A-Level Biology: The Immune System — T Cells, B Cells & Antibodies | 免疫系统：T细胞、B细胞与抗体

Introduction | 引言

The human body is constantly under attack from pathogens — bacteria, viruses, fungi, and parasites. Fortunately, we possess a sophisticated defence network: the immune system. For A-Level Biology, understanding the cellular and humoral arms of the adaptive immune response — particularly the roles of T lymphocytes and B lymphocytes — is essential. This article covers everything you need, from antigen presentation to clonal selection theory, with bilingual explanations to deepen your understanding.

人体无时无刻不在受到病原体——细菌、病毒、真菌和寄生虫的侵袭。幸运的是，我们拥有一套精密的防御网络：免疫系统。在A-Level生物学中，理解适应性免疫应答的细胞免疫和体液免疫机制——特别是T淋巴细胞和B淋巴细胞的作用——至关重要。本文涵盖从抗原呈递到克隆选择理论的所有核心知识点，并提供中英双语解释以加深你的理解。

1. Innate vs Adaptive Immunity | 先天免疫与适应性免疫

1.1 Innate Immunity (先天免疫)

Innate immunity is the first line of defence and responds rapidly — within minutes to hours. It is non-specific: it does not distinguish between different pathogens. Key components include:

先天免疫是第一道防线，反应迅速——在几分钟到几小时内。它是非特异性的：不区分不同病原体。主要组成部分包括：

Physical barriers: skin, mucous membranes, cilia in the respiratory tract.
物理屏障：皮肤、粘膜、呼吸道纤毛。
Chemical barriers: stomach acid (HCl), lysozyme in tears and saliva.
化学屏障：胃酸（盐酸）、泪液和唾液中的溶菌酶。
Phagocytes: neutrophils and macrophages that engulf and digest pathogens via phagocytosis.
吞噬细胞：中性粒细胞和巨噬细胞，通过吞噬作用吞噬和消化病原体。
Inflammatory response: histamine release causes vasodilation and increased capillary permeability, allowing more immune cells to reach the site of infection.
炎症反应：组胺释放引起血管扩张和毛细血管通透性增加，使更多免疫细胞到达感染部位。

1.2 Adaptive Immunity (适应性免疫)

Adaptive immunity is slower (days to weeks on first exposure) but highly specific. It also produces immunological memory, enabling a faster, stronger response upon re-exposure. The adaptive response has two branches:

适应性免疫较慢（初次接触需数天至数周），但具有高度特异性。它还产生免疫记忆，使再次接触时能够产生更快、更强的应答。适应性免疫有两个分支：

Cell-mediated immunity: driven by T lymphocytes (T cells) — targets intracellular pathogens (viruses, some bacteria).
细胞介导免疫：由T淋巴细胞驱动——针对胞内病原体（病毒、某些细菌）。
Humoral immunity: driven by B lymphocytes (B cells) — targets extracellular pathogens via antibody production.
体液免疫：由B淋巴细胞驱动——通过产生抗体针对胞外病原体。

2. Antigens and Antigen Presentation | 抗原与抗原呈递

2.1 What is an Antigen? (什么是抗原？)

An antigen is any molecule — typically a protein or polysaccharide — that the immune system can recognise as foreign (non-self). Antigens are usually found on the surface of pathogens, but they can also be present on cancer cells, transplanted tissues, or even harmless substances (allergens). Each antigen has specific epitopes — the precise regions recognised by immune receptors.

抗原是免疫系统能够识别为外来（非自身）的任何分子——通常是蛋白质或多糖。抗原通常存在于病原体表面，但也可以存在于癌细胞、移植组织，甚至无害物质（过敏原）上。每个抗原都有特定的表位（epitope）——即免疫受体识别的精确区域。

2.2 Antigen-Presenting Cells (APCs) | 抗原呈递细胞

For T cells to “see” an antigen, it must be presented to them by specialised cells. The three professional APCs are:

为了让T细胞”看到”抗原，必须由专门的细胞将抗原呈递给它们。三种专职抗原呈递细胞是：

Dendritic cells: the most potent APCs; they patrol tissues, capture antigens, and migrate to lymph nodes to activate naïve T cells.
树突状细胞：最强的抗原呈递细胞；它们在组织中巡逻，捕获抗原，并迁移到淋巴结以激活初始T细胞。
Macrophages: phagocytic cells that present antigens after digesting pathogens.
巨噬细胞：在消化病原体后呈递抗原的吞噬细胞。
B cells: can present antigens to helper T cells after binding them via their surface antibodies (BCRs).
B细胞：在通过其表面抗体（BCR）结合抗原后，可向辅助T细胞呈递抗原。

2.3 MHC Molecules | MHC分子

Antigens are presented on the surface of APCs bound to Major Histocompatibility Complex (MHC) molecules. There are two classes:

抗原被呈递到APC表面时，结合在主要组织相容性复合体（MHC）分子上。有两类：

Feature	MHC Class I	MHC Class II
Found on	All nucleated cells (体内所有有核细胞)	Professional APCs only (仅专职APC)
Presents to	CD8⁺ Cytotoxic T cells (细胞毒性T细胞)	CD4⁺ Helper T cells (辅助T细胞)
Antigen source	Endogenous (viral proteins made inside the cell — 内源性)	Exogenous (phagocytosed material — 外源性)

Exam tip: Remember the “8” in CD8 and “I” in MHC-I — both have single-digit numbers. Similarly, CD4 pairs with MHC-II (both even). This mnemonic saves marks in exam questions!

考试技巧：记住CD8中的”8″和MHC-I中的”I”——都是个位数。类似地，CD4与MHC-II配对（都是偶数）。这个助记法能在考试题中帮你拿分！

3. T Lymphocytes (T Cells) | T淋巴细胞

3.1 T Cell Development and Selection | T细胞的发育与选择

T lymphocytes mature in the thymus gland (hence “T”). During development, they undergo two critical selection processes:

T淋巴细胞在胸腺中成熟（因此称为”T细胞”）。在发育过程中，它们经历两个关键的选择过程：

Positive selection: T cells must recognise self-MHC molecules. Cells that fail to bind self-MHC undergo apoptosis.
阳性选择：T细胞必须识别自身MHC分子。无法结合自身MHC的细胞会凋亡。
Negative selection: T cells that bind too strongly to self-antigens are eliminated, preventing autoimmunity.
阴性选择：与自身抗原结合过强的T细胞被清除，从而防止自身免疫。

This dual selection produces a repertoire of T cells that are self-MHC restricted and self-tolerant.

这种双重选择产生了一个受自身MHC限制且自身耐受的T细胞库。

3.2 Helper T Cells (Th / CD4⁺) | 辅助T细胞

Helper T cells are the central coordinators of the adaptive immune response. Their TCR (T cell receptor) binds to the antigen-MHC-II complex on APCs. Upon activation, helper T cells:

辅助T细胞是适应性免疫应答的核心协调者。它们的TCR（T细胞受体）与APC上的抗原-MHC-II复合物结合。激活后，辅助T细胞：

Clonal expansion: undergo rapid mitosis, producing thousands of identical clones.
克隆扩增：进行快速有丝分裂，产生数千个完全相同的克隆。
Cytokine secretion: release interleukins (e.g., IL-2) that stimulate B cells, cytotoxic T cells, and macrophages.
细胞因子分泌：释放白细胞介素（如IL-2），刺激B细胞、细胞毒性T细胞和巨噬细胞。
B cell activation: provide “help” to B cells by binding CD40 ligand (on Th) to CD40 receptor (on B cell), triggering B cell proliferation and differentiation.
B细胞激活：通过将CD40配体（在Th上）与CD40受体（在B细胞上）结合，为B细胞提供”帮助”，触发B细胞增殖和分化。

3.3 Cytotoxic T Cells (Tc / CD8⁺) | 细胞毒性T细胞

Cytotoxic T cells are the killers of the immune system. Their TCR binds to antigen-MHC-I complexes on infected or cancerous cells. Once activated (with help from Th cells):

细胞毒性T细胞是免疫系统的杀手。它们的TCR与感染细胞或癌细胞上的抗原-MHC-I复合物结合。一旦激活（在Th细胞的帮助下）：

They release perforin, a protein that creates pores in the target cell membrane.
释放穿孔素（perforin），在靶细胞膜上形成孔洞。
They secrete granzymes, proteases that enter through the pores and trigger apoptosis (programmed cell death).
分泌颗粒酶（granzyme），通过孔洞进入并触发凋亡（程序性细胞死亡）。
This is highly precise — only infected cells expressing the specific antigen are killed.
这非常精确——只有表达特定抗原的感染细胞被杀死。

3.4 Memory T Cells | 记忆T细胞

After an infection is cleared, most effector T cells die by apoptosis. However, a small population of memory T cells persists — sometimes for decades. Upon re-exposure to the same antigen, memory T cells mount a secondary response that is faster, larger, and more effective than the primary response. This is the basis of vaccination.

感染清除后，大多数效应T细胞会凋亡。然而，一小部分记忆T细胞会持续存在——有时长达数十年。当再次接触相同抗原时，记忆T细胞会发动比初次应答更快、更强、更有效的二次应答。这就是疫苗接种的基础。

4. B Lymphocytes (B Cells) | B淋巴细胞

4.1 B Cell Development | B细胞的发育

B cells mature in the bone marrow (hence “B”). Each naïve B cell expresses approximately 100,000 identical B cell receptors (BCRs) — membrane-bound antibodies — on its surface. Each B cell has a unique BCR, creating a diverse repertoire capable of recognising virtually any antigen.

B细胞在骨髓中成熟（因此称为”B细胞”）。每个初始B细胞在其表面表达约100,000个完全相同的B细胞受体（BCR）——即膜结合抗体。每个B细胞拥有独特的BCR，创造出一个能够识别几乎任何抗原的多样化库。

4.2 B Cell Activation | B细胞的激活

B cell activation usually requires two signals:

B细胞激活通常需要两个信号：

Signal 1: The BCR binds to its specific antigen. The B cell internalises the antigen, processes it, and presents it on MHC-II molecules.
信号1：BCR与其特异性抗原结合。B细胞内化抗原，处理后在MHC-II分子上呈递。
Signal 2: A helper T cell (already activated by the same antigen) recognises the presented antigen-MHC-II complex and provides co-stimulation (CD40-CD40L interaction, plus cytokines).
信号2：已由相同抗原激活的辅助T细胞识别呈递的抗原-MHC-II复合物，并提供共刺激（CD40-CD40L相互作用，加上细胞因子）。

Once activated, B cells undergo clonal expansion and differentiate into two types:

激活后，B细胞进行克隆扩增并分化为两种类型：

Plasma cells (效应B细胞 / 浆细胞): short-lived antibody factories that secrete up to 2,000 antibody molecules per second. They do not express surface BCRs.
浆细胞：短命的抗体工厂，每秒可分泌高达2,000个抗体分子。它们不表达表面BCR。
Memory B cells (记忆B细胞): long-lived cells that persist in the body and mount rapid secondary responses upon re-exposure.
记忆B细胞：长寿命细胞，在体内存留，并在再次接触时发动快速二次应答。

5. Antibodies (Immunoglobulins) | 抗体（免疫球蛋白）

5.1 Antibody Structure | 抗体结构

Antibodies are Y-shaped glycoproteins composed of four polypeptide chains:

抗体是Y形糖蛋白，由四条多肽链组成：

Two heavy chains (重链): longer polypeptides forming the inner structure.
两条重链：较长的多肽链，构成内部结构。
Two light chains (轻链): shorter polypeptides on the outside.
两条轻链：较短的肽链，位于外侧。
Chains are held together by disulfide bonds (二硫键).

Each arm of the Y contains an antigen-binding site (Fab region) formed by the variable regions of both heavy and light chains. The stem is the constant region (Fc region) that determines the antibody class and binds to immune cell receptors.

Y的每条臂都包含一个抗原结合位点（Fab区），由重链和轻链的可变区组成。柄部是恒定区（Fc区），决定抗体类别并与免疫细胞受体结合。

5.2 Classes of Antibodies | 抗体类别

Class	Structure	Function
IgG	Monomer	Most abundant; crosses placenta (passive immunity); opsonisation, neutralisation (最主要的抗体；穿过胎盘提供被动免疫；调理作用、中和作用)
IgM	Pentamer	First antibody produced in primary response; strong agglutination (初次应答中最先产生的抗体；强凝集作用)
IgA	Dimer	Found in secretions (milk, saliva, tears); mucosal immunity (存在于分泌物中；粘膜免疫)
IgE	Monomer	Involved in allergic reactions and defence against parasites (参与过敏反应和抗寄生虫防御)

5.3 Mechanisms of Antibody Action | 抗体作用机制

Neutralisation (中和作用): antibodies bind to toxins or viral surface proteins, blocking them from interacting with host cells.
中和作用：抗体与毒素或病毒表面蛋白结合，阻止它们与宿主细胞相互作用。
Agglutination (凝集作用): antibodies cross-link pathogens, forming clumps that are easier for phagocytes to engulf.
凝集作用：抗体交联病原体，形成团块，使吞噬细胞更容易吞噬。
Opsonisation (调理作用): antibodies coat the pathogen, and the Fc region binds to receptors on phagocytes, enhancing phagocytosis.
调理作用：抗体包裹病原体，Fc区与吞噬细胞上的受体结合，增强吞噬作用。
Complement activation (补体激活): antigen-antibody complexes trigger the complement cascade, leading to lysis of the pathogen.
补体激活：抗原-抗体复合物触发补体级联反应，导致病原体溶解。

6. Primary vs Secondary Immune Response | 初次免疫应答与二次免疫应答

When the body encounters a pathogen for the first time:

当身体首次接触到某种病原体时：

There is a lag phase of several days while the correct T and B cell clones are activated.
有数天的滞后期，期间正确的T和B细胞克隆被激活。
Antibody levels rise slowly, peaking at 7–14 days.
抗体水平缓慢上升，在7-14天达到峰值。
IgM is produced first, followed by class-switching to IgG.
先产生IgM，随后类别转换至IgG。
Symptoms of illness appear during this period.
在此期间出现疾病症状。

Upon re-exposure (secondary response):

再次接触时（二次应答）：

The lag phase is much shorter (hours to 1–2 days) because memory cells are already present.
滞后期大幅缩短（数小时至1-2天），因为记忆细胞已存在。
Antibody production is faster, and the concentration is 10–100× higher.
抗体产生更快，浓度高出10-100倍。
Mainly IgG is produced (class-switching already primed).
主要产生IgG（类别转换已预启动）。
The pathogen is usually eliminated before symptoms develop.
病原体通常在症状出现之前就被清除。

This is the fundamental principle behind vaccination: expose the body to a harmless form of the antigen (attenuated pathogen, subunit protein, or mRNA) to generate memory cells without causing disease.

这就是疫苗接种的基本原理：让身体暴露于无害形式的抗原（减毒病原体、亚单位蛋白或mRNA），在不引起疾病的情况下产生记忆细胞。

7. Clonal Selection Theory | 克隆选择学说

The Clonal Selection Theory, proposed by Frank Macfarlane Burnet (Nobel Prize 1960), explains how the adaptive immune system can recognise an almost infinite variety of antigens:

克隆选择学说由弗兰克·麦克法兰·伯内特（1960年诺贝尔奖）提出，解释了适应性免疫系统如何识别几乎无限种类的抗原：

Each lymphocyte (B or T cell) carries a single, unique type of receptor on its surface.
每个淋巴细胞（B或T细胞）表面携带单一且独特类型的受体。
There are millions of different lymphocyte clones, each with a different receptor specificity, generated by V(D)J recombination during development.
有数百万个不同的淋巴细胞克隆，每个具有不同的受体特异性，通过发育过程中的V(D)J重组产生。
When an antigen enters the body, it selects the lymphocyte clone whose receptor matches it.
当抗原进入体内时，它会选择其受体与之匹配的淋巴细胞克隆。
The selected clone undergoes clonal expansion — rapid mitosis producing thousands of identical effector cells.
被选中的克隆经历克隆扩增——快速有丝分裂，产生数千个相同的效应细胞。
Some cells become memory cells, persisting long-term for future protection.
部分细胞成为记忆细胞，长期存在以供未来保护。

This theory elegantly explains specificity, memory, and the distinction between self and non-self (self-reactive clones are deleted during development).

该学说优雅地解释了特异性、记忆性以及自身与异己的区分（自身反应性克隆在发育过程中被清除）。

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

Feature	Active Immunity (主动免疫)	Passive Immunity (被动免疫)
How acquired	Body produces its own antibodies after exposure to antigen (body makes its own antibodies after antigen exposure 身体接触抗原后自己产生抗体)	Antibodies are transferred from another source (抗体从外部来源转移而来)
Onset	Slow (days to weeks) \| 慢（数天至数周）	Immediate \| 即时
Duration	Long-term (years to lifetime) \| 长期（数年至终生）	Short-term (weeks to months) \| 短期（数周至数月）
Memory cells	Yes — formed during response	No — antibodies degrade over time
Example	Vaccination; natural infection (疫苗接种；自然感染)	Maternal antibodies via placenta/breast milk; antivenom injection (母体抗体经胎盘/母乳传递；抗蛇毒血清注射)

9. Autoimmune Diseases and Allergies | 自身免疫疾病与过敏

When the immune system malfunctions, it can cause disease:

当免疫系统功能异常时，可能引起疾病：

Autoimmune diseases (自身免疫疾病): the immune system attacks self-tissues. Examples: Type 1 diabetes (destruction of pancreatic β-cells), rheumatoid arthritis, multiple sclerosis.
自身免疫疾病：免疫系统攻击自身组织。例如：1型糖尿病（破坏胰腺β细胞）、类风湿性关节炎、多发性硬化症。
Allergies (过敏): an exaggerated IgE-mediated response to harmless environmental antigens (allergens) like pollen or peanuts. Mast cells release histamine, causing symptoms from mild (sneezing, itching) to severe (anaphylaxis).
过敏：对无害环境抗原（过敏原，如花粉或花生）的过度IgE介导应答。肥大细胞释放组胺，引起从轻微（打喷嚏、瘙痒）到严重（过敏性休克）的症状。

10. Common Exam Questions | 常见考试题型

Q1: Describe the role of helper T cells in the immune response. (4 marks)

Model Answer: Helper T cells have CD4 receptors that bind to antigen-MHC-II complexes on APCs. Once activated, they (1) undergo clonal expansion, (2) secrete cytokines (e.g., IL-2) that stimulate B cell proliferation, (3) activate cytotoxic T cells, and (4) provide CD40-CD40L co-stimulation to B cells, enabling their differentiation into plasma cells.

标准答案：辅助T细胞具有CD4受体，与APC上的抗原-MHC-II复合物结合。激活后，它们(1)进行克隆扩增，(2)分泌细胞因子（如IL-2）刺激B细胞增殖，(3)激活细胞毒性T细胞，(4)向B细胞提供CD40-CD40L共刺激，使其分化为浆细胞。

Q2: Explain why the secondary immune response is faster and stronger than the primary response. (3 marks)

Model Answer: During the primary response, memory T and memory B cells are produced. These cells persist after the infection is cleared. Upon re-exposure to the same antigen, memory cells are already present at higher frequency and can be activated more quickly — there is no need to find and expand the correct clone from scratch. Memory B cells have also undergone class-switching and affinity maturation, producing high-affinity IgG immediately.

标准答案：在初次应答期间，产生了记忆T细胞和记忆B细胞。这些细胞在感染清除后持续存在。再次接触相同抗原时，记忆细胞已经以更高的频率存在，可以更快地被激活——无需从头寻找和扩增正确的克隆。记忆B细胞还经历了类别转换和亲和力成熟，能立即产生高亲和力IgG。

Q3: Compare MHC Class I and MHC Class II presentation. (4 marks)

Model Answer: MHC-I is found on all nucleated cells and presents endogenous antigens (e.g., viral proteins) to CD8⁺ cytotoxic T cells. MHC-II is found only on professional APCs (dendritic cells, macrophages, B cells) and presents exogenous antigens (phagocytosed material) to CD4⁺ helper T cells. MHC-I activates Tc cells to kill infected cells; MHC-II activates Th cells to orchestrate the overall response.

标准答案：MHC-I存在于所有有核细胞上，将内源性抗原（如病毒蛋白）呈递给CD8⁺细胞毒性T细胞。MHC-II仅存在于专职APC上（树突状细胞、巨噬细胞、B细胞），将外源性抗原（吞噬的物质）呈递给CD4⁺辅助T细胞。MHC-I激活Tc细胞杀死感染细胞；MHC-II激活Th细胞协调整体应答。

Summary | 总结

The adaptive immune system is a marvel of biological engineering — capable of recognising virtually any pathogen, coordinating a precise multi-cellular response, and remembering past infections for decades. For A-Level Biology, the key takeaways are:

适应性免疫系统是生物工程的奇迹——能够识别几乎任何病原体、协调精确的多细胞应答，并记忆过去的感染长达数十年。对于A-Level生物学，关键要点是：

T cells drive cell-mediated immunity; B cells drive humoral immunity.
Antigens are presented on MHC-I (to CD8⁺ Tc) or MHC-II (to CD4⁺ Th).
Helper T cells are the central coordinators, providing essential signals to both B cells and cytotoxic T cells.
B cells differentiate into plasma cells (antibody factories) and memory B cells (long-term protection).
Antibodies work through neutralisation, agglutination, opsonisation, and complement activation.
The secondary immune response is faster and stronger due to immunological memory — the basis of vaccination.
Clonal selection theory explains how a diverse repertoire of lymphocytes can respond specifically to a vast range of antigens.

Good luck with your studies! 祝学习顺利！🎓

Keywords | 关键词： adaptive immunity 适应性免疫, T cells T细胞, B cells B细胞, antibodies 抗体, MHC molecules MHC分子, antigen presentation 抗原呈递, clonal selection 克隆选择, immune memory 免疫记忆, vaccination 疫苗接种