📚 IB & Edexcel Biology: Immune System Key Points | IB Edexcel 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 and foreign substances. Understanding its mechanisms, from non-specific barriers to highly specific lymphocyte-mediated responses, is crucial for IB and Edexcel biology exams. This article provides a focused review of key concepts, including innate defences, phagocytosis, cell-mediated and humoral immunity, vaccination, monoclonal antibodies, and common immune disorders. Careful attention to processes like antigen presentation, clonal selection and immunological memory will help you answer exam questions with confidence.
免疫系统是一个复杂的细胞、组织和器官网络，负责抵御病原体及外来物质。理解其机制——从非特异性屏障到高度特异的淋巴细胞介导应答——对 IB 和 Edexcel 生物考试至关重要。本文聚焦复习核心概念，包括先天防御、吞噬作用、细胞介导与体液免疫、疫苗接种、单克隆抗体以及常见免疫失调。仔细掌握抗原呈递、克隆选择和免疫记忆等过程，能让你从容应对考试问题。
1. Overview of the Immune System | 免疫系统概述
The immune system distinguishes self from non-self to protect the body against infections by bacteria, viruses, fungi and parasites. It involves physical and chemical barriers, white blood cells (leukocytes) derived from bone marrow, the lymphatic system, and soluble defence proteins such as antibodies and the complement system. The response is divided into innate (non-specific) immunity and adaptive (specific) immunity.
免疫系统区分'自身'与'非自身'，保护机体免受细菌、病毒、真菌和寄生虫的感染。它包括物理与化学屏障、来自骨髓的白细胞、淋巴系统，以及抗体和补体系统等可溶性防御蛋白。免疫应答分为先天（非特异性）免疫和适应性（特异性）免疫。
The lymphatic system collects interstitial fluid and returns it to the blood; lymph nodes filter pathogens and contain high concentrations of lymphocytes and antigen-presenting cells. This is where many adaptive immune responses are initiated.
淋巴系统收集组织液并将其送回血液；淋巴结过滤病原体，含有高浓度的淋巴细胞和抗原呈递细胞，许多适应性免疫应答就是在这里启动的。
2. Physical and Chemical Barriers | 物理与化学屏障
The first line of defence consists of physical barriers that block pathogen entry. The skin is a tough, waterproof, slightly acidic physical barrier, with keratin providing strength. Mucous membranes in the respiratory, digestive and reproductive tracts secrete sticky mucus that traps microbes. Ciliated epithelial cells in the airways sweep mucus and trapped pathogens upward to be swallowed or expelled.
第一道防线由阻挡病原体入侵的物理屏障组成。皮肤坚韧、防水、呈微酸性，角蛋白赋予其强度。呼吸道、消化道和生殖道的黏膜分泌粘稠的黏液捕获微生物。气道中的纤毛上皮细胞将黏液和被捕获的病原体向上清扫，使其被吞咽或排出。
Chemical defences also play a key role. Lysozyme, an enzyme in tears, saliva and nasal secretions, hydrolyses bacterial cell walls. Hydrochloric acid in the stomach destroys most ingested pathogens. Sebum on the skin contains antimicrobial fatty acids. Urine flow and its acidity also help wash out microbes from the urethra.
化学防御同样重要。眼泪、唾液和鼻分泌物中的溶菌酶可水解细菌细胞壁。胃中的盐酸摧毁大多数摄入的病原体。皮肤上的皮脂含有抗菌脂肪酸。尿液的流动及其酸性也有助于冲刷出尿道中的微生物。
3. Phagocytosis and Inflammation | 吞噬作用与炎症
Phagocytes, such as neutrophils and macrophages, are key cells of innate immunity. They recognise pathogens via pattern recognition receptors, then engulf them by endocytosis to form a phagosome. Lysosomes fuse with the phagosome, releasing hydrolytic enzymes and reactive oxygen species to destroy the engulfed material.
吞噬细胞，如中性粒细胞和巨噬细胞，是先天免疫的关键细胞。它们通过模式识别受体识别病原体，然后通过内吞作用将其吞入，形成吞噬体。溶酶体与吞噬体融合，释放水解酶和活性氧，摧毁吞入的物质。
Inflammation is a localised response to infection or injury. Damaged mast cells release histamine, causing vasodilation and increased capillary permeability. This leads to redness, heat, swelling and pain. Cytokines attract more phagocytes to the site. The process isolates pathogens and enhances immune cell recruitment.
炎症是对感染或损伤的局部反应。受损的肥大细胞释放组胺，引起血管扩张和毛细血管通透性增加，导致红、热、肿、痛。细胞因子吸引更多吞噬细胞到达该部位。此过程隔离病原体并增强免疫细胞的募集。
4. Antigens, Antibodies and Lymphocytes | 抗原、抗体与淋巴细胞
Antigens are molecules – typically proteins or polysaccharides on the surface of pathogens, transplanted tissues or cancer cells – that elicit an adaptive immune response. Each antigen has specific epitopes recognised by lymphocyte receptors. Antibodies, or immunoglobulins, are Y-shaped glycoproteins secreted by plasma B cells. They bind specifically to matching antigens, neutralising them or marking them for destruction.
抗原是能够引发适应性免疫应答的分子——通常是病原体、移植组织或癌细胞表面的蛋白质或多糖。每个抗原具有特定的表位，被淋巴细胞受体识别。抗体（免疫球蛋白）是由浆 B 细胞分泌的 Y 形糖蛋白，能特异性地结合匹配的抗原，将其中和或标记以便摧毁。
Lymphocytes are white blood cells responsible for specific immunity. T lymphocytes (T cells) mature in the thymus and are responsible for cell-mediated immunity. B lymphocytes (B cells) mature in bone marrow and mediate humoral immunity. Each lymphocyte carries surface receptors specific to one particular antigen; this is the basis of clonal selection.
淋巴细胞是负责特异性免疫的白细胞。T 淋巴细胞（T 细胞）在胸腺中成熟，负责细胞介导免疫；B 淋巴细胞（B 细胞）在骨髓中成熟，介导体液免疫。每一个淋巴细胞表面携带针对某一种特定抗原的受体，这是克隆选择的基础。
5. Cell-Mediated Immunity (T-Cells) | 细胞介导免疫（T 细胞）
Cell-mediated immunity is directed against virus-infected cells, cancer cells and transplanted tissues. Antigen-presenting cells (APCs), such as dendritic cells and macrophages, process antigens and present peptide fragments on MHC class II molecules to helper T cells (CD4+). Infected body cells present endogenous antigens on MHC class I to cytotoxic T cells (CD8+).
细胞介导免疫针对病毒感染细胞、癌细胞和移植组织。抗原呈递细胞（如树突状细胞和巨噬细胞）处理抗原，并通过 MHC II 类分子将肽段呈递给辅助 T 细胞（CD4+)。受感染的体细胞则通过 MHC I 类分子将内源性抗原呈递给细胞毒性 T 细胞（CD8+)。
Upon activation, helper T cells secrete cytokines that stimulate B cells, cytotoxic T cells and macrophages. Cytotoxic T cells bind to infected cells via the TCR–MHC I interaction, then release perforin (forming pores in the target cell membrane) and granzymes (inducing apoptosis). Memory T cells persist for fast recall responses.
激活后的辅助 T 细胞分泌细胞因子，刺激 B 细胞、细胞毒性 T 细胞和巨噬细胞。细胞毒性 T 细胞通过 TCR–MHC I 相互作用与感染细胞结合，然后释放穿孔素（在靶细胞膜上形成孔洞）和颗粒酶（诱导凋亡）。记忆 T 细胞持续存在，以便快速回忆应答。
6. Humoral Immunity (B-Cells and Antibody Production) | 体液免疫（B 细胞与抗体产生）
Humoral immunity involves B lymphocytes that attack pathogens and toxins circulating in body fluids. A naive B cell is activated when its surface immunoglobulin (B cell receptor) binds to a specific antigen. Full activation usually requires co-stimulation from an activated helper T cell. The B cell then undergoes clonal expansion and differentiation into plasma cells and memory B cells.
体液免疫涉及 B 淋巴细胞，它们攻击在体液中循环的病原体和毒素。初始 B 细胞表面的免疫球蛋白（B 细胞受体）与特异抗原结合后被激活。完全激活通常需要活化辅助 T 细胞提供的共刺激。随后 B 细胞进行克隆扩增，并分化为浆细胞和记忆 B 细胞。
Plasma cells secrete large quantities of soluble antibodies. Antibodies neutralise toxins and viruses, agglutinate (clump) bacteria, and act as opsonins (enhancing phagocytosis). The antibody structure includes variable regions for antigen binding and a constant region that determines the class (IgM, IgG, IgA, IgE, IgD). The class of antibody changes during a response, with IgG providing long-term protection.
浆细胞分泌大量可溶性抗体。抗体中和毒素和病毒，凝集细菌，并充当调理素（增强吞噬作用）。抗体结构包括负责抗原结合的可变区和决定类别的恒定区（IgM、IgG、IgA、IgE、IgD）。在应答过程中抗体类别会转换，其中 IgG 提供长期保护。
The following table compares the key features of cell-mediated and humoral immunity:
下表比较了细胞介导免疫与体液免疫的关键特征：
| Feature | Cell-Mediated Immunity | Humoral Immunity |
|---|---|---|
| Main cells involved | T lymphocytes (helper and cytotoxic T cells) | B lymphocytes, plasma cells |
| Target | Infected cells, cancer cells, transplant tissues | Extracellular pathogens, toxins in body fluids |
| Recognition | TCR binds antigen–MHC complex | B cell receptor binds free antigen |
| Effector molecules | Perforin, granzymes, cytokines | Antibodies |
| Role in vaccination | Memory T cells for rapid cellular response | Memory B cells, long-lived antibody production |
7. Primary and Secondary Immune Responses | 初次与再次免疫应答
During the primary immune response, lag time occurs while lymphocytes undergo clonal selection and expansion. Antibody concentration rises slowly, peaks at a moderate level, then declines. Memory cells are produced and remain in circulation. The secondary immune response, upon re-exposure to the same antigen, is much faster and stronger because memory cells are already primed. Antibody levels skyrocket, often preventing disease symptoms entirely.
在初次免疫应答中，淋巴细胞进行克隆选择和扩增需要一段滞后时间。抗体浓度缓慢上升，达到中等峰值后下降。记忆细胞产生并留在体内循环。再次暴露于相同抗原时，二次免疫应答迅速且强度更高，因为记忆细胞已被致敏。抗体水平急剧升高，常常完全阻止疾病症状的出现。
This principle is fundamental to vaccination: the first dose primes the immune system, and booster shots generate a robust secondary response. Graph-plotting questions are common in exams, so make sure you can sketch and label the primary and secondary antibody curves with appropriate time and titre axes.
这一原理是疫苗接种的基础：首剂疫苗致敏免疫系统，加强针则激发强大的二次应答。考试中经常出现绘图题，务必能够绘制并标注初次和再次抗体曲线，标出合适的时间和滴度坐标轴。
8. Vaccination and Herd Immunity | 疫苗接种与群体免疫
Vaccination is the deliberate introduction of antigens (in a safe form) to stimulate adaptive immunity without causing disease. Vaccines may contain live attenuated pathogens, inactivated pathogens, subunit antigens, toxoids, or nucleic acids (mRNA, DNA). This process provides active artificially acquired immunity, generating memory cells that ensure long-term protection. Booster doses maintain immunity levels.
疫苗接种是指刻意引入安全形式的抗原来激发适应性免疫而不引起疾病。疫苗可能含有减毒活病原体、灭活病原体、亚单位抗原、类毒素或核酸（mRNA、DNA）。这一过程提供人工主动免疫，产生记忆细胞以确保长期保护。加强剂量可维持免疫水平。
Herd immunity occurs when a high proportion of a population is vaccinated, breaking the chain of transmission and protecting unvaccinated or immunocompromised individuals. The required threshold depends on the pathogen's basic reproduction number (R₀). Memory cells also confer individual protection even if antibody titres have waned.
群体免疫是指群体中大多数人接种疫苗后，阻断了传播链，从而保护未接种或免疫功能低下者。所需阈值取决于病原体的基本再生数(R₀)。即使抗体滴度下降，记忆细胞也能提供个体保护。
9. Monoclonal Antibodies: Production and Uses | 单克隆抗体的生产与应用
Monoclonal antibodies (mAbs) are identical antibodies produced from a single clone of plasma cells. They are generated by fusing a specific antibody-producing B cell from an immunised mouse with a myeloma (cancer) cell to create a hybridoma. The hybridoma can divide indefinitely while secreting the desired antibody. The antibodies are then harvested, purified and used for diagnostic and therapeutic purposes.
单克隆抗体是由单一浆细胞克隆产生的相同抗体。通过将免疫小鼠的特异性抗体产生 B 细胞与骨髓瘤（癌细胞）融合，产生杂交瘤。杂交瘤可无限分裂并分泌所需的抗体。随后收集、纯化抗体，用于诊断和治疗目的。
Key applications include pregnancy test kits (using anti-hCG antibodies), ELISA tests for detecting pathogens or biomarkers, targeted cancer therapy (antibodies conjugated to drugs or radioactive substances), and treatment of autoimmune diseases. The specificity of mAbs allows precise targeting, reducing side effects compared with conventional treatments.
主要应用包括验孕棒（使用抗 hCG 抗体）、检测病原体或生物标志物的 ELISA 测试、靶向癌症治疗（与药物或放射性物质结合的抗体）以及自身免疫病的治疗。单克隆抗体的特异性可精准靶向，与传统治疗相比减少副作用。
10. 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+) by binding to the CD4 receptor and a co-receptor (CCR5 or CXCR4). After entering the cell, viral RNA is reverse-transcribed into DNA and integrated into the host genome. The virus destroys helper T cells over time, progressively weakening the immune system.
人类免疫缺陷病毒（HIV）是一种逆转录病毒，通过结合 CD4 受体和共受体（CCR5 或 CXCR4），特异性感染辅助 T 细胞（CD4+)。进入细胞后，病毒 RNA 被逆转录为 DNA 并整合到宿主基因组中。随时间推移，病毒摧毁辅助 T 细胞，逐渐削弱免疫系统。
When the CD4 count drops below a critical level (~200 cells/µL), the individual develops Acquired Immune Deficiency Syndrome (AIDS) and becomes susceptible to opportunistic infections and rare cancers such as Kaposi's sarcoma. HIV is transmitted through blood, semen, vaginal fluids and breast milk. Although there is no cure, antiretroviral therapy (ART) can suppress viral load, restoring immune function and preventing progression to AIDS.
当 CD4 计数降至临界水平以下（约 200 个细胞/µL），患者会患上获得性免疫缺陷综合征（艾滋病），易受机会性感染和罕见癌症（如卡波西肉瘤）侵害。HIV 通过血液、精液、阴道分泌物和母乳传播。虽无法治愈，但抗逆转录病毒疗法（ART）可抑制病毒载量，恢复免疫功能并防止发展为艾滋病。
11. Allergies and Autoimmune Diseases | 过敏与自身免疫病
Allergies are hypersensitive immune responses to normally harmless environmental substances (allergens). On first exposure, B cells produce IgE antibodies which bind to mast cells. Upon re-exposure, the allergen cross-links IgE on mast cells, triggering degranulation and the release of histamine and other mediators. This causes symptoms ranging from mild itching and sneezing to severe anaphylactic shock, which requires adrenaline (epinephrine) administration.
过敏是对通常无害的环境物质（过敏原）发生的超敏免疫应答。首次暴露时，B 细胞产生 IgE 抗体，后者结合于肥大细胞表面。再次暴露时，过敏原与肥大细胞上的 IgE 交联，触发脱颗粒并释放组胺及其他介质。这导致从轻微瘙痒、打喷嚏到严重的过敏性休克等一系列症状，严重时需注射肾上腺素。
Autoimmune diseases occur when the immune system mistakenly attacks self-antigens. Examples include type 1 diabetes (insulin-producing β cells destroyed by T cells), multiple sclerosis (myelin sheath attacked) and rheumatoid arthritis (joint tissues targeted). The mechanisms may involve molecular mimicry, failure of self-tolerance, or genetic and environmental factors. Treatments often involve immunosuppressive drugs or monoclonal antibodies that block specific immune pathways.
自身免疫病是免疫系统错误攻击自身抗原所致。例如 1 型糖尿病（T 细胞摧毁产生胰岛素的 β 细胞）、多发性硬化（攻击髓鞘）和类风湿关节炎（靶向关节组织）。机制可能涉及分子模拟、自身耐受失败或遗传与环境因素。治疗常使用免疫抑制剂或阻断特定免疫途径的单克隆抗体。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图