Cells and Functions of the Immune System | 免疫系统的细胞构成与功能

📚 Cells and Functions of the Immune System | 免疫系统的细胞构成与功能

The immune system is a highly complex and dynamic defence network composed of specialised cells, tissues, and organs. For CIE A-Level Biology, a clear understanding of the cellular components of the immune system—their origins, roles, and interactions—is essential for tackling exam questions on disease and immunity. This article provides a structured breakdown of the key immune cells and their specific functions.

免疫系统是一个由特化细胞、组织和器官组成的高度复杂且动态的防御网络。对于 CIE A-Level 生物学而言，清晰理解免疫系统的细胞构成——它们的起源、角色和相互作用——是解答疾病与免疫相关考题的关键。本文将系统梳理免疫系统中的主要细胞及其特定功能。

1. Overview of Leukocytes | 白细胞概览

All immune cells originate from pluripotent haematopoietic stem cells in the bone marrow. From these stem cells, two main lineages arise: the myeloid lineage and the lymphoid lineage. Myeloid cells include phagocytes such as neutrophils and macrophages, as well as dendritic cells and mast cells. Lymphoid cells include T lymphocytes, B lymphocytes, and natural killer (NK) cells. Understanding this ancestry helps clarify the distinct yet overlapping roles these cells play in immunity.

所有免疫细胞均起源于骨髓中的多能造血干细胞。这些干细胞分化出两个主要谱系：髓系和淋巴系。髓系细胞包括吞噬细胞（如中性粒细胞和巨噬细胞）、树突状细胞和肥大细胞。淋巴系细胞包括 T 淋巴细胞、B 淋巴细胞和自然杀伤（NK）细胞。理解这一分化起源有助于厘清这些细胞在免疫中既不同又相互重叠的角色。

The immune response is traditionally divided into two arms: innate immunity and adaptive immunity. Phagocytes are the primary effectors of innate immunity, providing a rapid, non-specific response. Lymphocytes are the primary effectors of adaptive immunity, providing a slower but highly specific response with immunological memory.

免疫反应传统上分为两部分：先天免疫和适应性免疫。吞噬细胞是先天免疫的主要效应细胞，提供快速、非特异性的反应。淋巴细胞是适应性免疫的主要效应细胞，提供较慢但高度特异性的反应，并具有免疫记忆。

2. Phagocytes: First-Line Defenders | 吞噬细胞：第一道防线

Phagocytes are a type of white blood cell that engulf and digest pathogens and cellular debris. They are the body’s first line of cellular defence and play a crucial role in innate immunity. The two most important phagocytes are neutrophils and macrophages.

吞噬细胞是一种能够吞噬并消化病原体和细胞碎片的白细胞。它们是机体细胞防御的第一道防线，在先天免疫中起着至关重要的作用。两种最重要的吞噬细胞是中性粒细胞和巨噬细胞。

Neutrophils are the most abundant type of granulocyte and are rapidly recruited to sites of infection. They are short-lived and die after phagocytosing a few pathogens, forming pus. Macrophages are larger, longer-lived phagocytes that can be resident in tissues or recruited from monocytes. They not only phagocytose pathogens but also act as antigen-presenting cells, linking innate and adaptive immunity.

中性粒细胞是最丰富的粒细胞类型，会被快速招募到感染部位。它们寿命较短，在吞噬少量病原体后即死亡，形成脓液。巨噬细胞体积更大、寿命更长，既可定居于组织，也可由单核细胞招募而来。它们不仅能吞噬病原体，还充当抗原呈递细胞，连接先天免疫和适应性免疫。

The process of phagocytosis involves several key stages:

吞噬作用的过程涉及几个关键阶段：

Chemotaxis: The phagocyte moves towards the site of infection, guided by chemical signals such as cytokines and complement proteins.
化学趋向性：吞噬细胞在细胞因子和补体蛋白等化学信号的引导下向感染部位移动。
Adherence: The phagocyte attaches to the surface of the pathogen via non-specific receptors or opsonins.
附着：吞噬细胞通过非特异性受体或调理素附着到病原体表面。
Ingestion: The phagocyte extends pseudopodia to surround the pathogen, engulfing it into a vesicle called a phagosome.
摄入：吞噬细胞伸出伪足包裹病原体，将其吞入称为吞噬体的囊泡中。
Killing and Digestion: The phagosome fuses with a lysosome to form a phagolysosome. Lysozymes, reactive oxygen species, and hydrolytic enzymes destroy the pathogen.
杀伤与消化：吞噬体与溶酶体融合形成吞噬溶酶体。溶菌酶、活性氧物质和水解酶摧毁病原体。
Exocytosis: The digested debris is released from the cell via exocytosis.
胞吐：消化的碎片通过胞吐作用从细胞中释放出来。

3. Lymphocytes: The Adaptive Specialists | 淋巴细胞：适应性免疫专家

Lymphocytes are the principal cells of the adaptive immune system. They possess highly specific cell-surface receptors that recognise antigens. Each lymphocyte has a unique receptor specificity, allowing the immune system to recognise a vast array of pathogens. The two main types of lymphocytes are B cells and T cells.

淋巴细胞是适应性免疫系统的主要细胞。它们拥有高度特异性的细胞表面受体，能够识别抗原。每个淋巴细胞具有独特的受体特异性，使免疫系统能够识别大量病原体。两类主要的淋巴细胞是 B 细胞和 T 细胞。

B cells mature in the bone marrow and are responsible for humoral immunity. When activated, B cells differentiate into plasma cells that secrete antibodies. T cells mature in the thymus and are responsible for cell-mediated immunity. They do not produce antibodies but instead directly kill infected cells or regulate other immune cells.

B 细胞在骨髓中成熟，负责体液免疫。激活后，B 细胞分化为分泌抗体的浆细胞。T 细胞在胸腺中成熟，负责细胞介导免疫。它们不产生抗体，而是直接杀死受感染细胞或调节其他免疫细胞。

The specificity of lymphocytes is generated through a process called clonal selection. During development, lymphocytes that react to self-antigens are eliminated, preventing autoimmunity. The remaining lymphocytes are released into circulation, ready to respond to foreign antigens.

淋巴细胞的特异性通过称为克隆选择的过程产生。在发育过程中，对自身抗原反应的淋巴细胞被清除，从而防止自身免疫。剩余的淋巴细胞释放到循环中，准备对外来抗原作出反应。

4. T Cell Subsets: Helpers and Killers | T 细胞亚群：辅助者与杀伤者

T cells can be broadly divided into two major subsets based on their surface markers and functions: T helper cells and cytotoxic T cells.

根据表面标记和功能的不同，T 细胞可大致分为两个主要亚群：辅助 T 细胞和细胞毒性 T 细胞。

T helper cells, which express the CD4 glycoprotein, act as the “orchestrators” of the immune response. They do not kill pathogens directly. Instead, they recognise antigens presented on MHC class II molecules and release cytokines, which stimulate other immune cells such as B cells, macrophages, and cytotoxic T cells to mount a coordinated attack.

辅助 T 细胞表达 CD4 糖蛋白，充当免疫应答的“指挥家”。它们不直接杀死病原体。相反，它们识别 MHC II 类分子上呈递的抗原并释放细胞因子，刺激 B 细胞、巨噬细胞和细胞毒性 T 细胞等其他免疫细胞发起协调攻击。

Cytotoxic T cells, which express the CD8 glycoprotein, are the primary killers of the adaptive immune system. They recognise antigens presented on MHC class I molecules, which are found on all nucleated cells. When a cytotoxic T cell recognises a specific antigen on an infected cell, it releases perforin and granzymes. Perforin forms pores in the target cell membrane, allowing granzymes to enter and induce apoptosis, or programmed cell death.

细胞毒性 T 细胞表达 CD8 糖蛋白，是适应性免疫系统中的主要杀伤细胞。它们识别所有有核细胞表面 MHC I 类分子上呈递的抗原。当细胞毒性 T 细胞识别出受感染细胞上的特定抗原时，它会释放穿孔素和颗粒酶。穿孔素在靶细胞膜上形成孔道，使颗粒酶进入并诱导细胞凋亡，即程序性细胞死亡。

5. Antigen Presentation and MHC | 抗原呈递与 MHC

For a T cell to recognise an antigen, the antigen must be presented on a Major Histocompatibility Complex (MHC) molecule. This is a crucial concept in A-Level Biology, as it explains why T cells cannot recognise free-floating antigens, unlike B cells.

T 细胞要识别抗原，抗原必须呈递在主要组织相容性复合体（MHC）分子上。这是 A-Level 生物学中的一个关键概念，因为它解释了为什么 T 细胞不能像 B 细胞那样识别游离的抗原。

There are two main classes of MHC molecules:

MHC 分子主要有两类：

MHC class I molecules are found on the surface of all nucleated cells. They present endogenous antigens, such as viral proteins synthesised inside an infected cell, to cytotoxic T cells.
MHC I 类分子存在于所有有核细胞表面。它们将内源性抗原（如受感染细胞内合成的病毒蛋白）呈递给细胞毒性 T 细胞。
MHC class II molecules are found on the surface of professional antigen-presenting cells (APCs), such as macrophages, dendritic cells, and B cells. They present exogenous antigens, which have been engulfed and processed, to T helper cells.
MHC II 类分子存在于专业抗原呈递细胞（APCs）表面，如巨噬细胞、树突状细胞和 B 细胞。它们将已被吞噬和处理的外源性抗原呈递给辅助 T 细胞。

This phenomenon is known as MHC restriction: a T cell can only recognise an antigen if it is presented by the correct MHC molecule. Antigen-presenting cells, particularly dendritic cells, are essential for initiating the primary immune response by activating naive T cells.

这种现象称为 MHC 限制性：T 细胞只有在抗原由正确的 MHC 分子呈递时才能识别它。抗原呈递细胞，尤其是树突状细胞，通过激活初始 T 细胞对启动初次免疫应答至关重要。

6. B Cells, Antibodies and Humoral Immunity | B 细胞、抗体与体液免疫

Humoral immunity is mediated by antibodies, which are soluble proteins produced by plasma cells derived from B cells. B cells recognise native antigens directly through their B-cell receptors (BCRs), which are membrane-bound antibodies. However, most B cells require help from T helper cells to become fully activated, particularly for T-dependent antigens.

体液免疫由抗体介导，抗体是浆细胞产生的可溶性蛋白质，浆细胞由 B 细胞分化而来。B 细胞通过其 B 细胞受体（BCR）（即膜结合抗体）直接识别天然抗原。然而，大多数 B 细胞需要辅助 T 细胞的帮助才能被完全激活，尤其是针对 T 依赖性抗原时。

Upon activation, B cells proliferate and differentiate into two main cell types:

激活后，B 细胞增殖并分化为两种主要的细胞类型：

Plasma cells: These are antibody factories, producing and secreting thousands of antibodies per second. They have extensive rough endoplasmic reticulum to support high rates of protein synthesis.
浆细胞：它们是抗体工厂，每秒产生并分泌数千个抗体。它们拥有丰富的粗面内质网，以支持高水平的蛋白质合成。
Memory B cells: These are long-lived cells that provide immunological memory, enabling a rapid response upon secondary exposure to the same antigen.
记忆 B 细胞：这些细胞寿命长，提供免疫记忆，使机体在再次接触相同抗原时能迅速作出反应。

Antibodies, also known as immunoglobulins, are Y-shaped proteins. They neutralise pathogens through several mechanisms:

抗体，也称为免疫球蛋白，是 Y 形蛋白质。它们通过几种机制中和病原体：

Neutralisation: Antibodies bind to toxins or viruses, preventing them from attaching to host cells.
中和作用：抗体与毒素或病毒结合，阻止它们附着到宿主细胞上。
Opsonisation: Antibodies coat the pathogen, making it more recognisable to phagocytes, which have receptors for the antibody’s Fc region.
调理作用：抗体包被病原体，使其更容易被吞噬细胞识别，因为吞噬细胞具有针对抗体 Fc 区的受体。
Agglutination: Antibodies cross-link pathogens into clumps, reducing their ability to spread and making them easier for phagocytes to engulf.
凝集作用：抗体将病原体交联成团，降低其扩散能力，并使其更容易被吞噬细胞吞噬。
Complement activation: Antibody-antigen complexes activate the complement system, leading to inflammation and lysis of pathogens.
补体激活：抗体-抗原复合物激活补体系统，导致炎症反应和病原体裂解。

7. Cell-Mediated Immunity in Action | 细胞介导免疫的作用机制

Cell-mediated immunity is essential for eliminating intracellular pathogens, such as viruses, and for destroying abnormal cells, such as cancer cells. Unlike humoral immunity, it relies on the direct action of cytotoxic T cells rather than secreted antibodies.

细胞介导免疫对于清除细胞内病原体（如病毒）和摧毁异常细胞（如癌细胞）至关重要。与体液免疫不同，它依赖于细胞毒性 T 细胞的直接作用，而非分泌的抗体。

The process begins when a virus infects a host cell. The cell processes viral proteins and presents them on MHC class I molecules. A circulating cytotoxic T cell with a receptor specific to this antigen-peptide-MHC complex binds to the infected cell. This binding is strengthened by co-receptors like CD8. Once activated, the cytotoxic T cell releases cytotoxic granules containing perforin and granzymes at the immunological synapse.

该过程始于病毒感染宿主细胞。细胞处理病毒蛋白并将其呈递在 MHC I 类分子上。一个带有对该抗原肽-MHC 复合物特异性受体的循环细胞毒性 T 细胞与受感染细胞结合。CD8 等共受体增强了这种结合。一旦激活，细胞毒性 T 细胞在免疫突触处释放含有穿孔素和颗粒酶的细胞毒性颗粒。

Perforin polymerises to form pores in the target cell’s membrane, similar to the membrane attack complex of the complement system. Granzymes enter through these pores and activate caspases, a family of proteases that initiate apoptosis. Apoptosis is a controlled process of cell death that fragments the cell into apoptotic bodies, which are then engulfed by phagocytes, preventing the release of viral particles and minimising inflammation.

穿孔素聚合在靶细胞膜上形成孔道，类似于补体系统的膜攻击复合物。颗粒酶通过这些孔道进入并激活半胱天冬酶（caspases），这是一类启动细胞凋亡的蛋白酶。细胞凋亡是一种受控的细胞死亡过程，将细胞碎裂成凋亡小体，随后被吞噬细胞吞噬，从而防止病毒颗粒释放并最大限度地减少炎症。

8. Memory Cells and the Secondary Response | 记忆细胞与二次免疫应答

A defining feature of adaptive immunity is immunological memory. During an initial infection, some antigen-specific B and T cells differentiate into long-lived memory cells rather than effector cells. These memory cells persist in the body, often for decades, poised to respond to the same antigen if encountered again.

适应性免疫的一个决定性特征是免疫记忆。在初次感染期间，一些抗原特异性 B 和 T 细胞分化为寿命长的记忆细胞，而非效应细胞。这些记忆细胞在体内持续存在，通常可达数十年，随时准备在再次遇到相同抗原时作出反应。

The primary immune response occurs when the body encounters an antigen for the first time. It is characterised by a lag phase, during which naive lymphocytes are activated and proliferate, followed by a rise in antibody titre, reaching a peak in 1-2 weeks. This response often produces symptoms of illness.

初次免疫应答发生在身体首次接触抗原时。其特点是存在一个滞后期，在此期间初始淋巴细胞被激活并增殖，随后抗体滴度上升，在 1-2 周内达到峰值。这种反应通常会产生疾病的症状。

The secondary immune response occurs upon a subsequent encounter with the same antigen. Because memory cells are already present at high frequencies, the lag phase is much shorter, and the response is faster, larger, and longer-lasting. The antibody titre peaks at a much higher level, primarily due to the rapid differentiation of memory B cells into plasma cells. This is why vaccines are effective: they stimulate the formation of memory cells without causing the full-blown disease.

二次免疫应答发生在再次遇到相同抗原时。由于记忆细胞已经以高频率存在，滞后期要短得多，反应更快、更强、更持久。抗体滴度峰值要高得多，这主要是由于记忆 B 细胞迅速分化为浆细胞。这就是疫苗有效的原因：它们刺激记忆细胞的形成，而不会引起全面的疾病。

9. Summary and Exam Tips | 总结与考点提示

In summary, the immune system coordinates a wide range of specialised cells. Phagocytes like neutrophils and macrophages provide immediate, non-specific defence. Lymphocytes, including B cells and T cells, provide specific, long-lasting immunity. T helper cells coordinate the response, whilst cytotoxic T cells kill infected cells. B cells produce antibodies that neutralise extracellular pathogens. Memory cells ensure a rapid and robust secondary response.

总之，免疫系统协调着多种特化细胞。中性粒细胞和巨噬细胞等吞噬细胞提供即时、非特异性的防御。包括 B 细胞和 T 细胞在内的淋巴细胞提供特异性、持久的免疫。辅助 T 细胞协调应答，而细胞毒性 T 细胞杀死受感染细胞。B 细胞产生中和细胞外病原体的抗体。记忆细胞确保持久而强大的二次应答。

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Cells and Functions of the Immune System | 免疫系统的细胞构成与功能