A-Level Biology: Immune System Key Concepts | A-Level 生物：免疫系统 考点精讲

📚 A-Level Biology: Immune System Key Concepts | A-Level 生物：免疫系统 考点精讲

The immune system is a complex network of cells, tissues, and molecules that defends the body against pathogens. For A-Level Biology, you must understand both the non‑specific (innate) and specific (adaptive) responses, the roles of different white blood cells, the distinction between humoral and cell‑mediated immunity, and how vaccination exploits immunological memory. This article covers every core examination point with bilingual explanations.

免疫系统是一个由细胞、组织和分子构成的复杂网络，负责抵御病原体。在 A-Level 生物学中，你必须掌握非特异性（固有）与特异性（适应性）免疫应答、不同白细胞的作用、体液免疫与细胞免疫的区别，以及疫苗如何利用免疫记忆。本文以中英双语详解所有核心考点。

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1. Pathogens and the First Line of Defence | 病原体与第一道防线

Pathogens are disease‑causing microorganisms, including bacteria, viruses, fungi, and protists. The body’s first line of defence consists of physical and chemical barriers that prevent entry.

病原体是导致疾病的微生物，包括细菌、病毒、真菌和原生生物。人体的第一道防线由阻止病原体进入的物理与化学屏障组成。

• Physical barriers: intact skin, mucous membranes, cilia in airways, and the blood‑brain barrier.

  物理屏障：完整的皮肤、黏膜、气道中的纤毛以及血脑屏障。

• Chemical defences: lysozyme (in tears and saliva) digests bacterial cell walls; stomach acid (HCl) denatures pathogens; sebum on skin lowers pH.

  化学防御：溶菌酶（存在于泪液和唾液中）能分解细菌细胞壁；胃酸（HCl）使病原体变性；皮肤表面的皮脂降低 pH 值。

If a pathogen breaches these barriers, the innate immune system is activated immediately.

一旦病原体突破这些屏障，固有免疫系统会立即被激活。

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2. Innate Immune Response: Inflammation and Phagocytosis | 固有免疫应答：炎症与吞噬作用

The innate response is non‑specific and rapid. Key features are inflammation and the action of phagocytes, especially neutrophils and macrophages.

固有免疫是非特异性的快速应答。核心特征为炎症以及吞噬细胞（特别是中性粒细胞和巨噬细胞）的作用。

• Inflammation: damaged tissues release histamine, causing vasodilation and increased capillary permeability. This leads to redness, heat, swelling, and pain, and attracts more white blood cells to the site.

  炎症：受损组织释放组胺，引起血管舒张与毛细血管通透性增加，导致红、热、肿、痛，并吸引更多白细胞到达感染部位。

• Phagocytosis: phagocytes recognise non‑self molecules (e.g., lipopolysaccharides). The pathogen is engulfed into a phagosome, which fuses with a lysosome to form a phagolysosome. Enzymes and reactive oxygen species destroy the pathogen. Antigens from the pathogen are then presented on MHC Class II molecules (in macrophages and dendritic cells).

  吞噬作用：吞噬细胞识别非己分子（例如脂多糖）。病原体被包裹入吞噬体，与溶酶体融合形成吞噬溶酶体。酶和活性氧将病原体杀灭。病原体的抗原随后通过 MHC II 类分子（在巨噬细胞和树突状细胞表面）呈递。

Antigen presentation bridges the innate and adaptive responses.

抗原呈递是连接固有免疫和适应性免疫的桥梁。

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3. Adaptive Immunity: Specificity, Clonal Selection, and Memory | 适应性免疫：特异性、克隆选择与记忆

The adaptive immune response is specific to each antigen and involves B lymphocytes (humoral) and T lymphocytes (cell‑mediated). It relies on the process of clonal selection.

适应性免疫对每种抗原具有特异性，涉及 B 淋巴细胞（体液免疫）和 T 淋巴细胞（细胞免疫），并依赖于克隆选择过程。

• Each lymphocyte carries a unique receptor (B‑cell receptor or T‑cell receptor) specific for one antigen.

  每个淋巴细胞表面携带一个独特的受体（B 细胞受体或 T 细胞受体），对单一抗原具有特异性。

• When a matching antigen binds to the receptor, that lymphocyte is activated and undergoes rapid clonal expansion.

  当匹配的抗原与受体结合时，该淋巴细胞被激活并发生快速的克隆扩增。

• Some of the daughter cells become effector cells that fight the infection; others become long‑lived memory cells, providing lasting immunity.

  部分子代细胞成为效应细胞以对抗感染；另一部分成为长寿命的记忆细胞，提供持久免疫力。

This explains why secondary exposure to the same antigen triggers a faster, stronger response.

这解释了为何再次遇到同一抗原时，免疫应答会更快、更强。

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4. Cell‑Mediated Immunity: T Cells and Their Roles | 细胞免疫：T 细胞及其作用

Cell‑mediated immunity involves T lymphocytes, which mature in the thymus. They respond to antigens presented on MHC molecules rather than free antigens.

细胞免疫由 T 淋巴细胞介导，它们在胸腺中成熟。T 细胞识别的是由 MHC 分子呈递的抗原，而非游离抗原。

• Helper T cells (CD4⁺): activated by MHC Class II–antigen complexes on antigen‑presenting cells. They secrete cytokines (e.g., interleukins) that stimulate B cells, cytotoxic T cells, and macrophages.

  辅助性 T 细胞（CD4⁺）：由抗原呈递细胞表面 MHC II 类–抗原复合物激活。它们分泌细胞因子（如白细胞介素），激活 B 细胞、细胞毒性 T 细胞和巨噬细胞。

• Cytotoxic T cells (CD8⁺): activated mainly by cytokines and MHC Class I–antigen complexes on infected body cells. They release perforin and granzymes, inducing apoptosis of the infected cell.

  细胞毒性 T 细胞（CD8⁺）：主要被细胞因子以及受感染细胞表面 MHC I 类–抗原复合物激活。它们释放穿孔素和颗粒酶，诱导感染细胞凋亡。

MHC Class I is on all nucleated cells, while MHC Class II is only on professional antigen‑presenting cells.

MHC I 类分子存在于所有有核细胞表面，而 MHC II 类分子仅存在于专职抗原呈递细胞上。

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5. Humoral Immunity and B Cell Activation | 体液免疫与 B 细胞活化

Humoral immunity involves B lymphocytes, which mature in the bone marrow. Activated B cells differentiate into plasma cells that secrete antibodies.

体液免疫由 B 淋巴细胞介导，它们在骨髓中成熟。活化的 B 细胞分化为浆细胞，分泌抗体。

• T‑dependent antigens: most protein antigens. B cells internalise the antigen, present it on MHC Class II, and are then activated by helper T cells. This leads to class switching, affinity maturation, and memory B cell production.

  T 依赖性抗原：大多数蛋白质抗原。B 细胞内吞抗原后通过 MHC II 类分子呈递，随后在辅助性 T 细胞的帮助下活化，产生类别转换、亲和力成熟和记忆 B 细胞。

• T‑independent antigens: typically polysaccharides with repeating epitopes; they can cross‑link B‑cell receptors directly, but produce weaker memory and mainly IgM antibodies.

  T 非依赖性抗原：通常是带有重复表位的多糖，可直接交联 B 细胞受体，但产生的记忆较弱，抗体主要为 IgM 型。

Antibodies do not directly kill pathogens; they mark them for destruction (opsonisation) or neutralise toxins and viruses.

抗体并不直接杀死病原体，而是通过标记（调理作用）或中和毒素、病毒来促进清除。

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6. Antibody Structure and Function | 抗体的结构与功能

Antibodies (immunoglobulins) are Y‑shaped glycoproteins. Each monomer consists of two identical heavy chains and two identical light chains held together by disulfide bonds.

抗体（免疫球蛋白）是 Y 形的糖蛋白。每个单体由两条相同的重链和两条相同的轻链通过二硫键连接而成。

• Variable regions: tips of the Y, forming the antigen‑binding sites. Each B cell produces antibodies with a unique variable region.

  可变区：位于 Y 形两臂的末端，形成抗原结合位点。每个 B 细胞产生具有独特可变区的抗体。

• Constant regions: stem of the Y, determining the class (isotype): IgM, IgG, IgA, IgE, or IgD. Different classes have distinct effector functions.

  恒定区：Y 形的柄部，决定抗体的类别（同种型）：IgM、IgG、IgA、IgE 或 IgD。不同类别的抗体具有不同的效应功能。

Antibody Class	Function / Feature
IgM	First produced; pentamer; excellent at agglutination.
IgG	Most abundant in blood; crosses placenta; opsonisation + neutralisation.
IgA	Found in secretions (milk, saliva, tears); dimer; prevents mucosal attachment.
IgE	Involved in allergic responses and defence against parasites; binds to mast cells.

类 IgA 的中文描述：IgA 发现于分泌物（乳汁、唾液、泪液）中；二聚体；防止黏膜附着。

Understanding antibody structure is essential for explaining agglutination, neutralisation, and the basis of diagnostic tests like ELISA.

理解抗体结构对于解释凝集、中和作用以及 ELISA 等诊断检测的原理至关重要。

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7. Primary vs. Secondary Immune Response | 初次与再次免疫应答

The difference between the primary and secondary immune responses is a classic graph‑based exam question.

初次与再次免疫应答的区别是经典的图表分析考题。

• Primary response: on first exposure, there is a lag phase of several days. IgM is produced first, then IgG. The response is slower and smaller in magnitude.

  初次应答：首次接触抗原后，存在数天的滞后期。首先产生 IgM，而后 IgG；应答速度慢，抗体滴度较低。

• Secondary response: on re‑exposure, memory cells are rapidly activated. The lag phase is shorter, the antibody concentration rises faster and higher, and IgG dominates from the start.

  再次应答：再次接触抗原时，记忆细胞被迅速激活。滞后期更短，抗体浓度上升更快、更高，且从一开始就以 IgG 为主。

This principle underlies vaccination, where the first dose primes the system and booster doses reinforce the memory pool.

这一原理是疫苗接种的基础——初次剂量致敏免疫系统，加强剂量扩充记忆细胞池。

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8. Vaccination and Herd Immunity | 疫苗接种与群体免疫

Vaccines contain antigens from a pathogen (live‑attenuated, inactivated, subunit, or mRNA‑based) that trigger an adaptive immune response without causing disease.

疫苗含有来自病原体的抗原（减毒活疫苗、灭活疫苗、亚单位疫苗或 mRNA 疫苗），能在不引发疾病的情况下触发适应性免疫应答。

• Vaccination creates a pool of memory B and T cells specific to that pathogen, providing long‑term protection.

  疫苗接种产生对该病原体特异性的记忆 B 细胞和 T 细胞，提供长期保护。

• Herd immunity occurs when a high proportion of a population is immune (≥ threshold level), reducing the spread of the pathogen and protecting vulnerable individuals who cannot be vaccinated.

  群体免疫是指人群中免疫比例足够高（≥ 阈值），从而减少病原体传播，保护无法接种疫苗的弱势个体。

The threshold depends on the basic reproduction number (R₀): Threshold = 1 – (1/R₀).

群体免疫阈值取决于基本传染数 R₀：阈值 = 1 – (1/R₀)。

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9. Antigenic Variation and the Challenges to Immunity | 抗原变异与免疫挑战

Some pathogens evade the immune system through antigenic variation. The influenza virus undergoes antigenic drift (minor mutations in surface proteins haemagglutinin and neuraminidase) and antigenic shift (major genetic reassortment).

某些病原体通过抗原变异逃避免疫系统。流感病毒经历抗原漂移（表面蛋白血凝素和神经氨酸酶的微小突变）和抗原转变（重大基因重排）。

• Because pre‑existing memory cells no longer match the new antigens, annual re‑vaccination is needed for influenza.

  由于已有的记忆细胞不再匹配新抗原，因此每年都需要重新接种流感疫苗。

• Trypanosoma (sleeping sickness) regularly switches its surface glycoprotein coat, staying one step ahead of antibody responses.

  锥虫（昏睡病）定期更换表面糖蛋白外衣，永远领先于抗体应答一步。

This questions the feasibility of universal vaccines for such pathogens.

这对研制针对此类病原体的通用疫苗提出了挑战。

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10. Autoimmune Diseases and Allergies | 自身免疫病与过敏

When the immune system fails to distinguish self from non‑self, autoimmune disease or allergy can result.

当免疫系统无法区分“自己”与“非己”时，就会导致自身免疫病或过敏。

• Autoimmune disease: e.g., Type 1 diabetes (immune destruction of pancreatic β‑cells), rheumatoid arthritis. Often due to a breakdown of central or peripheral tolerance, or cross‑reactivity between pathogen and self antigens.

  自身免疫病：例如 1 型糖尿病（免疫系统破坏胰腺 β 细胞）、类风湿关节炎。常因中枢或外周耐受机制崩溃，或病原体与自身抗原的交叉反应所致。

• Allergy: an inappropriate IgE‑mediated response to harmless environmental antigens (allergens). Mast cells degranulate, releasing histamine, causing symptoms ranging from hay fever to anaphylaxis.

  过敏：对无害环境抗原（过敏原）产生的、不恰当的 IgE 介导应答。肥大细胞脱颗粒释放组胺，引起从花粉症到过敏性休克的一系列症状。

Anaphylaxis is a severe, systemic reaction that can be life‑threatening and requires adrenaline (epinephrine).

过敏性休克是一种严重的全身性反应，可能危及生命，需要使用肾上腺素。

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11. Monoclonal Antibodies in Medicine | 单克隆抗体在医学中的应用

Monoclonal antibodies are identical antibodies produced from a single clone of B cells (hybridoma technology). They are specific to a single epitope and have broad diagnostic and therapeutic uses.

单克隆抗体是由单一 B 细胞克隆（杂交瘤技术）产生的、完全相同的抗体，针对单一表位，具有广泛的诊断和治疗用途。

• Pregnancy tests: use monoclonal anti‑hCG antibodies to detect human chorionic gonadotrophin in urine.

  验孕：利用抗 hCG 单克隆抗体检测尿液中的人绒毛膜促性腺激素。

• ELISA tests: used to detect antibodies against HIV or other infections; rely on enzyme‑linked monoclonal antibodies for colour change.

  ELISA 检测：用于检测抗 HIV 等抗体；依赖酶联单克隆抗体显色。

• Cancer therapy: monoclonal antibodies can block growth factor receptors (e.g., Herceptin for HER2⁺ breast cancer) or deliver cytotoxic drugs specifically to tumour cells (antibody–drug conjugates).

  癌症治疗：单克隆抗体可阻断生长因子受体（例如赫赛汀用于 HER2 阳性乳腺癌），或作为抗体–药物偶联物将细胞毒性药物精准递送至肿瘤细胞。

Exam questions frequently ask you to describe the hybridoma method or interpret test‑strip results.

考题常要求描述杂交瘤技术方法，或解释试纸条检测结果。

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12. Key Exam Tips and Common Diagrams | 关键考试提示与常见示意图

A-Level examiners expect precise terminology and the ability to link concepts. Always use: antigen, antibody, MHC, phagocytosis, clonal selection, memory cells.

A-Level 考官要求使用精确的术语并具备概念串联能力。始终使用：抗原、抗体、MHC、吞噬作用、克隆选择、记忆细胞。

• Be ready to label diagrams of: antibody molecule (variable/constant regions, hinge region, disulfide bonds), phagocytosis steps, and the graph of primary vs. secondary response.

  准备好标注以下示意图：抗体分子（可变区/恒定区、铰链区、二硫键）、吞噬步骤、初次与再次应答曲线图。

• Compare humoral vs. cell‑mediated immunity clearly: table format often gains full marks.

  清晰比较体液免疫与细胞免疫：用表格形式常能获得满分。

• Link structure to function: e.g., flexible hinge region allows antibody to bind two antigens simultaneously; variable region diversity generated by V(D)J recombination.

  将结构与功能相联系：例如灵活的铰链区允许抗体同时结合两个抗原；可变区的多样性由 V(D)J 重组产生。

Reinforce your revision with past‑paper questions on vaccination, ELISA interpretation, and ethical issues surrounding monoclonal antibodies.

通过有关疫苗、ELISA 判读以及单克隆抗体伦理问题的历年真题巩固复习。

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