Immune System for A-Level CIE Biology | A-Level CIE 生物：免疫系统考点精讲

📚 Immune System for A-Level CIE Biology | A-Level CIE 生物：免疫系统考点精讲

The immune system is a complex network of cells, tissues, and organs that defends the body against pathogens, including bacteria, viruses, fungi, and parasites. In the CIE A-Level Biology syllabus, understanding both non‑specific (innate) and specific (adaptive) immunity is crucial. Students must be able to distinguish between humoral and cell‑mediated responses, describe the roles of B lymphocytes, T lymphocytes, antibodies, and memory cells, and relate these concepts to vaccination, immunological memory, and disorders such as autoimmunity.

免疫系统是一个由细胞、组织与器官构成的复杂网络，负责抵御细菌、病毒、真菌和寄生虫等病原体。在 CIE A-Level 生物学考纲中，理解非特异性（先天）免疫和特异性（适应性）免疫至关重要。考生需要能够区分体液免疫与细胞介导免疫，描述 B 淋巴细胞、T 淋巴细胞、抗体和记忆细胞的作用，并将这些概念与疫苗接种、免疫记忆以及自身免疫等疾病联系起来。

1. Overview of Defence Mechanisms | 防御机制概述

The human body has three lines of defence against pathogens. The first line consists of physical and chemical barriers such as skin, mucous membranes, and secretions like tears and stomach acid. The second line is non‑specific cellular and chemical responses, including inflammation, phagocytosis, and antimicrobial proteins. The third line is the specific immune response involving lymphocytes and the production of antibodies.

人体对病原体有三道防线。第一道防线由物理和化学屏障组成，如皮肤、黏膜以及泪液和胃酸等分泌物。第二道防线是炎症、吞噬作用和抗菌蛋白等非特异性细胞和化学反应。第三道防线是由淋巴细胞参与的、能产生抗体的特异性免疫应答。

2. Non‑Specific (Innate) Immunity | 非特异性（先天）免疫

Innate immunity provides immediate, general protection. Key components include phagocytes such as neutrophils and macrophages. Neutrophils are the first to arrive at infection sites; they engulf pathogens and die forming pus. Macrophages are larger, longer‑lived cells that engulf pathogens and present antigens on their surface to activate the specific immune system. Natural killer cells destroy virus‑infected cells and tumour cells.

先天免疫提供即时的、通用的保护。主要成分包括中性粒细胞和巨噬细胞等吞噬细胞。中性粒细胞最先到达感染部位，它们吞噬病原体后死亡，形成脓液。巨噬细胞体积更大、寿命更长，能够吞噬病原体并将抗原呈递在其表面，以激活特异性免疫系统。自然杀伤细胞则摧毁病毒感染细胞和肿瘤细胞。

Inflammation is a hallmark of innate immunity. Damaged tissue releases histamine, causing vasodilation and increased capillary permeability. This brings more phagocytes and plasma proteins to the site, causing redness, heat, swelling, and pain. Fever, triggered by pyrogens, inhibits pathogen growth and speeds up immune reactions.

炎症是先天免疫的标志。受损组织释放组胺，引起血管舒张和毛细血管通透性增加。这使得更多吞噬细胞和血浆蛋白到达感染部位，导致红、热、肿、痛。由热原质引发的发热能抑制病原体生长并加速免疫反应。

3. Antigens and Self / Non‑Self Recognition | 抗原与自身/非自身识别

Antigens are molecules (usually proteins or polysaccharides) on the surface of pathogens, cells, or toxins that are recognised by the immune system as foreign. The body distinguishes self‑cells from non‑self by the presence of major histocompatibility complex (MHC) molecules. In humans, these are called human leukocyte antigens (HLAs). Self‑cells display self‑antigens on MHC class I molecules, which are ignored by the immune system. Cells that present foreign antigens on MHC class I or II molecules trigger an immune response.

抗原是位于病原体、细胞或毒素表面的分子（通常是蛋白质或多糖），被免疫系统识别为外来物。机体通过主要组织相容性复合体（MHC）分子的存在来区分自身细胞与非自身细胞。在人类中，这些分子称为人类白细胞抗原（HLA）。自身细胞在 MHC I 类分子上展示自身抗原，被免疫系统忽略。而在 MHC I 类或 II 类分子上展示外来抗原的细胞会引发免疫反应。

4. Lymphocytes: B Cells and T Cells | 淋巴细胞：B 细胞与 T 细胞

Lymphocytes are white blood cells responsible for specific immunity. They originate from stem cells in the bone marrow. B lymphocytes (B cells) mature in the bone marrow, while T lymphocytes (T cells) mature in the thymus gland. Each lymphocyte carries specific receptors on its surface that can bind to a complementary antigen. When a lymphocyte encounters its specific antigen, it becomes activated and undergoes clonal selection and expansion, producing many identical effector cells and memory cells.

淋巴细胞是负责特异性免疫的白细胞。它们来源于骨髓中的干细胞。B 淋巴细胞（B 细胞）在骨髓中成熟，而 T 淋巴细胞（T 细胞）在胸腺中成熟。每个淋巴细胞表面携带特定的受体，能与互补的抗原结合。当淋巴细胞遇到其特定抗原时，便被激活，经历克隆选择和克隆扩增，产生大量相同的效应细胞和记忆细胞。

5. Cell‑Mediated Immunity | 细胞介导免疫

Cell‑mediated immunity involves T lymphocytes and is effective against intracellular pathogens, such as viruses that have entered host cells, as well as cancer cells and transplanted tissues. When a macrophage (or dendritic cell) engulfs a pathogen, it processes the antigens and presents them on its MHC class II molecules. This antigen‑presenting cell (APC) migrates to a lymph node and activates helper T cells (Tₕ cells) with complementary receptors.

细胞介导免疫涉及 T 淋巴细胞，对细胞内病原体有效，比如已侵入宿主细胞的病毒，以及癌细胞和移植组织。当巨噬细胞（或树突状细胞）吞噬病原体后，会加工抗原并将其呈递在 MHC II 类分子上。这个抗原呈递细胞（APC）迁移至淋巴结，激活具有互补受体的辅助性 T 细胞（Tₕ 细胞）。

Activated Tₕ cells secrete cytokines that stimulate the proliferation of more Tₕ cells, activate cytotoxic T cells (Tₖ cells), and stimulate B cells. Cytotoxic T cells recognise infected body cells displaying foreign antigens on MHC class I molecules and release perforin and granzymes, which induce apoptosis (programmed cell death). This destroys the infected cell before the pathogen can replicate further.

活化的 Tₕ 细胞分泌细胞因子，刺激更多 Tₕ 细胞增殖，激活细胞毒性 T 细胞（Tₖ 细胞），并刺激 B 细胞。细胞毒性 T 细胞识别在 MHC I 类分子上展示外来抗原的感染体细胞，释放穿孔素和颗粒酶，诱导细胞凋亡（程序性细胞死亡）。这在病原体进一步复制之前就摧毁了感染细胞。

6. Humoral Immunity | 体液免疫

Humoral immunity involves B lymphocytes and targets extracellular pathogens and toxins in body fluids. When a B cell with a complementary receptor binds to an antigen, it internalises the antigen, processes it, and presents fragments on its MHC class II molecules. A nearby activated Tₕ cell recognises the presented antigen and releases cytokines that fully activate the B cell. The B cell then undergoes clonal expansion and differentiates into plasma cells and memory B cells.

体液免疫涉及 B 淋巴细胞，针对存在于体液中的细胞外病原体和毒素。当带有互补受体的 B 细胞与抗原结合后，它会内吞抗原，进行加工，并将片段呈递在 MHC II 类分子上。邻近的活化 Tₕ 细胞识别呈递的抗原，并释放细胞因子以完全激活该 B 细胞。随后 B 细胞发生克隆扩增，分化成浆细胞和记忆 B 细胞。

Plasma cells are antibody factories that secrete millions of specific antibodies into the blood and lymph. These antibodies bind to antigens, neutralising pathogens, agglutinating (clumping) bacteria, precipitating toxins, and enhancing phagocytosis (opsonisation). The primary response peaks after about 10–14 days and produces a modest amount of antibodies, mainly IgM.

浆细胞是抗体的工厂，向血液和淋巴液中分泌数百万个特异性抗体。这些抗体与抗原结合，中和病原体，凝集细菌，沉淀毒素，并增强吞噬作用（调理作用）。初次应答在约 10–14 天后达到高峰，产生中等量的抗体，主要为 IgM。

7. Structure and Function of Antibodies | 抗体的结构与功能

Antibodies (immunoglobulins) are Y‑shaped proteins made up of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bonds. Each arm of the Y contains a variable region with unique amino acid sequences that form the antigen‑binding site, allowing specific recognition. The rest of the molecule, the constant region, determines the antibody class (IgG, IgM, IgA, IgE, IgD) and recruits other immune components such as phagocytes or complement proteins.

抗体（免疫球蛋白）是 Y 形蛋白，由四条多肽链组成：两条相同的重链和两条相同的轻链，通过二硫键连接。Y 形结构的每个臂含有一个可变区，其独特的氨基酸序列构成抗原结合位点，实现特异性识别。分子的其余部分即恒定区，决定了抗体的类别（IgG、IgM、IgA、IgE、IgD），并能够招募吞噬细胞或补体蛋白等其他免疫组分。

Functions include neutralisation (blocking pathogen binding sites), agglutination (clumping pathogens for easier phagocytosis), precipitation (making soluble toxins insoluble), opsonisation (coating pathogens to enhance phagocytosis), and activation of the complement system (a cascade of proteins that leads to lysis of microbes).

抗体功能包括中和作用（阻断病原体结合位点）、凝集作用（使病原体聚集以便于吞噬）、沉淀作用（使可溶性毒素变为不溶）、调理作用（包裹病原体以增强吞噬）以及激活补体系统（一系列蛋白质级联反应，最终导致微生物裂解）。

8. Primary and Secondary Immune Responses | 初次与二次免疫应答

The first exposure to an antigen triggers the primary response, which is slow (lag phase of several days) because few B and T cells exist with complementary receptors. After the infection resolves, memory B cells and memory T cells remain in the body for years or decades. These memory cells are long‑lived and can respond faster and more vigorously upon re‑exposure to the same antigen.

第一次接触抗原引发初次应答，该应答较慢（有数天的滞后期），因为带有互补受体的 B 细胞和 T 细胞很少。感染消退后，记忆 B 细胞和记忆 T 细胞在体内留存数年或数十年。这些记忆细胞寿命长，再次接触相同抗原时能更快、更强烈地作出应答。

The secondary response has a shorter lag phase, reaches a higher antibody concentration (mainly IgG), and lasts longer. This is the basis of immunological memory and explains why we usually do not suffer from the same disease twice. Vaccination exploits this principle by priming the immune system with harmless forms of an antigen.

二次应答的滞后期更短，抗体浓度达到更高水平（主要是 IgG），且持续时间更长。这正是免疫记忆的基础，也解释了为什么我们通常不会两次患上同一种疾病。疫苗接种正是利用这一原理，用无害的抗原形式预先激活免疫系统。

9. Active and Passive Immunity | 主动免疫与被动免疫

Active immunity results when the body produces its own antibodies after exposure to an antigen, either through natural infection or vaccination. It takes time to develop but provides long‑lasting protection because memory cells are generated. Passive immunity involves receiving antibodies produced by another organism. This occurs naturally when maternal antibodies cross the placenta or are present in breast milk, and artificially through injection of antiserum (e.g., tetanus antitoxin). Passive immunity is immediate but temporary, as the antibodies are eventually degraded and no memory cells are formed.

主动免疫是机体在接触抗原后（通过自然感染或疫苗接种）自身产生抗体而形成的。它需要一定时间建立，但因生成记忆细胞而提供长期保护。被动免疫则是接受其他生物体产生的抗体。这可以通过胎盘转移母体抗体或母乳喂养自然获得，也可以通过注射抗血清（如破伤风抗毒素）人工获得。被动免疫效果立竿见影，但只是暂时的，因为抗体会被最终降解，且不形成记忆细胞。

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

Vaccines contain antigens that have been attenuated (weakened), inactivated, or are subunit/toxoid preparations. They stimulate a primary response and the formation of memory cells without causing the disease. Booster doses are often required to maintain high levels of memory cells. Herd immunity occurs when a sufficient proportion of a population is immune (through vaccination or prior infection), reducing the spread of the pathogen and protecting unvaccinated individuals.

疫苗含有减毒（削弱）、灭活的抗原，或者是亚单位/类毒素制剂。它们刺激初次应答和记忆细胞的形成，而不会引发疾病。通常需要加强剂次以维持高水平的记忆细胞。当人群中足够比例的人具有免疫力（通过疫苗接种或既往感染）时，就会产生群体免疫，从而减少病原体的传播，并保护未接种者。

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

Sometimes the immune system fails to distinguish self from non‑self, resulting in autoimmune diseases. Examples include Type 1 diabetes (destruction of insulin‑producing β cells by cytotoxic T cells), rheumatoid arthritis (inflammation of joints), and multiple sclerosis (destruction of myelin sheaths). Allergies are hypersensitive responses to normally harmless environmental substances (allergens), such as pollen or peanuts. They involve IgE antibodies and the release of large amounts of histamine from mast cells, causing symptoms from mild (sneezing, itching) to severe anaphylaxis.

有时免疫系统无法区分自身与非自身，导致自身免疫病。例子包括 1 型糖尿病（细胞毒性 T 细胞破坏产生胰岛素的 β 细胞）、类风湿关节炎（关节炎症）和多发性硬化（髓鞘被破坏）。过敏是对通常无害的环境物质（过敏原，如花粉或花生）产生的超敏反应。其涉及 IgE 抗体和肥大细胞释放大量组胺，引起从轻微（打喷嚏、瘙痒）到严重过敏性休克等症状。

12. Key Examination Tips for CIE A-Level | CIE A-Level 考试要点提示

In CIE examinations, be precise with terminology. Use ‘phagocytosis’ and describe the steps: engulfment, formation of phagocytic vacuole, fusion with lysosomes, digestion, and antigen presentation. Distinguish clearly between B cell and T cell activation pathways. When describing antibody structure, mention variable and constant regions, light and heavy chains, and disulfide bridges. Link the primary and secondary responses to vaccination, and explain ethical considerations of vaccine use where required. Practice diagram questions on antibody structure and graph interpretation of antibody concentration over time.

在 CIE 考试中，术语必须精准。使用“吞噬作用”一词并描述步骤：包裹、形成吞噬泡、与溶酶体融合、消化以及抗原呈递。清楚区分 B 细胞和 T 细胞的激活途径。在描述抗体结构时，要提及可变区与恒定区、轻链和重链以及二硫键。将初次和二次应答与疫苗接种联系起来，并按要求解释疫苗使用的伦理考量。练习抗体结构示意图题以及抗体浓度随时间变化图的解读。

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Immune System for A-Level CIE Biology | A-Level CIE 生物：免疫系统 考点精讲