A-Level生物 免疫系统 适应性免疫

A-Level Biology: The Adaptive Immune System – T Cells, B Cells, and Immunological Memory

1. Introduction: Innate vs Adaptive Immunity

The vertebrate immune system operates on two interconnected levels: innate immunity and adaptive immunity. Innate immunity provides the first line of defence with physical barriers (skin, mucous membranes), chemical defences (lysozyme in tears, stomach acid), and non-specific cellular responses (phagocytes, natural killer cells). It responds immediately to infection but lacks specificity and memory. Adaptive immunity, by contrast, is highly specific to particular pathogens and retains immunological memory, enabling a faster and stronger response upon re-exposure to the same pathogen. The adaptive immune system is mediated by two types of lymphocytes: T lymphocytes (T cells) and B lymphocytes (B cells), which work together to eliminate pathogens through cell-mediated and humoral mechanisms respectively. 脊椎动物的免疫系统在两个相互关联的层面上运作：先天免疫和适应性免疫。先天免疫通过物理屏障（皮肤、黏膜）、化学防御（泪液中的溶菌酶、胃酸）和非特异性细胞反应（吞噬细胞、自然杀伤细胞）提供第一道防线。它对感染立即作出反应，但缺乏特异性和记忆。相比之下，适应性免疫对特定病原体具有高度特异性，并保留免疫记忆，使得再次接触同一病原体时能产生更快更强的反应。适应性免疫系统由两种淋巴细胞介导：T淋巴细胞（T细胞）和B淋巴细胞（B细胞），它们分别通过细胞介导和体液机制协同消除病原体。

2. Lymphocyte Origins and Maturation

Both T cells and B cells originate from haematopoietic stem cells in the bone marrow. B cells complete their maturation in the bone marrow (hence their name: B for bone marrow). T cell precursors, however, migrate from the bone marrow to the thymus gland, where they undergo a rigorous selection process known as thymic education. During this process, immature T cells that fail to recognise self-MHC molecules (positive selection) or that react too strongly to self-antigens (negative selection) are eliminated by apoptosis. This clonal deletion mechanism establishes self-tolerance and prevents autoimmune reactions. Only about 2-5% of developing T cells survive this selection and enter the bloodstream as mature, naive T cells, each bearing a unique T-cell receptor (TCR) capable of recognising a specific antigen. T细胞和B细胞均起源于骨髓中的造血干细胞。B细胞在骨髓中完成成熟过程（因此得名：B代表骨髓）。然而，T细胞前体从骨髓迁移到胸腺，在那里经历一个称为胸腺教育的严格筛选过程。在此过程中，未能识别自身MHC分子的未成熟T细胞（阳性选择）或对自身抗原有过强反应的T细胞（阴性选择）通过凋亡被清除。这种克隆删除机制建立了自身耐受性并防止自身免疫反应。只有约2-5%的发育中T细胞能通过这一筛选，作为成熟的初始T细胞进入血液循环，每个细胞携带能够识别特定抗原的独特T细胞受体（TCR）。

3. Antigen Presentation and MHC Molecules

For T cells to recognise an antigen, the antigen must first be processed and presented on the surface of an antigen-presenting cell (APC) bound to major histocompatibility complex (MHC) molecules. There are two classes of MHC: MHC Class I molecules are expressed on all nucleated cells and present endogenous antigens (from intracellular pathogens such as viruses) to CD8+ cytotoxic T cells. MHC Class II molecules are expressed only on professional APCs (dendritic cells, macrophages, and B cells) and present exogenous antigens (from extracellular pathogens that have been phagocytosed) to CD4+ helper T cells. Dendritic cells are the most potent APCs and serve as the critical bridge between innate and adaptive immunity. They capture antigens at the site of infection, migrate to lymph nodes, and present processed antigen fragments to naive T cells, triggering clonal expansion. 为了使T细胞识别抗原，抗原必须首先被加工并由抗原呈递细胞（APC）呈递在其表面，与主要组织相容性复合体（MHC）分子结合。MHC分为两类：MHC I类分子表达于所有有核细胞表面，将内源性抗原（来自胞内病原体如病毒）呈递给CD8+细胞毒性T细胞。MHC II类分子仅在专职APC（树突状细胞、巨噬细胞和B细胞）上表达，将外源性抗原（来自被吞噬的胞外病原体）呈递给CD4+辅助T细胞。树突状细胞是最强效的APC，充当先天免疫与适应性免疫之间的关键桥梁。它们在感染部位捕获抗原，迁移至淋巴结，并将加工后的抗原片段呈递给初始T细胞，触发克隆扩增。

4. Cell-Mediated Immunity: T Cells in Action

Cell-mediated immunity is orchestrated primarily by T lymphocytes and is essential for eliminating intracellular pathogens (viruses, some bacteria) and abnormal cells (cancer cells, transplant cells). The process begins when a dendritic cell presents an antigen on MHC Class I to a naive CD8+ T cell with a complementary TCR in the lymph node. This interaction, together with co-stimulatory signals and cytokines (particularly IL-2 from activated CD4+ helper T cells), activates the CD8+ T cell. The activated CD8+ T cell undergoes clonal expansion, producing a large population of effector cytotoxic T lymphocytes (CTLs) and a smaller pool of memory T cells. CTLs migrate to the site of infection and destroy infected cells by releasing cytotoxic granules containing perforin (which creates pores in the target cell membrane) and granzymes (which enter through the pores and induce apoptosis). CTLs also express Fas ligand (FasL) on their surface, which binds to Fas receptors on target cells, triggering the extrinsic apoptosis pathway. This targeted killing mechanism ensures that only infected cells are destroyed while healthy neighbouring cells remain unharmed. 细胞介导免疫主要由T淋巴细胞协调，对于清除胞内病原体（病毒、某些细菌）和异常细胞（癌细胞、移植细胞）至关重要。该过程始于树突状细胞在淋巴结中将MHC I类分子上的抗原呈递给具有互补TCR的初始CD8+ T细胞。这种相互作用与共刺激信号和细胞因子（特别是来自活化CD4+辅助T细胞的IL-2）共同激活CD8+ T细胞。活化的CD8+ T细胞经历克隆扩增，产生大量效应细胞毒性T淋巴细胞（CTL）和少量记忆T细胞。CTL迁移至感染部位，通过释放含有穿孔素（在靶细胞膜上形成孔洞）和颗粒酶（通过孔洞进入并诱导凋亡）的细胞毒性颗粒来摧毁受感染细胞。CTL还在其表面表达Fas配体（FasL），与靶细胞上的Fas受体结合，触发外源性凋亡通路。这种靶向杀伤机制确保只有受感染细胞被摧毁，而邻近健康细胞保持完好。

5. Humoral Immunity: B Cells and Antibody Production

Humoral immunity targets extracellular pathogens (bacteria, toxins, and viruses in the bloodstream or tissue fluid) through the production of antibodies by plasma cells derived from activated B cells. Each naive B cell expresses a unique B-cell receptor (BCR), which is a membrane-bound form of the antibody that the cell is programmed to produce. When a B cell encounters its specific antigen, it internalises the antigen through receptor-mediated endocytosis, processes it, and presents peptide fragments on MHC Class II molecules. A CD4+ helper T cell (specifically a T follicular helper cell, Tfh) with a complementary TCR recognises this antigen-MHC II complex and provides essential activation signals through CD40L-CD40 interaction and cytokine secretion (IL-4, IL-5, IL-6). This T-cell-dependent activation triggers B cell clonal expansion and differentiation. Most of the expanded B cells differentiate into short-lived plasma cells that secrete large quantities of antibodies (approximately 2000 molecules per second per cell). A smaller fraction differentiates into memory B cells that persist for years in the bone marrow and secondary lymphoid organs. Some B cells undergo class-switch recombination, changing the constant region of the antibody from IgM to IgG, IgA, or IgE, which alters the effector function without changing antigen specificity. Additionally, somatic hypermutation in the variable region genes fine-tunes antibody affinity, a process known as affinity maturation. 体液免疫通过由活化的B细胞分化而来的浆细胞产生抗体，靶向胞外病原体（血液或组织液中的细菌、毒素和病毒）。每个初始B细胞表达独特的B细胞受体（BCR），这是该细胞被编程产生的抗体的膜结合形式。当B细胞遇到其特定抗原时，通过受体介导的内吞作用内化抗原，对其进行加工，并在MHC II类分子上呈递肽段片段。具有互补TCR的CD4+辅助T细胞（特别是滤泡辅助T细胞Tfh）识别此抗原-MHC II复合物，并通过CD40L-CD40相互作用和细胞因子分泌（IL-4、IL-5、IL-6）提供必要的激活信号。这种T细胞依赖性激活触发B细胞克隆扩增和分化。大多数扩增的B细胞分化为短期存活的浆细胞，分泌大量抗体（每个细胞每秒约2000个分子）。少数分化为记忆B细胞，在骨髓和次级淋巴器官中持续存在多年。一些B细胞经历类别转换重组，将抗体恒定区从IgM转换为IgG、IgA或IgE，改变效应功能而不改变抗原特异性。此外，可变区基因中的体细胞超突变微调抗体亲和力，这一过程称为亲和力成熟。

6. Antibody Structure and Function

Antibodies, or immunoglobulins, are Y-shaped glycoproteins composed of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulphide bonds. Each chain has a variable region (V region) at the amino-terminal end, which forms the antigen-binding site, and a constant region (C region) that determines the antibody’s effector function and class. The five classes of human antibodies are IgM (first antibody produced in a primary response, excellent at agglutination and complement activation), IgG (the most abundant antibody in serum, crosses the placenta providing passive immunity to the fetus), IgA (found in mucosal secretions such as saliva, tears, and breast milk, protecting mucosal surfaces), IgE (involved in allergic responses and defence against parasitic worms), and IgD (functions primarily as a BCR on naive B cells). Antibodies neutralise pathogens through several mechanisms: neutralisation (blocking pathogen binding to host cells), opsonisation (coating pathogens to enhance phagocytosis), complement activation (triggering the classical complement pathway leading to pathogen lysis), and agglutination (clumping pathogens together for easier clearance). 抗体或免疫球蛋白是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。每条链在氨基端有一个可变区（V区），形成抗原结合位点；还有一个恒定区（C区），决定抗体的效应功能和类别。人类抗体的五种类别为：IgM（初次应答中最早产生的抗体，善于凝集和激活补体）、IgG（血清中最丰富的抗体，穿过胎盘为胎儿提供被动免疫）、IgA（存在于黏膜分泌物中如唾液、泪液和母乳，保护黏膜表面）、IgE（参与过敏反应和防御寄生虫）和IgD（主要在初始B细胞上作为BCR发挥作用）。抗体通过多种机制中和病原体：中和（阻断病原体与宿主细胞结合）、调理（包被病原体以增强吞噬作用）、补体激活（触发经典补体途径导致病原体裂解）和凝集（将病原体聚集成团以便更易清除）。

7. Primary and Secondary Immune Responses

The primary immune response occurs when the adaptive immune system encounters a pathogen for the first time. There is a lag phase of several days during which antigen-presenting cells process and present antigens, and specific B and T cell clones are selected and expanded. The primary response is relatively slow, with peak antibody levels reached approximately 10-14 days after initial exposure, and the predominant antibody class is IgM. During this primary response, memory B cells and memory T cells are generated and remain in the body for decades. The secondary immune response occurs upon re-exposure to the same pathogen. Memory cells are rapidly activated without the need for APC processing and T-cell help (in the case of B cells), resulting in a much shorter lag phase (1-3 days), higher peak antibody concentrations, and class switching predominantly to IgG. This is the immunological basis of vaccination: exposing the immune system to a harmless form of a pathogen (attenuated, inactivated, or subunit vaccine) generates memory cells, so that subsequent exposure to the actual pathogen triggers a rapid, robust secondary response that prevents disease. The secondary response can be 100-1000 times more effective than the primary response in terms of antibody titre. 初次免疫应答发生在适应性免疫系统首次遇到病原体时。存在数天的潜伏期，在此期间抗原呈递细胞加工和呈递抗原，特定的B和T细胞克隆被选择和扩增。初次应答相对缓慢，抗体水平在初次暴露后约10-14天达到峰值，主要抗体类别为IgM。在此初次应答期间，记忆B细胞和记忆T细胞产生并在体内保留数十年。再次暴露于同一病原体时发生二次免疫应答。记忆细胞被迅速激活，无需APC加工和T细胞辅助（对B细胞而言），导致潜伏期大大缩短（1-3天）、抗体峰值浓度更高，且类别转换主要向IgG。这是疫苗接种的免疫学基础：将无害形式的病原体（减毒、灭活或亚单位疫苗）暴露于免疫系统以产生记忆细胞，使后续接触真正病原体时触发快速、强大的二次应答以防止疾病。在抗体滴度方面，二次应答可比初次应答有效100-1000倍。

8. Active and Passive Immunity

Immunity can be categorised as active or passive, and as natural or artificial. Active immunity results from the immune system’s own response to an antigen, generating memory cells that provide long-term protection. Natural active immunity occurs after recovering from an infection. Artificial active immunity is achieved through vaccination, where a vaccine containing antigens (but not causing disease) stimulates the production of memory cells. Passive immunity involves the transfer of pre-formed antibodies from one individual to another, providing immediate but temporary protection as the transferred antibodies are gradually degraded and no memory cells are produced. Natural passive immunity occurs when maternal IgG antibodies cross the placenta to the fetus, and when IgA antibodies are transferred through breast milk to the newborn. Artificial passive immunity involves the injection of antibodies, such as antivenom for snake bites or anti-tetanus immunoglobulin for suspected tetanus infection. Passive immunity is crucial when immediate protection is needed and there is insufficient time for the recipient to mount an active immune response. 免疫可分为主动或被动，以及天然或人工。主动免疫源于免疫系统对抗原的自身反应，产生记忆细胞提供长期保护。天然主动免疫在感染恢复后发生。人工主动免疫通过疫苗接种实现，含有抗原但不引起疾病的疫苗刺激记忆细胞的产生。被动免疫涉及将预先形成的抗体从一个个体转移至另一个个体，提供即时但暂时的保护，因为转移的抗体会逐渐降解且不产生记忆细胞。天然被动免疫发生在母体IgG抗体穿过胎盘进入胎儿以及IgA抗体通过母乳传递给新生儿时。人工被动免疫涉及注射抗体，如蛇咬抗蛇毒血清或疑似破伤风感染的抗破伤风免疫球蛋白。当需要即时保护且受体没有足够时间产生主动免疫应答时，被动免疫至关重要。

9. Monoclonal Antibodies in Medicine

Monoclonal antibodies (mAbs) are laboratory-produced antibodies that are all identical, derived from a single clone of B cells, and bind to a single specific epitope on an antigen. The production of mAbs involves fusing an antibody-producing B cell from an immunised mouse with a myeloma (cancer) cell to create a hybridoma that is both immortal and capable of secreting the desired antibody. Modern techniques use recombinant DNA technology to humanise the antibodies, replacing mouse constant regions with human sequences to reduce immunogenicity. Monoclonal antibodies have revolutionised medicine with applications including: cancer therapy (e.g., trastuzumab/Herceptin targeting HER2 receptors in breast cancer), autoimmune disease treatment (e.g., adalimumab/Humira targeting TNF-alpha in rheumatoid arthritis), diagnostic testing (ELISA pregnancy tests using anti-hCG antibodies), and targeted drug delivery (antibody-drug conjugates that deliver cytotoxic drugs specifically to cancer cells). The AQA specification particularly explores the use of monoclonal antibodies in pregnancy testing and in cancer treatment, making these important application examples for exam questions. 单克隆抗体（mAb）是实验室生产的完全相同的抗体，源自单一B细胞克隆，与抗原上的单一特定表位结合。mAb的生产涉及将来自免疫小鼠的产生抗体的B细胞与骨髓瘤（癌）细胞融合，以创建既永生化又能分泌所需抗体的杂交瘤。现代技术使用重组DNA技术人源化抗体，用人类序列替换小鼠恒定区以减少免疫原性。单克隆抗体已经革新了医学，应用包括：癌症治疗（如曲妥珠单抗/赫赛汀靶向乳腺癌中的HER2受体）、自身免疫病治疗（如阿达木单抗/修美乐靶向类风湿关节炎中的TNF-alpha）、诊断测试（使用抗hCG抗体的ELISA验孕测试）以及靶向药物递送（将细胞毒性药物特异性递送至癌细胞的抗体-药物偶联物）。AQA考试大纲特别探讨了单克隆抗体在验孕和癌症治疗中的应用，使其成为考试题目的重要应用示例。

10. Exam Tips for A-Level Biology

When answering questions on the adaptive immune system, pay careful attention to the precise terminology required by the mark scheme. Distinguish clearly between cell-mediated and humoral immunity: specify T cells for the former and B cells/antibodies for the latter. Always state the role of MHC molecules and the distinction between MHC Class I (endogenous pathway, all nucleated cells, CD8+ T cells) and MHC Class II (exogenous pathway, professional APCs only, CD4+ T cells). When describing the primary versus secondary response, quote quantitative data if provided ： for example, “the secondary response produced antibody concentrations 100 times higher than the primary response”. Use specific terms: “clonal selection” and “clonal expansion” rather than generic “cells multiply”, and distinguish between “plasma cells” (effector B cells that secrete antibodies) and “memory cells” (long-lived cells that enable rapid secondary response). For monoclonal antibody questions, describe the hybridoma method step by step, and be prepared to explain how monoclonal antibodies are used in ELISA tests and in targeted cancer therapy. Common exam pitfalls include confusing BCR with antibody (the BCR is the membrane-bound form), forgetting to mention MHC in antigen presentation, and failing to distinguish between active and passive immunity. 回答关于适应性免疫系统的题目时，要特别注意评分方案所要求的精确术语。明确区分细胞介导免疫和体液免疫：前者指定T细胞，后者指定B细胞/抗体。始终说明MHC分子的作用以及MHC I类（内源性途径、所有有核细胞、CD8+ T细胞）与MHC II类（外源性途径、仅专职APC、CD4+ T细胞）之间的区别。描述初次应答与二次应答时，如有给出定量数据请引用，例如”二次应答产生的抗体浓度比初次应答高100倍”。使用特定术语：”克隆选择”和”克隆扩增”而非笼统的”细胞增殖”，并区分”浆细胞”（分泌抗体的效应B细胞）和”记忆细胞”（使快速二次应答成为可能的长寿细胞）。对于单克隆抗体题目，逐步描述杂交瘤方法，并准备解释单克隆抗体如何在ELISA测试和靶向癌症治疗中使用。常见考试失分点包括混淆BCR与抗体（BCR是膜结合形式）、在抗原呈递中忘记提及MHC，以及未能区分主动免疫和被动免疫。