📚 The Immune System: Essential Revision for IB and OCR Biology | 免疫系统：IB与OCR生物核心考点精讲
The immune system is one of the most prominent topics in both IB and OCR Biology, bridging cell biology, biochemistry, and human physiology. This article breaks down the innate and adaptive defences, the roles of B and T lymphocytes, antibody structure, vaccination, monoclonal antibodies, and key disorders. Exam-focused explanations are paired to help you master the essential concepts and confidently tackle data-analysis and long-answer questions.
免疫系统是 IB 和 OCR 生物课程中最突出的主题之一，连接了细胞生物学、生物化学和人体生理学。本文拆解了先天性和适应性防御、B 与 T 淋巴细胞的作用、抗体结构、疫苗接种、单克隆抗体以及关键疾病。以考点为导向的中英对照讲解，帮助你掌握核心概念，自信应对数据分析和长篇问答。
1. Overview of the Immune System | 免疫系统概述
The immune system is a complex network of cells, tissues, and molecules that defends the body against pathogens such as bacteria, viruses, fungi, and parasites. It is traditionally divided into innate (non-specific) immunity and adaptive (specific) immunity. Innate immunity provides immediate but generic protection, while adaptive immunity develops slower but produces a highly specific response and immunological memory. Key organs include bone marrow, thymus, lymph nodes, and spleen.
免疫系统是由细胞、组织和分子组成的复杂网络，抵抗细菌、病毒、真菌和寄生虫等病原体。传统上分为先天（非特异性）免疫和适应性（特异性）免疫。先天免疫提供即时但通用的保护，而适应性免疫发育较慢，但能产生高度特异性的应答和免疫记忆。关键器官包括骨髓、胸腺、淋巴结和脾脏。
2. Non-Specific Defences (Innate Immunity) | 非特异性防御（先天免疫）
The first line of defence consists of physical and chemical barriers. The skin acts as a tough, waterproof physical barrier, while mucous membranes trap pathogens. Chemical defences include lysozyme in tears and saliva, which breaks down bacterial cell walls, and stomach acid (HCl) that denatures proteins and kills most ingested microbes. If pathogens breach these barriers, the second line of innate defence is activated, involving phagocytes, inflammation, and antimicrobial proteins.
第一道防线由物理和化学屏障组成。皮肤充当坚韧防水的物理屏障，黏膜则捕获病原体。化学防御包括眼泪和唾液中的溶菌酶，可分解细菌细胞壁，以及胃酸（HCl）使蛋白质变性并杀死大多数摄入的微生物。若病原体突破这些屏障，第二道先天防御启动，涉及吞噬细胞、炎症和抗微生物蛋白。
- Phagocytes (neutrophils and macrophages) engulf pathogens by phagocytosis and digest them with lysosomal enzymes.
- 吞噬细胞（中性粒细胞和巨噬细胞）通过吞噬作用吞入病原体，并用溶酶体酶消化它们。
- Inflammation increases blood flow and capillary permeability, recruiting immune cells to the site of infection. Histamine released by mast cells is a key mediator.
- 炎症增加血流和毛细血管通透性，将免疫细胞招募至感染部位。肥大细胞释放的组胺是关键介质。
- Interferons are proteins released by virus-infected cells that ‘interfere’ with viral replication in neighbouring cells.
- 干扰素是由病毒感染细胞释放的蛋白质，可“干扰”邻近细胞中的病毒复制。
3. Phagocytosis and Antigen Presentation | 吞噬作用与抗原呈递
Phagocytosis is a hallmark of innate immunity but also bridges to adaptive immunity. A phagocyte recognises a pathogen via its surface receptors, engulfs it into a phagosome, which then fuses with a lysosome to form a phagolysosome. The pathogen is digested, and fragments of its antigens are displayed on the phagocyte’s surface using major histocompatibility complex (MHC) class II molecules. This turns the phagocyte into an antigen-presenting cell (APC), such as a dendritic cell or macrophage, which subsequently activates helper T cells.
吞噬作用是先天免疫的标志，但也衔接适应性免疫。吞噬细胞通过表面受体识别病原体，将其吞入吞噬体，随后与溶酶体融合形成吞噬溶酶体。病原体被消化，其抗原片段通过主要组织相容性复合体（MHC）II 类分子呈递在吞噬细胞表面。这使吞噬细胞转变为抗原呈递细胞（APC），如树突状细胞或巨噬细胞，随后激活辅助T细胞。
Phagocytosis → Phagosome → Phagolysosome → Antigen presentation on MHC II
吞噬作用 → 吞噬体 → 吞噬溶酶体 → 抗原在 MHC II 上呈递
4. Specific Immune Responses: An Overview | 特异性免疫应答概述
Adaptive immunity is characterised by specificity and memory. It relies on lymphocytes: B cells (mature in bone marrow) and T cells (mature in the thymus). Each lymphocyte bears receptors for a single specific antigen. The adaptive response is triggered when an APC presents an antigen to a helper T cell with a complementary receptor. This leads to clonal selection: the specific lymphocyte is activated, proliferates, and differentiates into effector cells and memory cells. The two branches are humoral immunity (B cells, antibodies) and cell-mediated immunity (cytotoxic T cells).
适应性免疫的特点在于特异性和记忆性。它依赖于淋巴细胞：B细胞（在骨髓成熟）和T细胞（在胸腺成熟）。每个淋巴细胞带有针对单一特定抗原的受体。当APC将抗原呈递给具有互补受体的辅助T细胞时，适应性应答被触发。这导致克隆选择：特定淋巴细胞被激活、增殖，并分化为效应细胞和记忆细胞。两个分支为体液免疫（B细胞、抗体）和细胞介导免疫（细胞毒性T细胞）。
5. Humoral Immunity: B Cells and Antibodies | 体液免疫：B细胞与抗体
Humoral immunity targets extracellular pathogens and toxins. When a B cell encounters its specific antigen, it engulfs and presents it on MHC II. A helper T cell with a complementary receptor binds to this complex and releases cytokines (e.g., interleukins) that activate the B cell. The B cell then undergoes clonal expansion and differentiates into plasma cells, which secrete large quantities of antibodies, and memory B cells for long-term protection.
体液免疫针对胞外病原体和毒素。当B细胞遇到其特异性抗原时，将其吞入并通过MHC II呈递。具有互补受体的辅助T细胞与该复合物结合，并释放细胞因子（如白细胞介素）激活B细胞。B细胞随后进行克隆扩增，分化为浆细胞（大量分泌抗体）和记忆B细胞（提供长期保护）。
Antibodies (immunoglobulins) are Y-shaped glycoproteins. They neutralise pathogens by binding to antigens, causing agglutination (clumping), and marking them for phagocytosis (opsonisation). The antigen-antibody complex can also activate the complement system, leading to lysis of the pathogen.
抗体（免疫球蛋白）是 Y 形糖蛋白。它们通过与抗原结合来中和病原体，引起凝集，并标记病原体供吞噬（调理作用）。抗原-抗体复合物还可激活补体系统，导致病原体裂解。
6. Cell-Mediated Immunity: T Cells in Action | 细胞介导免疫：T细胞的作用
Cell-mediated immunity is essential for destroying host cells that are infected by viruses or have become cancerous. Cytotoxic T cells (CD8⁺) recognise antigens presented on MHC class I molecules, which are found on all nucleated cells. When a cytotoxic T cell binds to a non-self antigen on MHC I, it releases perforin and granzymes. Perforin creates pores in the target cell membrane, and granzymes enter to induce apoptosis (programmed cell death).
细胞介导免疫对于消灭被病毒感染的宿主细胞或癌变的细胞至关重要。细胞毒性T细胞（CD8⁺）识别MHC I类分子上呈递的抗原，MHC I存在于所有有核细胞上。当细胞毒性T细胞与MHC I上的非己抗原结合时，它释放穿孔素和颗粒酶。穿孔素在靶细胞膜上形成孔洞，颗粒酶进入并诱导细胞凋亡（程序性细胞死亡）。
Helper T cells (CD4⁺) orchestrate both humoral and cell-mediated responses by secreting cytokines. They are the primary target of HIV. Memory T cells persist after infection, enabling a faster secondary response.
辅助T细胞（CD4⁺）通过分泌细胞因子协调体液和细胞介导应答。它们是HIV的主要靶标。记忆T细胞在感染后持续存在，使二次应答更快。
7. Antibody Structure and Function | 抗体的结构与功能
Antibodies consist of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bonds. Each chain has a variable (V) region at the tip of the ‘Y’, which forms the antigen-binding site, and a constant (C) region that determines the antibody class (IgM, IgG, IgA, IgE, IgD). The variable region is highly specific due to the unique amino acid sequence shaped by V(D)J recombination during B cell development.
抗体由四条多肽链组成：两条相同的重链和两条相同的轻链，通过二硫键连接。每条链在“Y”形尖端有一个可变区（V区），构成抗原结合位点，以及决定抗体类别（IgM、IgG、IgA、IgE、IgD）的恒定区（C区）。由于B细胞发育过程中V(D)J重组形成的独特氨基酸序列，可变区具有高度特异性。
| Region | Function |
| Variable region | Binds specific antigen epitope |
| Constant region | Interacts with immune cells and complement |
| Hinge region | Allows flexibility for binding two antigens |
中文对照：
| 区域 | 功能 |
| 可变区 | 结合特定抗原表位 |
| 恒定区 | 与免疫细胞和补体相互作用 |
| 铰链区 | 提供灵活性以结合两个抗原 |
8. Immunological Memory and Vaccination | 免疫记忆与疫苗接种
During the primary immune response, the lag phase is relatively long, and antibody concentration rises slowly, peaking at a lower level. Memory B and T cells generated during this phase persist for years or a lifetime. Upon re-exposure to the same antigen, the secondary response is much faster, stronger, and predominantly involves IgG antibodies. This principle underpins vaccination: introducing a weakened, inactivated, or subunit form of a pathogen to stimulate immunity without causing disease.
在初次免疫应答中，滞后期较长，抗体浓度缓慢上升，峰值较低。此阶段产生的记忆B细胞和T细胞可存留多年甚至终生。再次接触相同抗原时，二次应答更快、更强，主要产生IgG抗体。这一原理是疫苗接种的基础：引入减毒、灭活或亚单位形式的病原体，在不引发疾病的情况下刺激免疫。
- Vaccines may contain live attenuated, inactivated, toxoid, subunit, or conjugate antigens.
- 疫苗可包含减毒活疫苗、灭活疫苗、类毒素、亚单位或结合疫苗。
- Herd immunity occurs when a high percentage of the population is immunised, protecting those who cannot be vaccinated.
- 当人口中很高比例获得免疫时，即产生群体免疫，保护无法接种的个体。
- Smallpox eradication and polio control are key examples.
- 天花根除和脊髓灰质炎控制是典型例子。
9. Monoclonal Antibodies: Production and Uses | 单克隆抗体：制备与应用
Monoclonal antibodies (mAbs) are identical antibodies produced by a single clone of B cells, all specific to the same epitope. They are produced by fusing a myeloma (cancer) cell with a B cell from an immunised animal (usually a mouse) to form a hybridoma. Hybridoma cells divide indefinitely and secrete the desired antibody. mAbs are used in diagnostics (e.g., pregnancy tests, ELISA), and in therapy (e.g., targeting cancer cells, treating autoimmune diseases).
单克隆抗体（mAbs）是由单一B细胞克隆产生的相同抗体，都针对同一表位。它们通过将骨髓瘤（癌细胞）与免疫动物（通常是小鼠）的B细胞融合形成杂交瘤细胞而制备。杂交瘤细胞可无限分裂并分泌所需抗体。单克隆抗体用于诊断（如妊娠试验、ELISA）和治疗（如靶向癌细胞、治疗自身免疫病）。
In pregnancy tests, mobile monoclonal antibodies tagged with a coloured marker bind to hCG, and immobilised antibodies capture the complex, producing a visible line. In cancer therapy, mAbs can deliver cytotoxic drugs directly to tumour cells, or block growth factor receptors.
在妊娠检测中，带有颜色标记的移动单克隆抗体结合hCG，固定抗体捕获复合物，产生可见线条。在癌症治疗中，单克隆抗体可将细胞毒性药物直接递送到肿瘤细胞，或阻断生长因子受体。
10. Immune System Disorders: Allergies and Autoimmune Diseases | 免疫系统疾病：过敏与自身免疫病
Allergies are hypersensitive responses to harmless environmental antigens (allergens). Upon first exposure, B cells produce IgE antibodies that bind to mast cells. On subsequent exposure, the allergen cross-links IgE, triggering mast cell degranulation and release of histamine, causing symptoms from mild (rhinitis, urticaria) to severe anaphylactic shock.
过敏是对无害环境抗原（过敏原）的超敏反应。初次接触时，B细胞产生IgE抗体并结合在肥大细胞上。再次接触时，过敏原交联IgE，触发肥大细胞脱颗粒并释放组胺，引起从轻微（鼻炎、荨麻疹）到严重过敏性休克的症状。
Autoimmune diseases occur when the immune system fails to distinguish self from non-self and attacks body tissues. Examples include Type 1 diabetes (destruction of pancreatic beta cells by cytotoxic T cells), rheumatoid arthritis, and multiple sclerosis. The underlying mechanisms may involve molecular mimicry, genetic predisposition, and environmental triggers.
自身免疫病是因免疫系统无法区分自身与非己，攻击自身组织而发生的。包括1型糖尿病（细胞毒性T细胞破坏胰腺β细胞）、类风湿关节炎和多发性硬化症。其机制可能涉及分子模拟、遗传易感性和环境诱因。
11. HIV and the Immune System | HIV与免疫系统
Human Immunodeficiency Virus (HIV) is a retrovirus that specifically infects helper T cells (CD4⁺), macrophages, and dendritic cells. The viral envelope glycoprotein gp120 binds to CD4 receptors and a co-receptor (CCR5 or CXCR4) on the host cell. After entry, the viral RNA is reverse-transcribed into DNA, which integrates into the host genome. The virus gradually depletes CD4⁺ cells, weakening the immune system until it progresses to Acquired Immunodeficiency Syndrome (AIDS), where opportunistic infections and cancers occur.
人类免疫缺陷病毒（HIV）是一种逆转录病毒，特异性地感染辅助T细胞（CD4⁺）、巨噬细胞和树突状细胞。病毒包膜糖蛋白gp120结合宿主细胞上的CD4受体和辅助受体（CCR5或CXCR4）。进入后，病毒RNA逆转录为DNA，整合入宿主基因组。病毒逐渐耗竭CD4⁺细胞，削弱免疫系统，直至进展为获得性免疫缺陷综合征（AIDS），出现机会性感染和癌症。
The progression is monitored by measuring CD4⁺ cell counts and viral load. Antiretroviral therapy (ART) targets different stages of the viral life cycle, such as reverse transcriptase inhibitors and protease inhibitors, to reduce viral replication and preserve immune function.
通过测量CD4⁺细胞计数和病毒载量来监测疾病进展。抗逆转录病毒治疗（ART）靶向病毒生命周期的不同阶段，如逆转录酶抑制剂和蛋白酶抑制剂，以减少病毒复制并保护免疫功能。
12. Ethical Considerations and Global Impact | 伦理考量与全球影响
Immunology raises ethical questions in both IB and OCR syllabi. Topics include the use of animals in monoclonal antibody production, mandatory vaccination policies, and equitable access to vaccines and HIV treatments worldwide. While animal models have been crucial in developing life-saving therapies, they raise welfare concerns. Monoclonal antibodies produced in mice can trigger human anti-mouse antibody responses, leading to the development of chimeric or humanised antibodies. Balancing public health with individual autonomy is a recurring debate in vaccination ethics.
免疫学在IB和OCR大纲中引发伦理问题。主题包括单克隆抗体生产中使用动物、强制疫苗接种政策，以及全球范围内疫苗和HIV治疗的公平获取。虽然动物模型对于开发拯救生命的疗法至关重要，但它们引发了福利担忧。小鼠产生的单克隆抗体可能引发人抗鼠抗体反应，促使嵌合或人源化抗体的开发。在公共卫生与个人自主之间取得平衡是疫苗接种伦理中的反复辩论。
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