A-Level生物 免疫系统 细胞免疫 体液免疫

A-Level生物 免疫系统 细胞免疫 体液免疫

1. Introduction to the Immune System

The immune system is the body’s defence network against pathogens: bacteria, viruses, fungi, and parasites. It distinguishes self from non-self and mounts coordinated responses to eliminate threats. A-Level biology focuses on the mammalian immune system, covering both non-specific (innate) and specific (adaptive) branches. Understanding immunity is foundational for topics including vaccination, monoclonal antibodies, and autoimmune disease.

免疫系统是人体抵抗病原体（细菌、病毒、真菌、寄生虫）的防御网络。它通过区分”自己”与”非己”来启动协调的免疫应答，清除外来威胁。A-Level生物课程重点关注哺乳动物免疫系统，涵盖非特异性（先天）和特异性（适应性）两个分支。理解免疫原理是疫苗、单克隆抗体和自身免疫病等后续话题的基础。

The key cells involved include phagocytes (neutrophils and macrophages), lymphocytes (B cells and T cells), and antigen-presenting cells. These cells originate from haematopoietic stem cells in the bone marrow. B lymphocytes mature in the bone marrow, while T lymphocytes mature in the thymus gland: a distinction that names both cell types.

参与免疫的主要细胞包括吞噬细胞（中性粒细胞和巨噬细胞）、淋巴细胞（B细胞和T细胞）以及抗原呈递细胞。这些细胞均起源于骨髓中的造血干细胞。B淋巴细胞在骨髓中成熟，而T淋巴细胞在胸腺中成熟：这一区别正是两种细胞命名的由来。

2. Lines of Defence: Barriers and Innate Immunity

The body maintains three lines of defence. The first line comprises physical and chemical barriers: skin, mucous membranes, stomach acid, lysozyme in tears and saliva. These barriers prevent most pathogens from entering the body. The second line is the innate (non-specific) immune response, which activates within minutes of infection without requiring prior exposure. The third line is the adaptive (specific) immune response, discussed in later sections.

人体具有三道防线。第一道防线包括物理和化学屏障：皮肤、黏膜、胃酸、泪液和唾液中的溶菌酶。这些屏障阻止绝大多数病原体进入体内。第二道防线是先天（非特异性）免疫应答，在感染后数分钟内即可启动，无需预先接触。第三道防线是适应性（特异性）免疫应答，将在后续章节讨论。

Phagocytosis is the hallmark of innate immunity. Neutrophils are the first responders, migrating to infection sites via chemotaxis. Macrophages are larger, longer-lived phagocytes that engulf pathogens and also serve as antigen-presenting cells (APCs). The process of phagocytosis involves chemotaxis toward the pathogen, attachment to its surface, engulfment via pseudopodia to form a phagosome, fusion with lysosomes to create a phagolysosome, and enzymatic digestion of the pathogen.

吞噬作用是先天免疫的核心机制。中性粒细胞是第一批响应者，通过趋化作用迁移至感染部位。巨噬细胞是体积更大、寿命更长的吞噬细胞，既能吞噬病原体，又可作为抗原呈递细胞（APC）。吞噬作用的过程包括：向病原体趋化移动、附着于病原体表面、通过伪足包围吞噬形成吞噬体、与溶酶体融合形成吞噬溶酶体、最后通过酶解消化病原体。

Inflammation is another key innate response. Damaged mast cells release histamine, which causes vasodilation and increased capillary permeability. This produces the characteristic signs of inflammation: redness, heat, swelling, and pain. Vasodilation increases blood flow to the area, while increased permeability allows phagocytes and plasma proteins to enter the tissue.

炎症反应是另一关键的先天免疫反应。受损的肥大细胞释放组胺，引起血管舒张和毛细血管通透性增加。这产生了炎症的典型体征：红、热、肿、痛。血管舒张增加了流向该区域的血流量，而通透性增加则允许吞噬细胞和血浆蛋白进入组织。

3. Adaptive Immunity: An Overview

Adaptive immunity is specific, diverse, and possesses immunological memory. It targets particular antigens: molecules (usually proteins or polysaccharides) on the surface of pathogens that trigger an immune response. The two branches of adaptive immunity are humoral immunity, mediated by B lymphocytes and antibodies in body fluids, and cell-mediated immunity, mediated by T lymphocytes.

适应性免疫具有特异性、多样性和免疫记忆三大特征。它针对特定的抗原：即病原体表面触发免疫应答的分子（通常为蛋白质或多糖）。适应性免疫的两个分支分别为：由B淋巴细胞和体液中的抗体介导的体液免疫，以及由T淋巴细胞介导的细胞免疫。

Antigen presentation is the critical bridge between innate and adaptive immunity. When a macrophage or dendritic cell phagocytoses a pathogen, it processes the pathogen’s antigens and displays peptide fragments on its surface bound to major histocompatibility complex (MHC) class II molecules. This antigen-MHC complex is then recognised by helper T cells, which initiate the adaptive response.

抗原呈递是连接先天免疫与适应性免疫的关键桥梁。当巨噬细胞或树突状细胞吞噬病原体后，它们会处理病原体的抗原，并将肽段展示在细胞表面，与MHC II类分子结合。这一抗原-MHC复合物随后被辅助T细胞识别，从而启动适应性免疫应答。

4. Humoral Immunity: B Cells and Antibody Production

Humoral immunity targets pathogens in body fluids (blood, lymph, interstitial fluid). The process begins when a specific B cell encounters its complementary antigen. The B cell has membrane-bound antibodies (B cell receptors) that bind to the antigen. This binding, combined with stimulation from helper T cells (via cytokines such as interleukin-2), activates the B cell to undergo clonal selection.

体液免疫针对体液（血液、淋巴、组织液）中的病原体。该过程始于特定的B细胞遇到与其互补的抗原。B细胞表面具有膜结合抗体（B细胞受体），可与抗原结合。这种结合在辅助T细胞（通过IL-2等细胞因子）的刺激下，激活B细胞进行克隆选择。

Clonal selection produces two types of daughter cells. Plasma cells are antibody factories: they secrete large quantities of antibodies (approximately 2000 per second) into the blood and lymph. These antibodies have the same specificity as the parent B cell’s receptor. Memory B cells, in contrast, are long-lived cells that do not secrete antibodies but remain dormant in the body, ready to mount a rapid response upon re-exposure to the same antigen.

克隆选择产生两种子代细胞。浆细胞是抗体工厂：它们以每秒约2000个的速度向血液和淋巴中大量分泌抗体，这些抗体的特异性与父代B细胞的受体完全相同。记忆B细胞则是长寿细胞，不分泌抗体，但以休眠状态存留在体内，一旦再次接触相同抗原则可迅速启动免疫应答。

Antibodies (immunoglobulins) are Y-shaped proteins composed of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bridges. Each antibody has two identical antigen-binding sites at the tips of the variable regions. The constant region determines the antibody class (IgM, IgG, IgA, IgE, IgD) and interacts with effector cells. Antibodies neutralise pathogens through several mechanisms: agglutination (clumping pathogens together), neutralisation (blocking pathogen binding sites), opsonisation (tagging pathogens for phagocytosis), and activation of the complement system.

抗体（免疫球蛋白）是由四条多肽链组成的Y形蛋白质：两条相同的重链和两条相同的轻链，通过二硫键连接。每个抗体在可变区末端有两个相同的抗原结合位点。恒定区决定了抗体类别（IgM、IgG、IgA、IgE、IgD），并与效应细胞相互作用。抗体通过多种机制中和病原体：凝集（使病原体聚集）、中和（阻断病原体结合位点）、调理（标记病原体以便吞噬细胞识别）以及激活补体系统。

5. Cell-Mediated Immunity: T Cells at Work

Cell-mediated immunity targets intracellular pathogens: viruses that have invaded host cells, certain bacteria, and cancer cells. This branch involves T lymphocytes that mature in the thymus and express T cell receptors (TCRs) on their surface. There are two main types: helper T cells (CD4+) and cytotoxic T cells (CD8+).

细胞免疫针对细胞内病原体：入侵宿主细胞的病毒、某些细菌以及癌细胞。这一分支涉及在胸腺中成熟、表面表达T细胞受体（TCR）的T淋巴细胞。T细胞主要有两种类型：辅助T细胞（CD4+）和细胞毒性T细胞（CD8+）。

Helper T cells (Th cells) are the master regulators of the immune response. When a Th cell’s TCR recognises an antigen displayed on MHC class II molecules on an APC, the Th cell becomes activated. Activated Th cells secrete cytokines that stimulate B cells to produce antibodies, activate cytotoxic T cells, and enhance macrophage activity. This central coordinating role explains why HIV, which depletes CD4+ T cells, is so devastating: it disables the entire adaptive immune system.

辅助T细胞（Th细胞）是免疫应答的总调控者。当Th细胞的TCR识别APC表面MHC II类分子上展示的抗原后，Th细胞被激活。激活的Th细胞分泌多种细胞因子，刺激B细胞产生抗体、激活细胞毒性T细胞、增强巨噬细胞活性。这一核心协调角色解释了为何HIV（攻击CD4+ T细胞）如此具有破坏性：它使整个适应性免疫系统瘫痪。

Cytotoxic T cells (Tc cells) directly kill infected or abnormal cells. They recognise antigens presented on MHC class I molecules, which are found on all nucleated cells. When a cell is infected by a virus, it displays viral peptides on MHC I. The Tc cell binds via its TCR and CD8 co-receptor, then releases perforin (which creates pores in the target cell membrane) and granzymes (which enter through the pores and trigger apoptosis). This targeted killing spares neighbouring healthy cells.

细胞毒性T细胞（Tc细胞）直接杀伤受感染或异常细胞。它们识别MHC I类分子上呈递的抗原，MHC I类分子存在于所有有核细胞表面。当细胞被病毒感染后，会在MHC I上展示病毒肽段。Tc细胞通过其TCR和CD8共受体结合目标细胞，随后释放穿孔素（在靶细胞膜上形成孔道）和颗粒酶（经孔道进入并触发细胞凋亡）。这种精准杀伤不会波及邻近的健康细胞。

6. Primary and Secondary Immune Responses

The primary immune response occurs when the body encounters a pathogen for the first time. It is relatively slow: the lag phase (before detectable antibodies appear) lasts 5 to 10 days. IgM is the first antibody class produced, followed by IgG. The peak antibody concentration is moderate, and symptoms of the disease typically develop before the response clears the infection.

初次免疫应答发生在身体首次接触某种病原体时。其反应相对缓慢：潜伏期（检测到抗体之前）持续5至10天。IgM是最先产生的抗体类别，随后为IgG。抗体峰值浓度中等，通常在免疫应答清除感染之前，疾病症状已经出现。

The secondary immune response occurs upon re-exposure to a previously encountered pathogen. It is much faster (lag phase of 1 to 2 days), produces a much higher antibody concentration (predominantly IgG), and often clears the infection before any symptoms develop. This enhanced response is due to memory B cells and memory T cells that persist after the primary response. The secondary response is the scientific basis for vaccination.

再次免疫应答发生在再次接触已接触过的病原体时。它速度更快（潜伏期仅1至2天），产生的抗体浓度远高于初次（主要为IgG），通常在症状出现前就已清除感染。这种增强反应归因于初次应答后存留的记忆B细胞和记忆T细胞。再次免疫应答是疫苗接种的科学基础。

7. Vaccination and Herd Immunity

Vaccination exploits immunological memory. A vaccine contains antigens (live attenuated, inactivated, or subunit preparations) derived from a pathogen, administered to stimulate a primary immune response without causing disease. The resulting memory cell population provides long-term protection: upon actual infection, the secondary response rapidly eliminates the pathogen.

疫苗接种利用了免疫记忆原理。疫苗含有源自病原体的抗原（减毒活疫苗、灭活疫苗或亚单位疫苗），接种后可激发初次免疫应答而不引起疾病。产生的记忆细胞群可提供长期保护：当真正感染发生时，再次免疫应答迅速清除病原体。

Herd immunity occurs when a sufficiently large proportion of a population is vaccinated, reducing pathogen transmission so that even unvaccinated individuals are protected. The threshold for herd immunity depends on the pathogen’s basic reproduction number (R0). For measles, with an R0 of 12 to 18, herd immunity requires approximately 95% vaccine coverage. The concept of herd immunity explains why vaccination programmes benefit entire communities, not just vaccinated individuals.

群体免疫是指当人群中足够大比例的人接种疫苗后，病原体传播被削弱，使得即使是未接种个体也受到保护。群体免疫的阈值取决于病原体的基本传染数（R0）。以麻疹为例，其R0为12至18，群体免疫需要约95%的疫苗接种覆盖率。群体免疫这一概念解释了为什么疫苗接种计划造福的是整个社区，而不仅仅是接种个体。

8. Exam Tips for A-Level Biology

When describing phagocytosis, always use the correct sequence: chemotaxis, attachment, engulfment (phagosome formation), lysosome fusion (phagolysosome), digestion, and exocytosis of waste. Examiners look for precise terminology: “pseudopodia” for the membrane extensions and “phagolysosome” for the fused vesicle. Do not write “eats” or “digests” without mechanistic detail.

描述吞噬作用时，务必使用正确顺序：趋化、附着、吞噬（吞噬体形成）、溶酶体融合（吞噬溶酶体）、消化、废物胞吐。考官看重精确术语：用”pseudopodia”（伪足）指代膜延伸，用”phagolysosome”（吞噬溶酶体）指代融合后的囊泡。不要仅写”eats”或”digests”而不提供机制细节。

For antibody structure, draw and label the key features: variable region, constant region, antigen-binding site, disulfide bridges, heavy chains, and light chains. Be explicit about the distinction between the variable region (which determines antigen specificity) and the constant region (which determines antibody class). A common exam question asks you to explain why an antibody’s structure makes it specific to one antigen.

关于抗体结构，应绘制并标注关键特征：可变区、恒定区、抗原结合位点、二硫键、重链和轻链。请明确区分可变区（决定抗原特异性）和恒定区（决定抗体类别）。常见的考试题目是要求解释抗体结构如何使其对一种抗原具有特异性。

When comparing primary and secondary immune responses, focus on three key differences: lag time (long vs short), antibody concentration (lower peak vs higher peak), and antibody class (IgM first, then IgG vs predominantly IgG). Use the correct vocabulary: “immunological memory” rather than “the body remembers”. Link your answer to the clinical context by referencing memory cells specifically.

比较初次与再次免疫应答时，聚焦三个关键差异：潜伏期（长与短）、抗体浓度（较低峰值与较高峰值）、抗体类别（先IgM后IgG与主要为IgG）。使用正确词汇：用”免疫记忆”而非”身体记住”。在答题时通过引用记忆细胞将答案与临床背景联系起来。

Questions about vaccination often require you to explain the mechanism before discussing the social dimension. Start with: vaccines contain antigens that stimulate a primary response and generate memory cells, enabling a rapid secondary response on infection. Then discuss herd immunity: high vaccination coverage protects vulnerable individuals (newborns, immunocompromised) who cannot be vaccinated. Avoid simply stating “vaccines prevent disease” without explaining the immunological mechanism.

关于疫苗接种的题目通常要求先解释机制再讨论社会层面。开头应写：疫苗含有抗原，可激发初次免疫应答并产生记忆细胞，使机体在真正感染时能启动快速的再次应答。然后讨论群体免疫：高接种覆盖率可保护无法接种疫苗的脆弱人群（新生儿、免疫功能低下者）。避免仅写”疫苗预防疾病”而不解释免疫机制。