A-Level生物 免疫系统 特异性免疫

A-Level生物 免疫系统 特异性免疫

Introduction to the Immune System

The immune system is the body’s defence network against pathogens including bacteria, viruses, fungi, and parasites. It comprises a complex array of cells, tissues, and molecules that work together to recognise and eliminate foreign invaders while sparing the body’s own cells. Understanding how this system distinguishes self from non-self is fundamental to A-Level Biology. 免疫系统是人体抵御病原体（包括细菌、病毒、真菌和寄生虫）的防御网络。它由一系列复杂的细胞、组织和分子组成，协同识别并清除外来入侵者，同时不伤害自身细胞。理解免疫系统如何区分自身与非自身是A-Level生物的基础。

Non-Specific vs Specific Immunity

The immune response is divided into two broad categories: non-specific (innate) immunity and specific (adaptive) immunity. Non-specific defences are present from birth and respond to all pathogens in the same way, without memory. These include physical barriers like skin, chemical barriers like stomach acid, phagocytosis by macrophages and neutrophils, and the inflammatory response. 免疫反应分为两大类：非特异性（先天）免疫和特异性（适应性）免疫。非特异性防御是与生俱来的，对所有病原体以相同方式反应，没有免疫记忆。这些包括物理屏障如皮肤、化学屏障如胃酸、巨噬细胞和中性粒细胞的吞噬作用，以及炎症反应。

Specific immunity, by contrast, is acquired through exposure to specific pathogens and produces a tailored response. It involves lymphocytes (B cells and T cells) and generates immunological memory, which is the basis for vaccination. This is the focus of A-Level specifications. 相比之下，特异性免疫通过接触特定病原体获得，并产生针对性反应。它涉及淋巴细胞（B细胞和T细胞），并产生免疫记忆，这是疫苗接种的基础。这是A-Level考试的重点。

Antigens and Antibodies

An antigen is any substance capable of provoking an immune response, typically a protein or polysaccharide on the surface of a pathogen. Each antigen has specific molecular shapes called epitopes that are recognised by receptors on lymphocytes. Self-antigens are normally tolerated, while non-self antigens trigger an immune attack. 抗原是任何能够引发免疫反应的物质，通常是病原体表面的蛋白质或多糖。每个抗原具有称为表位的特定分子形状，被淋巴细胞上的受体识别。自身抗原通常被耐受，而非自身抗原则触发免疫攻击。

Antibodies (immunoglobulins) are Y-shaped proteins produced by plasma cells (differentiated B cells). Each antibody has a variable region that binds specifically to a complementary antigen, and a constant region that interacts with other immune components. The binding of antibody to antigen can neutralise toxins, agglutinate pathogens to make them easier targets for phagocytes, and activate the complement system. 抗体（免疫球蛋白）是由浆细胞（分化的B细胞）产生的Y形蛋白质。每个抗体有一个可变区，与互补抗原特异性结合，以及一个恒定区，与其他免疫成分相互作用。抗体与抗原的结合可以中和毒素、凝集病原体使其更容易被吞噬细胞靶向，并激活补体系统。

B Lymphocytes and Humoral Immunity

B lymphocytes mature in the bone marrow and are responsible for humoral immunity ： the antibody-mediated response that targets pathogens in body fluids (blood, lymph, interstitial fluid). Each B cell carries membrane-bound antibodies (B cell receptors) specific to one antigen. When a B cell encounters its complementary antigen, it becomes activated. B淋巴细胞在骨髓中成熟，负责体液免疫 ： 即抗体介导的反应，靶向体液（血液、淋巴液、组织液）中的病原体。每个B细胞表面携带针对一种抗原的膜结合抗体（B细胞受体）。当B细胞遇到其互补抗原时，它被激活。

Activation of B cells typically requires two signals. The first is antigen binding to the B cell receptor. The second is usually provided by helper T cells, which release cytokines (interleukins) that stimulate B cell division. This is called T-dependent activation and applies to most protein antigens. Some polysaccharide antigens can activate B cells without T cell help (T-independent activation), but this produces a weaker response with no memory cells. B细胞的激活通常需要两个信号。第一个是抗原与B细胞受体的结合。第二个通常由辅助T细胞提供，它们释放细胞因子（白细胞介素）刺激B细胞分裂。这称为T依赖性激活，适用于大多数蛋白质抗原。一些多糖抗原可以在没有T细胞帮助的情况下激活B细胞（T非依赖性激活），但这产生的反应较弱，没有记忆细胞。

Once activated, B cells undergo clonal selection and clonal expansion: the specific B cell with the matching receptor is selected and divides rapidly by mitosis to produce a large clone of identical cells. These daughter cells differentiate into two types: plasma cells, which are short-lived antibody factories secreting up to 2000 antibodies per second, and memory B cells, which persist for years and enable a rapid response upon re-exposure. 一旦激活，B细胞经历克隆选择和克隆扩增：具有匹配受体的特定B细胞被选中，并通过有丝分裂快速分裂，产生大量相同细胞克隆。这些子细胞分化为两种类型：浆细胞，即短命的抗体工厂，每秒分泌多达2000个抗体；以及记忆B细胞，持续多年，在再次接触时实现快速反应。

T Lymphocytes and Cell-Mediated Immunity

T lymphocytes mature in the thymus gland and are responsible for cell-mediated immunity. Unlike B cells, T cells do not recognise free antigens directly. Instead, they recognise antigen fragments displayed on the surface of body cells by major histocompatibility complex (MHC) molecules. There are two classes of MHC: MHC class I is present on all nucleated cells and presents endogenous antigens (from intracellular pathogens like viruses). MHC class II is found only on professional antigen-presenting cells (APCs) such as macrophages, dendritic cells, and activated B cells, and presents exogenous antigens (from engulfed pathogens). T淋巴细胞在胸腺中成熟，负责细胞介导的免疫。与B细胞不同，T细胞不直接识别游离抗原。相反，它们识别由主要组织相容性复合体（MHC）分子展示在体细胞表面的抗原片段。MHC有两类：MHC I类存在于所有有核细胞上，呈递内源性抗原（来自细胞内病原体如病毒）。MHC II类仅存在于专业抗原呈递细胞（APC）如巨噬细胞、树突状细胞和活化的B细胞上，呈递外源性抗原（来自被吞噬的病原体）。

There are several types of T cell. Helper T cells (CD4+ cells) recognise antigens presented on MHC class II and, upon activation, secrete cytokines that stimulate B cells to divide, activate cytotoxic T cells, and enhance macrophage activity. They are the central coordinators of the immune response. Cytotoxic T cells (CD8+ cells) recognise antigens on MHC class I and directly kill infected cells by releasing perforin (which creates pores in the target cell membrane) and granzymes (which induce apoptosis). Memory T cells persist long-term and enable a faster response on re-exposure. Suppressor (regulatory) T cells help shut down the immune response once the pathogen is cleared, preventing autoimmunity. 有几种T细胞类型。辅助T细胞（CD4+细胞）识别MHC II类上呈递的抗原，激活后分泌细胞因子刺激B细胞分裂、激活细胞毒性T细胞并增强巨噬细胞活性。它们是免疫反应的核心协调者。细胞毒性T细胞（CD8+细胞）识别MHC I类上的抗原，通过释放穿孔素（在靶细胞膜上形成孔洞）和颗粒酶（诱导凋亡）直接杀死感染细胞。记忆T细胞长期存在，在再次接触时实现更快反应。抑制性（调节性）T细胞帮助在病原体清除后关闭免疫反应，防止自身免疫。

Primary and Secondary Immune Response

When the body encounters a pathogen for the first time, the primary immune response is initiated. This is relatively slow (taking 5-10 days to produce detectable antibody levels) because the specific B and T cell clones are rare and must undergo clonal selection and expansion. The antibody concentration rises, peaks, and then declines as plasma cells die. IgM is typically the first antibody class produced, followed by a class switch to IgG. 当身体首次遇到病原体时，启动初级免疫反应。这相对较慢（需要5-10天才能产生可检测的抗体水平），因为特定的B和T细胞克隆很少，必须经历克隆选择和扩增。抗体浓度上升、达到峰值，然后随着浆细胞死亡而下降。IgM通常是首先产生的抗体类别，随后类别转换为IgG。

If the same pathogen invades again, the secondary immune response is triggered. Memory B and T cells, which were generated during the primary response and persist in the body, recognise the antigen immediately. The secondary response is much faster (1-2 days), produces far higher antibody concentrations, and is dominated by IgG. This is why we typically do not get sick from the same pathogen twice ： the memory cells eliminate it before symptoms develop. This principle underlies vaccination. 如果同一病原体再次入侵，触发次级免疫反应。在初级反应期间产生并持续存在于体内的记忆B细胞和T细胞立即识别抗原。次级反应更快（1-2天），产生更高的抗体浓度，并以IgG为主。这就是为什么我们通常不会同一病原体感染两次 ： 记忆细胞在症状出现前就将其清除。这一原理是疫苗接种的基础。

Active and Passive Immunity

Active immunity results from the body’s own immune system producing antibodies and memory cells in response to an antigen. This can be natural (through infection) or artificial (through vaccination). It takes time to develop but provides long-lasting protection because memory cells are generated. 主动免疫由身体自身免疫系统针对抗原产生抗体和记忆细胞而来。这可以是自然的（通过感染）或人工的（通过疫苗接种）。它需要时间建立，但由于产生了记忆细胞，能提供持久保护。

Passive immunity involves receiving pre-formed antibodies from an external source rather than producing them. Natural passive immunity occurs when a fetus receives maternal antibodies (IgG) across the placenta, and a newborn receives IgA in colostrum and breast milk. Artificial passive immunity involves injecting antibodies, such as antivenom for snake bites or pooled human immunoglobulin. Passive immunity provides immediate but temporary protection because the antibodies are eventually broken down and no memory cells are generated. 被动免疫涉及从外部来源获得预先形成的抗体，而不是自身产生。自然被动免疫发生在胎儿通过胎盘获得母体抗体（IgG），以及新生儿从初乳和母乳中获得IgA时。人工被动免疫涉及注射抗体，例如蛇咬伤的抗蛇毒血清或混合人免疫球蛋白。被动免疫提供即时但暂时的保护，因为抗体会被最终分解，且不产生记忆细胞。

Vaccination and Herd Immunity

Vaccination is the deliberate introduction of antigens to stimulate active immunity without causing disease. Vaccines may contain killed or inactivated pathogens, attenuated (weakened) live pathogens, toxoids (inactivated toxins), subunit antigens (specific proteins), or recombinant antigens. The success of vaccination requires a sufficient proportion of the population to be immunised to achieve herd immunity, which protects vulnerable individuals who cannot be vaccinated, such as newborns and immunocompromised patients. 疫苗接种是有意引入抗原来刺激主动免疫而不引起疾病。疫苗可能包含灭活病原体、减毒活病原体、类毒素（灭活毒素）、亚单位抗原（特定蛋白质）或重组抗原。疫苗接种的成功需要足够比例的人口免疫以实现群体免疫，保护无法接种疫苗的脆弱个体，如新生儿和免疫功能低下患者。

Common Misconceptions

A common exam error is confusing the roles of B and T cells. Remember: B cells produce antibodies (humoral response) and T cells attack infected cells and coordinate the response (cell-mediated response). Another common mistake is stating that antibiotics treat viral infections ： antibiotics only target bacteria, not viruses. Students also confuse antigens with antibodies: antigens are the foreign molecules (usually on pathogen surfaces), while antibodies are the Y-shaped proteins our body produces in response. 常见的考试错误是混淆B细胞和T细胞的作用。记住：B细胞产生抗体（体液反应），T细胞攻击受感染细胞并协调反应（细胞介导反应）。另一个常见错误是说抗生素治疗病毒感染 ： 抗生素只针对细菌，不针对病毒。学生还混淆抗原和抗体：抗原是外来分子（通常在病原体表面），而抗体是我们身体在反应中产生的Y形蛋白质。

Exam Tips

When answering questions on the immune system, always include the key terms from the specification: clonal selection, clonal expansion, plasma cells, memory cells, antigen-presenting cells, MHC, cytokines, and the distinction between humoral and cell-mediated immunity. Use precise language: T helper cells “activate” B cells through cytokines, not by direct contact. Phagocytes “engulf” pathogens, not “eat” them. 在回答免疫系统问题时，始终包含考试大纲中的关键术语：克隆选择、克隆扩增、浆细胞、记忆细胞、抗原呈递细胞、MHC、细胞因子，以及体液免疫和细胞介导免疫的区别。使用精确语言：辅助T细胞通过细胞因子”激活”B细胞，而非直接接触。吞噬细胞”吞噬”病原体，而非”吃掉”它们。

For the A-Level exam, you should be able to draw and label the structure of an antibody molecule, describe the sequence of events in both humoral and cell-mediated responses, explain the differences between primary and secondary responses with reference to a graph, and discuss the ethical considerations surrounding vaccination programmes. Practice comparing the properties of active versus passive immunity in a table format. 在A-Level考试中，你应该能够绘制并标注抗体分子的结构，描述体液和细胞介导反应的事件顺序，参照图表解释初级和次级反应的区别，并讨论疫苗接种计划的伦理考量。练习以表格形式比较主动与被动免疫的特性。