📚 Immune System Key Points Review for IB & CIE Biology | 免疫系统考点精讲 (IB & CIE 生物)
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. For IB and CIE biology candidates, understanding both innate (non-specific) and adaptive (specific) immunity is essential. This article breaks down the key concepts, mechanisms and applications, from physical barriers to monoclonal antibodies and HIV, in a structured revision format. Each section pairs English explanations with Chinese translations to support bilingual learners.
免疫系统是由细胞、组织和分子组成的复杂网络，负责防御细菌、病毒、真菌和寄生虫等病原体。对于 IB 和 CIE 生物考生来说，理解先天（非特异性）免疫和适应（特异性）免疫至关重要。本文以结构化复习形式拆解关键概念、机制和应用，从物理屏障到单克隆抗体再到 HIV，每部分都提供中英对照讲解，方便双语学习者掌握。
1. Overview of the Immune System | 免疫系统概述
The immune system is divided into two main arms: innate (non-specific) immunity, which provides immediate but general defence, and adaptive (specific) immunity, which is slower but highly targeted and generates immunological memory. Innate defences include physical barriers, chemical secretions, phagocytic cells and the inflammatory response. Adaptive immunity involves lymphocytes – B cells and T cells – that recognise specific antigens.
免疫系统分为两大分支：先天（非特异性）免疫提供即时但非针对性的防御，适应（特异性）免疫反应较慢但高度特异，并能产生免疫记忆。先天防御包括物理屏障、化学分泌物、吞噬细胞和炎症反应。适应性免疫涉及识别特定抗原的淋巴细胞——B 细胞和 T 细胞。
- Innate immunity: Present from birth, same response to all pathogens, no memory.
- 先天免疫：与生俱来，对所有病原体反应相同，无记忆。
- Adaptive immunity: Acquired, specific, slower on first exposure, memory cells enable faster secondary response.
- 适应性免疫：获得性，特异性，初次接触反应慢，记忆细胞使二次反应更快更强。
2. First Line of Defence – Physical and Chemical Barriers | 第一道防线——物理和化学屏障
The body’s initial protective surfaces prevent pathogen entry. The skin, with its tough keratinised layer, acts as a physical barrier, while sebum and sweat contain antimicrobial substances that lower pH. Mucous membranes lining the respiratory, digestive and urogenital tracts trap microbes, and cilia sweep mucus towards openings. Lysozyme in tears and saliva destroys bacterial cell walls by hydrolysing peptidoglycan.
人体最初防护面阻止病原体入侵。皮肤具有坚韧的角质层，构成物理屏障，皮脂和汗液含有降低 pH 的抗菌物质。呼吸道、消化道和泌尿生殖道的黏膜能粘附微生物，纤毛将黏液向开口处摆动。泪液和唾液中的溶菌酶通过水解肽聚糖破坏细菌细胞壁。
In the stomach, gastric acid (HCl) creates a highly acidic environment (pH 1.5–2.5) that denatures proteins and kills most ingested microbes. Commensal bacteria on skin and in the gut also compete with pathogens for space and nutrients, providing an additional biological barrier.
在胃中，胃酸（HCl）创造了强酸性环境（pH 1.5–2.5），使蛋白质变性并杀死大多数摄入的微生物。皮肤和肠道中的共生细菌还与病原体竞争空间和营养，提供了额外的生物屏障。
3. Second Line of Defence – Innate Cellular and Chemical Responses | 第二道防线——先天细胞和化学应答
If pathogens breach the barriers, innate immune cells such as phagocytes are recruited. Neutrophils and macrophages engulf and digest pathogens by phagocytosis. The pathogen is enclosed in a phagosome, which fuses with lysosomes to form a phagolysosome where hydrolytic enzymes and reactive oxygen species destroy it. Macrophages also act as antigen-presenting cells (APCs) for the adaptive system.
如果病原体突破屏障，先天免疫细胞如吞噬细胞会被募集。中性粒细胞和巨噬细胞通过吞噬作用吞入并消化病原体。病原体被包裹在吞噬体中，吞噬体与溶酶体融合形成吞噬溶酶体，其中水解酶和活性氧将其破坏。巨噬细胞还作为抗原呈递细胞（APC）与适应性系统衔接。
Interferons are signalling proteins released by virus-infected cells. They diffuse to neighbouring cells, triggering synthesis of enzymes that inhibit viral replication. The complement system, a cascade of plasma proteins, opsonises pathogens, promotes inflammation and can directly lyse microbes. The inflammatory response – characterised by redness, heat, swelling and pain – increases capillary permeability and attracts phagocytes to the site of infection.
干扰素是受病毒感染的细胞释放的信号蛋白。它们扩散到邻近细胞，触发抑制病毒复制的酶合成。补体系统是一系列血浆蛋白，可调理病原体、促进炎症并直接裂解微生物。炎症反应——以红、热、肿、痛为特征——增加毛细血管通透性并吸引吞噬细胞到达感染部位。
4. Antigens and the Basis of Specific Immunity | 抗原与特异性免疫的基础
An antigen is any molecule (usually a protein or polysaccharide) that is recognised by the immune system as foreign and elicits an adaptive response. Each lymphocyte carries a unique receptor that binds a specific antigen. The adaptive response relies on clonal selection: when a B or T cell receptor binds its complementary antigen, that lymphocyte is activated, proliferates and differentiates into effector and memory cells.
抗原是任何被免疫系统识别为外来物并引发适应性免疫反应的分子（通常为蛋白质或多糖）。每个淋巴细胞都带有独特的受体，能与特定抗原结合。适应性反应依赖于克隆选择：当 B 或 T 细胞受体与互补抗原结合时，该淋巴细胞被激活，增殖并分化为效应细胞和记忆细胞。
Self-tolerance is crucial. During lymphocyte maturation, cells that react strongly to self-antigens are eliminated (clonal deletion) or inactivated, preventing autoimmune diseases. The major histocompatibility complex (MHC) proteins present antigen fragments on cell surfaces, enabling T cells to recognise infected or abnormal cells.
自身耐受至关重要。淋巴细胞成熟期间，对自身抗原反应强烈的细胞会被清除（克隆删除）或失活，从而防止自身免疫病。主要组织相容性复合体（MHC）蛋白在细胞表面呈递抗原片段，使 T 细胞能够识别被感染或异常的细胞。
5. Humoral Immunity – B Cells and Antibody Production | 体液免疫——B 细胞与抗体产生
Humoral immunity targets extracellular pathogens and toxins. B cells bear membrane-bound antibodies (B cell receptors). When a B cell encounters its specific antigen, it engulfs, processes and presents antigen fragments on MHC class II molecules. Activated helper T cells (TH cells) recognise this complex and release cytokines that stimulate the B cell to undergo clonal expansion. Most daughter cells become plasma cells, which secrete vast quantities of soluble antibodies. Some become memory B cells, which persist for years and enable a rapid secondary response.
体液免疫针对胞外病原体和毒素。B 细胞表面带有膜结合抗体（B 细胞受体）。当 B 细胞遇到其特异抗原时，会吞噬、处理并在 MHC II 类分子上呈递抗原片段。活化的辅助 T 细胞（TH 细胞）识别这一复合物并释放细胞因子，刺激 B 细胞进行克隆扩增。大多数子细胞成为浆细胞，分泌大量可溶性抗体。一些成为记忆 B 细胞，存活多年，使二次应答迅速发生。
The primary immune response is slow, with a lag phase of several days before antibody levels peak, and mainly produces IgM. The secondary response, upon re-exposure, is faster and stronger, dominated by IgG, because memory cells are already present. This principle underpins vaccination.
初次免疫应答较慢，有数天滞后期抗体水平才达峰值，且主要产生 IgM。再次接触时的二次应答更快更强，以 IgG 为主，因为记忆细胞已经存在。这一原理是疫苗接种的基础。
6. Cell-Mediated Immunity – T Lymphocytes | 细胞免疫——T 淋巴细胞
Cell-mediated immunity deals with intracellular pathogens, such as viruses hiding inside host cells, and with cancerous cells. T cells mature in the thymus and express T cell receptors (TCRs) that recognise antigen fragments bound to MHC molecules. Cytotoxic T cells (TC cells, CD8+) recognise antigens presented on MHC class I molecules, which are found on almost all nucleated cells. Upon activation, they release perforin and granzymes that induce apoptosis in the target cell.
细胞免疫处理胞内病原体（如藏匿于宿主细胞内的病毒）以及癌细胞。T 细胞在胸腺中成熟，表达 T 细胞受体（TCR），可识别结合于 MHC 分子的抗原片段。细胞毒性 T 细胞（TC 细胞，CD8+）识别几乎在所有有核细胞上存在的 MHC I 类分子呈递的抗原。激活后，它们释放穿孔素和颗粒酶，诱导靶细胞凋亡。
Helper T cells (TH cells, CD4+) recognise antigens on MHC class II molecules presented by professional APCs (dendritic cells, macrophages, B cells). Activated TH cells secrete cytokines that orchestrate both humoral and cell-mediated responses, including activation of B cells, cytotoxic T cells and macrophages. Memory T cells are also generated for long-term immunity.
辅助 T 细胞（TH 细胞，CD4+）识别专职抗原呈递细胞（树突状细胞、巨噬细胞、B 细胞）上 MHC II 类分子呈递的抗原。活化的 TH 细胞分泌细胞因子，协调体液和细胞介导的免疫应答，包括激活 B 细胞、细胞毒性 T 细胞和巨噬细胞。同时产生记忆 T 细胞以提供长期免疫力。
7. Antibody Structure and Function | 抗体结构与功能
Antibodies (immunoglobulins) are Y-shaped glycoproteins composed 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 arms, which contains the antigen-binding site and determines specificity, and a constant (C) region that defines the antibody class and effector function.
抗体（免疫球蛋白）是 Y 形糖蛋白，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。每条链臂端各有一个可变区（V 区），包含抗原结合位点并决定特异性，恒定区（C 区）决定抗体类别和效应功能。
| Class | Structure | Main Role |
| IgG | Monomer | Main antibody in secondary response; crosses placenta |
| IgM | Pentamer | First produced in primary response; agglutinates pathogens |
| IgA | Dimer | Secreted in milk, tears, saliva; mucosal defence |
| IgE | Monomer | Binds mast cells; involved in allergies and parasite defence |
抗体类别表：IgG 二次应答主要抗体，可穿过胎盘；IgM 初次应答最早产生，凝集病原体；IgA 分泌至乳汁、泪液、唾液中，参与黏膜防御；IgE 与肥大细胞结合，参与过敏和抗寄生虫。
Antibodies neutralise pathogens by blocking binding sites, agglutinate microbes to enhance phagocytosis, precipitate soluble toxins, and activate complement. The hinge region provides flexibility, allowing both arms to bind two identical antigens simultaneously.
抗体通过阻断结合位点来中和病原体、凝集微生物以增强吞噬、沉淀可溶性毒素，并激活补体。铰链区提供柔韧性，使双臂可同时结合两个相同抗原。
8. Active vs Passive Immunity and Vaccination | 主动与被动免疫及疫苗接种
Immunity can be acquired actively or passively, and through natural or artificial means.
免疫可通过主动或被动方式获得，且分为自然和人工来源。
- Natural active: Infection triggers an immune response; memory cells form. 自然主动：感染引发免疫应答，产生记忆细胞。
- Artificial active: Vaccination with killed/attenuated pathogens, toxoids or subunit antigens stimulates adaptive immunity without causing disease. 人工主动：接种灭活/减毒病原体、类毒素或亚单位抗原，刺激适应性免疫而不致病。
- Natural passive: Antibodies transferred across placenta (IgG) or in colostrum/breast milk (IgA) provide temporary protection to infant. 自然被动：抗体通过胎盘（IgG）或初乳/母乳（IgA）传递给婴儿，提供短暂保护。
- Artificial passive: Injection of preformed antibodies (antisera, monoclonal antibodies) for immediate but short-lived protection. 人工被动：注射预制的抗体（抗血清、单克隆抗体），提供即时但短暂的保护。
Vaccines prime the immune system by exposing it to harmless forms of the antigen. The secondary response upon real infection is swift and robust. Herd immunity occurs when a high proportion of a population is vaccinated, protecting unvaccinated individuals by interrupting pathogen transmission.
疫苗通过让免疫系统接触无害形式的抗原，使其预先做好准备。真实感染时二次应答迅速且强大。当群体中高比例接种疫苗时，可通过阻断病原体传播来保护未接种者，称为群体免疫。
9. Monoclonal Antibodies – Production and Uses | 单克隆抗体——生产与应用
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 B-lymphocyte (that produces the desired antibody) with a myeloma (cancer) cell to form a hybridoma. The hybridoma divides indefinitely in culture and secretes large quantities of the single antibody type.
单克隆抗体是由单个 B 细胞克隆产生的相同抗体，全都针对同一表位。通过将产生所需抗体的 B 淋巴细胞与骨髓瘤（癌）细胞融合形成杂交瘤细胞来生产。杂交瘤细胞在培养中无限分裂，并大量分泌单一抗体类型。
Uses include: pregnancy testing (anti-hCG antibodies), cancer therapy (mAbs conjugated to drugs or radioisotopes target tumour markers), diagnosis of infections (ELISA), and treatment of autoimmune diseases (e.g. anti-TNFα). The high specificity minimises damage to healthy cells.
应用包括：验孕（抗 hCG 抗体）、癌症治疗（与药物或放射性同位素偶联的单抗靶向肿瘤标志物）、感染诊断（ELISA）以及自身免疫病治疗（如抗 TNFα）。高特异性可最大限度减少对健康细胞的损伤。
10. HIV and Immunodeficiency | HIV 与免疫缺陷
Human Immunodeficiency Virus (HIV) is a retrovirus that primarily infects CD4+ helper T cells, macrophages and dendritic cells. The virus uses reverse transcriptase to convert its RNA genome into DNA, which integrates into the host cell chromosome. Over time, HIV destroys TH cells, crippling the immune system and leading to Acquired Immunodeficiency Syndrome (AIDS).
人类免疫缺陷病毒（HIV）是一种逆转录病毒，主要感染 CD4+ 辅助 T 细胞、巨噬细胞和树突状细胞。病毒利用逆转录酶将其 RNA 基因组转换为 DNA，整合入宿主细胞染色体。随时间推移，HIV 破坏 TH 细胞，使免疫系统瘫痪，导致获得性免疫缺陷综合征（AIDS）。
With a critically low TH cell count (below 200 cells/µl), patients become susceptible to opportunistic infections (e.g. tuberculosis, candidiasis) and certain cancers (e.g. Kaposi’s sarcoma). The virus evades immunity through high mutation rates and by targeting the very cells that coordinate defence. Antiretroviral therapy (ART) targets different stages of the HIV life cycle, such as reverse transcriptase and protease, but no cure currently exists.
当 TH 细胞计数极低（低于 200 个/µL）时，患者易受机会性感染（如结核、念珠菌病）和某些癌症（如卡波西肉瘤）的侵袭。病毒通过高突变率和攻击协调防御的细胞本身来逃避免疫。抗逆转录病毒疗法（ART）靶向 HIV 生命周期的不同阶段，如逆转录酶和蛋白酶，但目前尚无法根治。
11. Cooperation Between Innate and Adaptive Arms | 先天与适应性免疫的协作
Innate and adaptive immunity are not separate; they interact continuously. Dendritic cells and macrophages act as bridges: after engulfing a pathogen, they migrate to lymph nodes and present antigen on MHC class II to activate naive TH cells. Cytokines released by TH cells amplify phagocytic activity. Antibodies produced by B cells often rely on phagocytes and complement for pathogen elimination. This integration is critical for a robust immune response and highlights why both systems are examined together.
先天免疫和适应性免疫不是分离的，它们持续交互。树突状细胞和巨噬细胞充当桥梁：吞噬病原体后，它们迁移至淋巴结并在 MHC II 类上呈递抗原以激活初始 TH 细胞。TH 细胞释放的细胞因子增强吞噬活性。B 细胞产生的抗体常依赖吞噬细胞和补体来清除病原体。这种整合对强大的免疫应答至关重要，也解释了为何考试常将两系统结合考查。
12. Common Exam Pitfalls and Key Summary | 常见考试失分点与核心总结
Students often confuse the roles of B cells and T cells, or fail to link antibody structure to function. Remember: plasma cells secrete antibodies; memory cells do not. Cytotoxic T cells kill infected host cells directly, while helper T cells coordinate the immune response. Clonal selection and clonal deletion are recurring concepts in both IB and CIE syllabi. Always use precise terminology: agglutination, opsonisation, neutralisation. In essay questions, include a graph of primary vs secondary antibody response and explain the role of memory cells.
学生常混淆 B 细胞和 T 细胞的功能，或未能将抗体结构与功能联系起来。记住：浆细胞分泌抗体，记忆细胞不分泌。细胞毒性 T 细胞直接杀死感染的宿主细胞，而辅助 T 细胞协调免疫应答。克隆选择和克隆删除是 IB 和 CIE 课程中反复出现的概念。务请使用精确术语：凝集、调理、中和。问答题中应绘制初次与二次抗体反应曲线并解释记忆细胞的作用。
A final checklist: distinguish innate from adaptive; know the first and second lines; describe phagocytosis in detail; explain the sequence of B and T cell activation; outline monoclonal antibody production; discuss vaccination and active/passive immunity; and illustrate how HIV undermines the immune system. Connections between topics, such as how a vaccine mimics a primary response, impress examiners and secure top marks.
最终复习清单：区分先天与适应性免疫；掌握第一、二道防线；详细描述吞噬作用；解释 B 细胞和 T 细胞激活顺序；概述单克隆抗体生产；讨论疫苗接种和主动/被动免疫；并说明 HIV 如何破坏免疫系统。将知识点联系起来，例如疫苗如何模拟初次应答，能给考官留下深刻印象并确保高分。
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