📚 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。
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。
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。
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。
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。
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。
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. |
