immune response by binding to specific receptors on lymphocytes. Antigens are typically found on the surface of pathogens, but they can also be present on cancer cells, transplanted tissues, or even harmless substances like pollen (in allergic individuals). The immune system distinguishes self from non-self through the major histocompatibility complex (MHC) ： cell-surface proteins that display peptide fragments. Every nucleated cell in the body expresses MHC class I molecules, which present samples of intracellular proteins. If a cell is infected by a virus or becomes cancerous, abnormal peptides are displayed, flagging the cell for destruction by cytotoxic T cells. Professional antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells also express MHC class II molecules, which are essential for activating helper T cells. 抗原是任何能够通过与淋巴细胞上的特异性受体结合而触发免疫反应的分子（通常是蛋白质或多糖）。抗原通常存在于病原体表面，但也可能存在于癌细胞、移植组织，甚至无害物质如花粉上（在过敏个体中）。免疫系统通过主要组织相容性复合体（MHC）：展示肽片段的细胞表面蛋白：来区分自我与非自我。体内每个有核细胞都表达MHC I类分子，它们呈递细胞内蛋白质的样本。如果细胞被病毒感染或癌变，就会展示异常肽段，标记该细胞以供细胞毒性T细胞消灭。专业抗原呈递细胞（APC）如树突状细胞、巨噬细胞和B细胞还表达MHC II类分子，这对于激活辅助T细胞至关重要。

5. Cell-Mediated Immunity: T Lymphocytes

Cell-mediated immunity involves T lymphocytes (T cells) and is particularly effective against intracellular pathogens ： viruses that have invaded host cells, certain bacteria (e.g., Mycobacterium tuberculosis), and cancer cells. T cells mature in the thymus gland, where they undergo positive and negative selection to ensure they can recognise self-MHC but do not react strongly to self-antigens. There are two main types: helper T cells (CD4+) and cytotoxic T cells (CD8+). When an APC presents a foreign antigen on MHC class II, a helper T cell with a complementary T-cell receptor (TCR) binds to it. This interaction, along with co-stimulatory signals, activates the helper T cell, causing it to proliferate by clonal expansion and differentiate into effector cells that secrete cytokines ： signalling molecules that orchestrate the broader immune response, including the activation of B cells and cytotoxic T cells. 细胞免疫涉及T淋巴细胞，对细胞内病原体特别有效：包括侵入宿主细胞的病毒、某些细菌（如结核分枝杆菌）和癌细胞。T细胞在胸腺中成熟，在此过程中经历阳性和阴性选择，以确保它们能识别自身MHC但不会对自身抗原产生强烈反应。主要有两种类型：辅助T细胞（CD4+）和细胞毒性T细胞（CD8+）。当APC在MHC II类分子上呈递外来抗原时，具有互补T细胞受体（TCR）的辅助T细胞与之结合。这种相互作用以及共刺激信号激活辅助T细胞，使其通过克隆扩增增殖并分化为效应细胞，分泌细胞因子：协调更广泛免疫反应的信号分子，包括激活B细胞和细胞毒性T细胞。

6. Cytotoxic T Cells and Perforin/Granzyme

Cytotoxic T cells (Tc cells) directly kill infected or abnormal cells. Once activated ： a process that requires both antigen presentation by an infected cell via MHC class I and cytokine stimulation from helper T cells ： the cytotoxic T cell releases cytotoxic granules containing perforin and granzymes. Perforin creates pores in the target cell’s membrane, while granzymes enter through these pores and trigger apoptosis (programmed cell death) by activating caspases. This targeted destruction mechanism ensures that only infected cells are eliminated, leaving healthy neighbouring cells intact. The cytotoxic T cell itself survives the encounter and can go on to kill multiple target cells. After the infection is cleared, most effector T cells undergo apoptosis, but a small population of memory T cells persists, enabling a faster and stronger response upon re-infection. 细胞毒性T细胞直接杀死受感染或异常的细胞。一旦被激活：这一过程既需要受感染细胞通过MHC I类分子呈递抗原，也需要辅助T细胞的细胞因子刺激：细胞毒性T细胞释放含有穿孔素和颗粒酶的细胞毒性颗粒。穿孔素在靶细胞膜上形成孔洞，而颗粒酶通过这些孔洞进入并触发细胞凋亡（程序性细胞死亡），通过激活半胱天冬酶实现。这种靶向破坏机制确保仅清除受感染细胞，使健康的邻近细胞保持完整。细胞毒性T细胞本身在遭遇后存活下来，并可继续杀死多个靶细胞。感染清除后，大多数效应T细胞发生凋亡，但少量记忆T细胞持续存在，使再感染时能产生更快更强的反应。

7. Humoral Immunity: B Lymphocytes and Antibodies

Humoral immunity is mediated by B lymphocytes (B cells) and targets extracellular pathogens ： bacteria, toxins, and viruses in the blood and lymph before they enter host cells. B cells mature in the bone marrow and each carries a unique B-cell receptor (BCR), which is essentially a membrane-bound antibody specific to one antigen epitope. When a B cell encounters its complementary antigen, it engulfs it by receptor-mediated endocytosis, processes it, and presents antigen fragments on MHC class II molecules. A helper T cell activated by the same antigen (via an APC) then binds to this MHC-antigen complex and releases cytokines ： most importantly interleukin-4 (IL-4) ： that stimulate the B cell to undergo clonal expansion. This is known as T-dependent activation; most protein antigens require this pathway. Some antigens, such as polysaccharides with repeating subunits, can activate B cells directly without T-cell help ： this is T-independent activation, though it produces a weaker response and does not generate memory cells. 体液免疫由B淋巴细胞介导，针对细胞外病原体：血液和淋巴液中进入宿主细胞之前的细菌、毒素和病毒。B细胞在骨髓中成熟，每个细胞携带独特的B细胞受体（BCR），本质上是针对一个抗原表位的膜结合抗体。当B细胞遇到其互补抗原时，通过受体介导的内吞作用将其吞噬，加工处理，并在MHC II类分子上呈递抗原片段。然后，由相同抗原（通过APC）激活的辅助T细胞与这个MHC-抗原复合物结合，释放细胞因子：最重要的是白细胞介素-4（IL-4）：刺激B细胞进行克隆扩增。这称为T依赖性激活；大多数蛋白质抗原需要此途径。某些抗原，如具有重复亚基的多糖，可以在没有T细胞帮助的情况下直接激活B细胞：这是T非依赖性激活，但产生的反应较弱且不产生记忆细胞。

8. Plasma Cells and Antibody Production

After activation and clonal expansion, most B cells differentiate into plasma cells ： antibody factories that secrete large quantities of a specific antibody (up to 2000 molecules per second). Antibodies, also known as immunoglobulins, are Y-shaped glycoproteins composed of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bridges. Each antibody has two identical antigen-binding sites at the tips of the variable regions, which are specific to one epitope. The constant region determines the antibody’s class (IgM, IgG, IgA, IgE, or IgD) and its effector function. Antibodies neutralise pathogens by several mechanisms: agglutination (clumping pathogens together for easier phagocytosis), opsonisation (coating pathogens to enhance phagocyte recognition), neutralisation (blocking pathogen binding sites), and complement activation (triggering the complement cascade that leads to lysis of bacterial cells). A small proportion of activated B cells become memory B cells instead of plasma cells, persisting in the body for years and enabling the secondary immune response. 激活和克隆扩增后，大多数B细胞分化为浆细胞：抗体工厂，大量分泌特定抗体（每秒多达2000个分子）。抗体，也称为免疫球蛋白，是由四条多肽链组成的Y形糖蛋白：两条相同的重链和两条相同的轻链，通过二硫键连接在一起。每个抗体在可变区的末端有两个相同的抗原结合位点，对一个表位具有特异性。恒定区决定抗体的类别（IgM、IgG、IgA、IgE或IgD）及其效应功能。抗体通过多种机制中和病原体：凝集作用（将病原体聚集在一起以便于吞噬）、调理作用（包被病原体以增强吞噬细胞识别）、中和作用（阻断病原体结合位点）以及补体激活（触发导致细菌细胞裂解的补体级联反应）。一小部分激活的B细胞成为记忆B细胞而非浆细胞，在体内持续存在多年，并实现二次免疫应答。

9. Primary and Secondary Immune Responses

The primary immune response occurs when the body first encounters a specific pathogen. There is a lag phase of several days while the appropriate B and T cells are selected and undergo clonal expansion. Antibody levels rise slowly, peaking after about 10-14 days, and the first antibodies produced are typically IgM. Symptoms of the disease often appear during this lag period. After the infection is cleared, antibody levels decline, but memory B and T cells remain. The secondary immune response occurs upon re-exposure to the same pathogen. Memory cells recognise the antigen immediately, bypassing the lengthy selection process. The lag phase is much shorter (1-2 days), antibody production is faster and more abundant (predominantly IgG), and the antibody concentration reaches a much higher peak. Most importantly, the response is so rapid that the pathogen is eliminated before symptoms develop ： this is the principle underlying vaccination. 初次免疫应答发生在身体首次遇到特定病原体时。有一个持续数天的滞后期，在此期间选择合适的B细胞和T细胞并进行克隆扩增。抗体水平缓慢上升，大约10-14天后达到峰值，最初产生的抗体通常是IgM。疾病症状常在此滞后期出现。感染清除后，抗体水平下降，但记忆B细胞和T细胞仍然存在。二次免疫应答发生在再次接触同一病原体时。记忆细胞立即识别抗原，绕过了漫长的选择过程。滞后期大大缩短（1-2天），抗体产生更快、更丰富（主要是IgG），抗体浓度达到更高的峰值。最重要的是，反应如此迅速，以至于病原体在症状发展之前就被清除了：这正是疫苗接种的基本原理。

10. Vaccination and Herd Immunity

Vaccination is the deliberate exposure of the immune system to a harmless form of a pathogen (or its antigens) to stimulate the production of memory cells without causing disease. Vaccines can be live attenuated (weakened but alive, e.g., MMR), inactivated (killed, e.g., polio), subunit (containing only specific antigens, e.g., hepatitis B), or mRNA-based (e.g., some COVID-19 vaccines, which instruct host cells to produce the antigen protein). The resulting memory cell population provides long-term immunity. Herd immunity occurs when a sufficiently high proportion of a population is immune (either through vaccination or prior infection), making person-to-person transmission unlikely and protecting those who cannot be vaccinated (e.g., immunocompromised individuals). The threshold for herd immunity depends on the pathogen’s basic reproduction number (R₀); for measles (R₀ ≈ 12-18), approximately 95% coverage is needed, while for influenza (R₀ ≈ 1.3), about 25% suffices. Ethical considerations include balancing individual choice against public health, ensuring equitable vaccine distribution globally, and maintaining public trust through transparent communication about risks and benefits. 疫苗接种是有意将免疫系统暴露于无害形式的病原体（或其抗原），以刺激记忆细胞的产生而不引起疾病。疫苗可以是减毒活疫苗（减弱但存活，如MMR）、灭活疫苗（杀死，如脊髓灰质炎）、亚单位疫苗（仅含特定抗原，如乙型肝炎）或mRNA疫苗（如某些COVID-19疫苗，指导宿主细胞产生抗原蛋白）。由此产生的记忆细胞群提供长期免疫力。群体免疫发生在人口中足够高比例的人具有免疫力（通过接种疫苗或先前感染）的情况下，使人与人之间的传播变得不可能，并保护那些无法接种疫苗的人（如免疫功能低下者）。群体免疫的阈值取决于病原体的基本繁殖数（R₀）；对于麻疹（R₀ ≈ 12-18），需要约95%的覆盖率，而对于流感（R₀ ≈ 1.3），约25%就足够了。伦理考虑包括在个人选择与公共卫生之间取得平衡、确保全球疫苗公平分配，以及通过透明沟通风险与益处来维持公众信任。

11. Active vs Passive Immunity

Active immunity results from the body’s own immune system producing antibodies and memory cells in response to antigen exposure, whether through natural infection or vaccination. It provides long-lasting protection because memory cells persist. Passive immunity, by contrast, involves the transfer of pre-formed antibodies from another source. Natural passive immunity occurs when maternal antibodies cross the placenta (IgG) or are passed in breast milk (IgA), protecting newborns during the first months of life. Artificial passive immunity involves the injection of antibodies ： for example, antivenom for snake bites or tetanus antitoxin. While passive immunity provides immediate protection, it is temporary (lasting weeks to months) because the antibodies are eventually broken down and no memory cells are generated. Understanding this distinction is important for evaluating treatment strategies: post-exposure prophylaxis for rabies combines passive (antibody injection) and active (vaccination) approaches to provide both immediate and long-term protection. 主动免疫是身体自身免疫系统在接触抗原（无论是通过自然感染还是疫苗接种）后产生抗体和记忆细胞的结果。它提供持久的保护，因为记忆细胞持续存在。相比之下，被动免疫涉及从其他来源转移预先形成的抗体。天然被动免疫发生在母体抗体穿过胎盘（IgG）或通过母乳传递（IgA）时，在新生儿出生后的最初几个月提供保护。人工被动免疫涉及注射抗体：例如蛇咬伤的抗蛇毒血清或破伤风抗毒素。虽然被动免疫提供即时保护，但它是暂时的（持续数周至数月），因为抗体最终会被分解，且不产生记忆细胞。理解这一区别对于评估治疗策略很重要：狂犬病的暴露后预防结合了被动（抗体注射）和主动（疫苗接种）方法，以提供即时和长期保护。

12. Exam Tips and Common Misconceptions

A common exam misconception is confusing the roles of T and B cells: T cells handle cell-mediated immunity against intracellular pathogens, while B cells produce antibodies for humoral immunity against extracellular pathogens. Note that helper T cells are essential for most B-cell activation (T-dependent pathway), so the two systems are interconnected, not isolated. Students often misidentify phagocytes as part of specific immunity; remember that phagocytosis is non-specific and part of the innate response ： though macrophages also function as APCs to bridge innate and adaptive immunity. In data-analysis questions, be prepared to interpret graphs showing primary vs secondary antibody response curves, distinguishing IgM from IgG by production timing and abundance. When describing the immune response in extended answers, always specify cell types precisely (e.g., “helper T cell” rather than just “T cell”) and link each step to its purpose. Finally, note that antibiotics are ineffective against viruses; this is a classic trick question that tests understanding of the difference between bacterial and viral infections. 常见的考试误解是混淆T细胞和B细胞的作用：T细胞负责针对细胞内病原体的细胞免疫，而B细胞产生抗体，负责针对细胞外病原体的体液免疫。请注意，辅助T细胞对大多数B细胞激活（T依赖性途径）至关重要，因此这两个系统是相互关联的，而不是孤立的。学生常将吞噬细胞误认为是特异性免疫的一部分；请记住，吞噬作用是非特异性的，属于先天反应的一部分：尽管巨噬细胞也可作为APC，连接先天免疫和适应性免疫。在数据分析题中，准备解读显示初次和二次抗体反应曲线的图表，从产生时间和丰度上区分IgM和IgG。在扩展答案中描述免疫反应时，始终精确指定细胞类型（如”辅助T细胞”而非仅”T细胞”），并将每个步骤与其目的联系起来。最后，请注意抗生素对病毒无效；这是一个经典的陷阱题，测试对细菌和病毒感染区别的理解。