The Immune System: Exam Essentials | 免疫系统：考点精讲

📚 The Immune System: Exam Essentials | 免疫系统：考点精讲

The immune system is the body’s sophisticated defense network against pathogens, toxins, and abnormal cells. In IB Biology, understanding both the innate and adaptive arms of immunity is crucial, as are the molecular details of antibody action, vaccination, and immune disorders. This article distills the core concepts into a clear, bilingual revision guide.

免疫系统是人体抵御病原体、毒素和异常细胞的精密防御网络。在 IB 生物学中，理解先天免疫与适应性免疫这两大支柱至关重要，同样关键的是抗体作用的分子细节、疫苗接种以及免疫紊乱。本文将核心概念提炼为清晰的双语复习指南。

1. Overview of the Immune System | 免疫系统概述

The immune system distinguishes self from non-self. It uses a layered defense: physical and chemical barriers, the innate immune response, and the highly specific adaptive immune response. The innate response is rapid and non-specific, while the adaptive response is slower but provides long-lasting immunological memory.

免疫系统区分自身与非自身。它采用分层防御：物理与化学屏障、先天免疫应答以及高度特异的适应性免疫应答。先天免疫应答快速且非特异性，而适应性免疫应答较慢，但可提供持久的免疫记忆。

Leukocytes (white blood cells) are the main cellular players. Phagocytes such as macrophages and neutrophils engulf pathogens, while lymphocytes (B cells and T cells) coordinate the adaptive response. These cells communicate via signaling molecules called cytokines.

白细胞是主要细胞角色。吞噬细胞（如巨噬细胞和中性粒细胞）吞食病原体，而淋巴细胞（B 细胞和 T 细胞）协调适应性应答。这些细胞通过称为细胞因子的信号分子进行通讯。

2. Non-Specific Defences: Barriers | 非特异性防御：屏障

The first line of defense prevents pathogen entry. Physical barriers include the skin and mucous membranes. Skin has a tough keratinized outer layer, while mucous membranes line the respiratory, digestive, and urogenital tracts, trapping pathogens in sticky mucus.

第一道防线阻止病原体进入。物理屏障包括皮肤和粘膜。皮肤具有坚韧的角质化外层，而粘膜衬在呼吸道、消化道和泌尿生殖道内，用粘性粘液捕获病原体。

Chemical barriers support these physical structures. Sebum and sweat lower skin pH, inhibiting microbial growth. Lysozyme, an enzyme in tears and saliva, destroys bacterial cell walls. Stomach acid (HCl) denatures proteins and kills most ingested microbes.

化学屏障辅助这些物理结构。皮脂和汗液降低皮肤 pH，抑制微生物生长。泪液和唾液中的溶菌酶可破坏细菌细胞壁。胃酸 (HCl) 使蛋白质变性并杀死大多数摄入的微生物。

Physical barriers: skin, mucous membranes, cilia in airways
Chemical barriers: sebum, sweat, lysozyme, gastric acid

物理屏障：皮肤、粘膜、气道纤毛
化学屏障：皮脂、汗液、溶菌酶、胃酸

3. Non-Specific Defences: Phagocytosis and Inflammation | 非特异性防御：吞噬作用与炎症

When barriers are breached, the innate immune response engages. Phagocytic cells recognize pathogens via generic molecular patterns. Phagocytosis involves chemotaxis, adhesion, engulfment, and intracellular digestion by lysosomal enzymes. The phagocyte then presents antigen fragments on its surface using MHC class II molecules, acting as an antigen-presenting cell (APC).

当屏障被突破时，先天免疫应答启动。吞噬细胞通过通用分子模式识别病原体。吞噬作用包括趋化、粘附、吞入以及由溶酶体酶进行的胞内消化。随后吞噬细胞利用 MHC II 类分子在其表面呈递抗原片段，充当抗原呈递细胞 (APC)。

Inflammation is a local response to tissue damage or infection. Mast cells release histamine, causing vasodilation and increased capillary permeability. This leads to redness, heat, swelling, and pain. Phagocytes migrate to the site and eliminate pathogens. Clotting factors help seal the wound.

炎症是对组织损伤或感染的局部应答。肥大细胞释放组胺，引起血管舒张和毛细血管通透性增加。这导致红、热、肿、痛。吞噬细胞迁移到该部位清除病原体。凝血因子有助于封闭伤口。

The complement system, a group of plasma proteins, also participates non-specifically. It enhances phagocytosis (opsonization), recruits inflammatory cells, and can even lyse pathogens via the membrane attack complex.

补体系统（一组血浆蛋白）也非特异地参与。它增强吞噬作用（调理作用）、招募炎症细胞，甚至可通过膜攻击复合物裂解病原体。

4. Antigens and Antibodies | 抗原与抗体

An antigen is any molecule (often a foreign protein or polysaccharide) that can trigger an adaptive immune response by binding to specific receptors on lymphocytes. Pathogens, pollen, transplanted tissues, and even self-cells in autoimmune conditions can serve as antigens.

抗原是指任何能够通过与淋巴细胞上特异受体结合而引发适应性免疫应答的分子（通常是外来蛋白质或多糖）。病原体、花粉、移植组织，甚至自身免疫状况下的自身细胞，都可以充当抗原。

Antibodies (immunoglobulins) are Y-shaped proteins produced by plasma B cells. They specifically bind to epitopes on antigens. The five classes in humans are IgM, IgG, IgA, IgE, and IgD, each with distinct roles. IgG is the most abundant in blood and can cross the placenta.

抗体（免疫球蛋白）是由浆 B 细胞产生的 Y 形蛋白。它们特异性地结合抗原上的表位。人体内五类抗体分别是 IgM、IgG、IgA、IgE 和 IgD，各自扮演不同角色。IgG 在血液中含量最高，并能穿过胎盘。

5. Antibody Structure and Function | 抗体结构与功能

Each antibody monomer consists of two identical heavy chains and two identical light chains, held together by disulfide bonds. The tips of the Y are the variable regions that form the antigen-binding sites, conferring specificity. The stem is the constant region, which determines the antibody class and mediates effector functions such as opsonization and complement activation.

每个抗体单体由两条相同的重链和两条相同的轻链组成，通过二硫键连接在一起。Y 形顶端为可变区，形成抗原结合位点，赋予特异性。主干为恒定区，决定抗体类别并介导调理作用、补体激活等效应功能。

Antibodies can neutralize toxins and viruses by blocking binding to host cells, agglutinate pathogens, and precipitate soluble antigens. They also mark pathogens for destruction (opsonization) and activate the classical complement pathway.

抗体能通过阻断与宿主细胞的结合来中和毒素和病毒、凝集病原体并沉淀可溶性抗原。它们还标记病原体以供摧毁（调理作用）并激活经典补体途径。

Antibody = 2 Heavy + 2 Light chains; Variable region binds antigen; Constant region binds receptors

抗体 = 2 条重链 + 2 条轻链；可变区结合抗原；恒定区结合受体

6. Specific Immune Response: Cell-Mediated Immunity | 特异性免疫应答：细胞介导免疫

Cell-mediated immunity targets infected cells, cancer cells, and transplanted tissues. It relies on T lymphocytes. When an APC presents an antigen on MHC II, helper T cells (CD4⁺) become activated. Helper T cells proliferate and differentiate, secreting cytokines that stimulate cytotoxic T cells and B cells.

细胞介导免疫针对被感染的细胞、癌细胞和移植组织。它依赖于 T 淋巴细胞。当 APC 在 MHC II 上呈递抗原时，辅助 T 细胞 (CD4⁺) 被激活。辅助 T 细胞增殖并分化，分泌细胞因子以刺激细胞毒性 T 细胞和 B 细胞。

Cytotoxic T cells (CD8⁺) recognize antigens presented on MHC I molecules by all nucleated cells. Upon activation, they release perforin and granzymes, which induce apoptosis in the target cell. This is crucial for eliminating virus-infected cells.

细胞毒性 T 细胞 (CD8⁺) 识别所有有核细胞上 MHC I 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。这对于清除病毒感染细胞至关重要。

Suppressor/regulatory T cells later downregulate the immune response, preventing autoimmunity. Memory T cells remain for years, enabling a faster response upon re-exposure.

抑制性/调节性 T 细胞随后下调免疫应答，防止自身免疫。记忆 T 细胞可存活多年，确保再次暴露时做出更快的应答。

7. Specific Immune Response: Humoral Immunity | 特异性免疫应答：体液免疫

Humoral immunity involves B cells and the production of antibodies, which are soluble in blood and lymph. B cells have specific antibodies on their surface acting as receptors. When a B cell binds its cognate antigen, and receives help from activated helper T cells (cytokine signals), it is fully activated.

体液免疫涉及 B 细胞和抗体的产生，抗体可溶于血液和淋巴液。B 细胞表面带有作为受体的特异性抗体。当 B 细胞与其同源抗原结合，并从活化的辅助 T 细胞获得帮助（细胞因子信号）后，便被完全激活。

Activated B cells undergo clonal expansion, producing plasma cells that secrete large quantities of the same antibody, and memory B cells. This process is called clonal selection. Plasma cells are short-lived but highly productive antibody factories.

活化的 B 细胞进行克隆扩增，产生浆细胞（分泌大量相同抗体）和记忆 B 细胞。这一过程称为克隆选择。浆细胞寿命短但却是高效的抗体工厂。

The primary immune response is slower and produces fewer antibodies, while the secondary response is faster, stronger, and mainly IgG-based, due to memory cells. This forms the basis of vaccination.

初次免疫应答较慢，产生抗体较少；而二次应答则更快、更强，且主要基于 IgG，这得益于记忆细胞。这构成了疫苗接种的基础。

8. Active and Passive Immunity | 主动免疫与被动免疫

Active immunity occurs when the body produces its own antibodies and memory cells following exposure to an antigen. It can be natural (infection) or artificial (vaccination). It provides long-term protection due to immunological memory.

主动免疫发生在机体暴露于抗原后产生自身抗体和记忆细胞时。它可以是天然的（感染）或人工的（疫苗接种）。由于免疫记忆，它能提供长期保护。

Passive immunity involves the transfer of pre-formed antibodies from another source. This can be natural (mother to fetus via placenta, or in breast milk) or artificial (injection of antiserum). Protection is immediate but short-lived, as no memory cells are produced.

被动免疫涉及从另一来源转移已形成的抗体。这可以是天然的（母亲经胎盘传给胎儿，或通过母乳）或人工的（注射抗血清）。保护即时但短暂，因为没有产生记忆细胞。

Type 类型	Acquired by 获得方式	Duration 持续时间	Memory 记忆
Active natural 天然主动	Infection 感染	Long-term 长期	Yes 是
Active artificial 人工主动	Vaccination 疫苗接种	Long-term 长期	Yes 是
Passive natural 天然被动	Placenta/Breast milk 胎盘/母乳	Short-term 短期	No 否
Passive artificial 人工被动	Antiserum injection 抗血清注射	Short-term 短期	No 否

9. Vaccination and Herd Immunity | 疫苗接种与群体免疫

Vaccination stimulates an active immune response without causing disease. Vaccines contain weakened or inactivated pathogens, antigenic proteins, or genetic material (mRNA). They induce the production of memory cells, providing future protection.

疫苗接种能在不引发疾病的情况下激发主动免疫应答。疫苗含有减毒或灭活的病原体、抗原蛋白或遗传物质 (mRNA)。它们诱导记忆细胞的产生，提供未来的保护。

Herd immunity arises when a high proportion of a population is immune (through vaccination or prior infection), thereby protecting individuals who cannot be vaccinated. The threshold varies with the pathogen’s basic reproduction number (R₀). For measles, it is about 95%.

当人群中相当高的比例具有免疫力时（通过疫苗接种或既往感染），就会产生群体免疫，从而保护无法接种疫苗的个体。该阈值因病原体的基本再生数 (R₀) 而异。对于麻疹，约为 95%。

Vaccines have eradicated smallpox and greatly reduced polio and measles incidence. However, vaccine hesitancy can erode herd immunity, leading to outbreaks.

疫苗已消灭了天花，并大幅降低了脊髓灰质炎和麻疹的发病率。然而，疫苗犹豫可能削弱群体免疫，导致暴发。

10. Monoclonal Antibodies | 单克隆抗体

Monoclonal antibodies (mAbs) are identical antibodies produced by hybridoma cells. They are made by fusing a specific antibody-producing B cell with a myeloma (cancer) cell. The resulting hybridoma divides indefinitely and secretes a single, specific antibody in large quantities.

单克隆抗体 (mAb) 是由杂交瘤细胞产生的完全相同的抗体。它们通过将特定的产生抗体的 B 细胞与骨髓瘤（癌细胞）融合而制成。所得的杂交瘤能无限分裂并分泌大量的单一、特异性抗体。

Applications include pregnancy testing (detecting hCG), targeted cancer therapy (attaching a drug or radioactive isotope to a tumor-specific antibody), and treating autoimmune diseases (blocking TNF-α in rheumatoid arthritis). They are also used in diagnostics (ELISA) and for identifying blood groups.

应用包括验孕（检测 hCG）、靶向癌症治疗（将药物或放射性同位素连接到肿瘤特异性抗体上）以及治疗自身免疫性疾病（在类风湿关节炎中阻断 TNF-α）。它们在诊断 (ELISA) 和血型鉴定中也有使用。

11. Immune System Disorders: Allergies and Autoimmunity | 免疫系统疾病：过敏与自身免疫

Allergies are hypersensitivity reactions to normally harmless environmental antigens called allergens. On first exposure, IgE antibodies are produced and bind to mast cells. Upon re-exposure, allergens cross-link the IgE, triggering mast cell degranulation and the release of histamine, causing symptoms ranging from sneezing to anaphylaxis.

过敏是对通常无害的环境抗原（称为过敏原）的超敏反应。初次暴露时，机体产生 IgE 抗体并结合在肥大细胞上。再次暴露时，过敏原交联 IgE，触发肥大细胞脱颗粒并释放组胺，引起从打喷嚏到过敏性休克的各种症状。

Autoimmune diseases occur when the immune system fails to distinguish self from non-self and attacks the body’s own tissues. Examples include Type 1 diabetes (destruction of pancreatic β cells), multiple sclerosis (attack on myelin sheaths), and rheumatoid arthritis (attack on joint synovium).

自身免疫性疾病发生在免疫系统未能区分自身与非自身并攻击自身组织时。实例包括 1 型糖尿病（破坏胰腺 β 细胞）、多发性硬化（攻击髓鞘）和类风湿关节炎（攻击关节滑膜）。

12. Human Immunodeficiency Virus (HIV) | 人类免疫缺陷病毒 (HIV)

HIV is a retrovirus that specifically infects helper T cells (CD4⁺). It uses the CD4 receptor and a co-receptor (CCR5 or CXCR4) to enter the cell. Viral RNA is reverse-transcribed into DNA and integrated into the host genome, leading to the production of new virions and eventual depletion of helper T cells.

HIV 是一种专门感染辅助 T 细胞 (CD4⁺) 的逆转录病毒。它利用 CD4 受体和辅助受体（CCR5 或 CXCR4）进入细胞。病毒 RNA 逆转录为 DNA 并整合到宿主基因组中，导致新病毒颗粒的产生和辅助 T 细胞的最终耗竭。

When the CD4⁺ count falls below a critical level (around 200 cells/:mu:l), the immune system is severely compromised, leading to Acquired Immunodeficiency Syndrome (AIDS). Opportunistic infections such as Pneumocystis pneumonia and Kaposi’s sarcoma become life-threatening.

当 CD4⁺ 计数降至临界水平以下（约 200 个细胞/μl）时，免疫系统严重受损，导致获得性免疫缺陷综合征 (AIDS)。肺孢子菌肺炎和卡波西肉瘤等机会性感染会危及生命。

Antiretroviral therapy (ART) can suppress viral replication but does not eliminate the integrated provirus. Therefore, treatment is lifelong, and as yet there is no cure. Preventive measures include safe sex practices, needle exchange programs, and pre-exposure prophylaxis (PrEP).

抗逆转录病毒疗法 (ART) 可抑制病毒复制，但不能清除已整合的原病毒。因此，治疗是终身的，且目前尚无治愈方法。预防措施包括安全性行为、针具交换计划和暴露前预防 (PrEP)。

HIV RNA → DNA (reverse transcriptase) → Integration → Viral assembly → CD4⁺ cell death

HIV RNA → DNA（逆转录酶）→ 整合 → 病毒装配 → CD4⁺ 细胞死亡

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The Immune System: Exam Essentials | 免疫系统：考点精讲