📚 Immunity | 免疫
The immune system is a complex network of cells, tissues and organs that protects the body against pathogens such as bacteria, viruses, fungi and parasites. In A-Level Biology, understanding immunity means distinguishing between non-specific and specific defences, exploring the roles of phagocytes and lymphocytes, and appreciating how vaccination harnesses the body's own adaptive responses to prevent disease.
免疫系统是一个由细胞、组织和器官组成的复杂网络，保护身体免受细菌、病毒、真菌和寄生虫等病原体的侵害。在A-Level生物学中，理解免疫意味着要区分非特异性防御和特异性防御，探究吞噬细胞与淋巴细胞的作用，并理解疫苗接种如何利用人体自身的适应性反应来预防疾病。
1. What is Immunity? | 什么是免疫？
Immunity is the ability of an organism to resist infection by pathogens. It can be categorised as innate (non-specific) or adaptive (specific). Innate immunity provides immediate, general protection, while adaptive immunity develops more slowly but provides long-lasting, targeted defence.
免疫是指生物体抵抗病原体感染的能力。免疫可分为先天性（非特异性）免疫和适应性（特异性）免疫。先天性免疫提供即时、通用的保护，而适应性免疫发展较慢，但能提供长效、针对性的防御。
The key cells involved in the immune response are produced in the bone marrow and include phagocytes (neutrophils and macrophages) and lymphocytes (T cells and B cells). These cells circulate in the blood and lymph, patrolling for signs of infection.
参与免疫反应的关键细胞产生于骨髓，包括吞噬细胞（中性粒细胞和巨噬细胞）以及淋巴细胞（T细胞和B细胞）。这些细胞在血液和淋巴中循环，巡查感染的迹象。
2. Non-Specific Defences | 非特异性防御
The body's first line of defence is physical and chemical barriers. The skin acts as an impermeable barrier, while mucous membranes trap pathogens. Tears, saliva and stomach acid contain enzymes and low pH that destroy microbes.
人体的第一道防线是物理和化学屏障。皮肤起着不透水屏障的作用，而黏膜则能捕获病原体。眼泪、唾液和胃酸含有酶以及低pH环境，能够破坏微生物。
If pathogens breach these barriers, the second line of non-specific defence is activated: the inflammatory response. Damaged tissues release histamine, causing vasodilation and increased capillary permeability. This brings more phagocytes to the site of infection and causes swelling, redness and heat.
如果病原体突破了这些屏障，非特异性防御的第二道防线就会被激活：炎症反应。受损组织释放组胺，引起血管扩张和毛细血管通透性增加。这使更多吞噬细胞到达感染部位，并引起肿胀、发红和发热。
Interferons are proteins released by virus-infected cells that "warn" neighbouring cells to increase their antiviral defences. Complement proteins in the blood can also directly lyse bacteria.
干扰素是由病毒感染细胞释放的蛋白质，能“警告”邻近细胞增强抗病毒防御。血液中的补体蛋白也能直接溶解细菌。
3. Phagocytosis | 吞噬作用
Phagocytosis is a crucial non-specific cellular defence. Phagocytes such as neutrophils and macrophages recognise pathogens via non-specific receptors that bind to common microbial molecules. They engulf the pathogen into a phagosome, which then fuses with a lysosome to form a phagolysosome.
吞噬作用是一种关键的非特异性细胞防御。中性粒细胞和巨噬细胞等吞噬细胞通过非特异性受体识别病原体，这些受体与常见的微生物分子结合。它们将病原体吞入吞噬体，吞噬体随后与溶酶体融合形成吞噬溶酶体。
Inside the phagolysosome, digestive enzymes and reactive oxygen species destroy the pathogen. The phagocyte may then present fragments of the pathogen (antigens) on its surface using MHC class II molecules, acting as an antigen-presenting cell (APC) to link the non-specific and specific immune systems.
在吞噬溶酶体内，消化酶和活性氧物质破坏病原体。吞噬细胞随后可通过MHC II类分子在其表面呈递病原体片段（抗原），充当抗原呈递细胞（APC），从而将非特异性免疫系统与特异性免疫系统联系在一起。
4. Antigens and Antibodies | 抗原与抗体
An antigen is any molecule (often a protein or polysaccharide on the surface of a pathogen) that can be recognised by the immune system as foreign and trigger an immune response. Self-antigens on our own cells are usually tolerated, but in autoimmune diseases this self-tolerance breaks down.
抗原是指任何能被免疫系统识别为外来物质并触发免疫反应的分子（通常是病原体表面的蛋白质或多糖）。我们自身细胞上的自身抗原通常被耐受，但在自身免疫病中，这种自身耐受被破坏。
An antibody (immunoglobulin) is a Y-shaped protein produced by B lymphocytes (plasma cells) that binds specifically to an antigen. Each antibody has a unique binding site complementary to a specific antigen epitope.
抗体（免疫球蛋白）是由B淋巴细胞（浆细胞）产生的Y形蛋白质，能特异性地与抗原结合。每种抗体都有一个独特的结合位点，与特定的抗原表位互补。
5. Cell-Mediated Immunity | 细胞介导免疫
Cell-mediated immunity involves T lymphocytes and is particularly important against intracellular pathogens such as viruses and some bacteria. It begins when an APC (e.g. a macrophage) presents an antigen on its MHC class II molecules to a helper T cell (TH cell) with a complementary receptor.
细胞介导免疫涉及T淋巴细胞，对于抵抗细胞内病原体（如病毒和某些细菌）尤为重要。当抗原呈递细胞（例如巨噬细胞）通过其MHC II类分子将抗原呈递给具有互补受体的辅助性T细胞（TH细胞）时，细胞介导免疫即启动。
Once activated, the TH cell divides to form a clone of active helper T cells and memory T cells. Active TH cells release cytokines that stimulate B cells to divide and differentiate, activate cytotoxic T cells, and enhance phagocytosis.
一旦被激活，辅助性T细胞就会分裂，形成一群活跃的辅助T细胞和记忆T细胞。活跃的辅助性T细胞释放细胞因子，刺激B细胞分裂和分化，激活细胞毒性T细胞，并增强吞噬作用。
Cytotoxic T cells (TC cells) recognise antigens presented on MHC class I molecules, which are found on all nucleated cells. When a body cell is infected by a virus, it displays viral antigens on MHC I. The cytotoxic T cell binds and releases perforin, which creates pores in the target cell membrane, leading to cell lysis.
细胞毒性T细胞（TC细胞）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上。当体细胞被病毒感染时，它会在MHC I上展示病毒抗原。细胞毒性T细胞结合后释放穿孔素，在靶细胞膜上形成孔洞，导致细胞裂解。
6. Humoral Immunity | 体液免疫
Humoral immunity involves B lymphocytes and antibodies in the blood and lymph. A B cell has specific antibody molecules on its surface that act as receptors. When a B cell binds to its complementary antigen, and receives cytokines from an activated helper T cell, it becomes activated.
体液免疫涉及B淋巴细胞以及血液和淋巴中的抗体。B细胞表面有特异性抗体分子作为受体。当B细胞与其互补抗原结合，并接受来自活化的辅助性T细胞的细胞因子时，它即被激活。
The activated B cell divides by mitosis to produce a clone of plasma cells and memory B cells. Plasma cells are short-lived but secrete massive quantities of antibodies specific to the antigen. These antibodies circulate in the blood and lymph, neutralising pathogens and marking them for destruction by phagocytes.
活化的B细胞通过有丝分裂分裂，产生一群浆细胞和记忆B细胞。浆细胞寿命较短，但会分泌大量特异性针对该抗原的抗体。这些抗体在血液和淋巴中循环，中和病原体并标记它们以供吞噬细胞清除。
Memory B cells remain in the body for many years. If the same antigen enters the body again, these cells can rapidly divide and differentiate, providing a faster and stronger secondary response.
记忆B细胞在体内存留多年。如果相同的抗原再次进入体内，这些细胞能迅速分裂并分化，从而提供更快更强的二次应答。
7. Antibody Structure and Function | 抗体的结构与功能
An antibody molecule consists of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulfide bonds. The molecule has a constant region, which determines the class of antibody and the mechanism of pathogen destruction, and a variable region that forms the antigen-binding site.
抗体分子由四条多肽链组成：两条相同的重链和两条相同的轻链，通过二硫键连接在一起。该分子有一个恒定区，决定抗体的类别和病原体破坏机制；还有一个可变区，形成抗原结合位点。
The variable regions of different antibodies have unique amino acid sequences, creating a vast diversity of binding sites. This is why the immune system can recognise millions of different antigens. The hinge region of the antibody provides flexibility, allowing both arms to bind antigens simultaneously.
不同抗体可变区的氨基酸序列独一无二，产生了种类繁多的结合位点。这就是免疫系统能识别数百万种不同抗原的原因。抗体的铰链区提供灵活性，使两臂能同时结合抗原。
Antibodies work by agglutination (clumping pathogens together to prevent spread), neutralisation (binding to toxins or viruses and blocking their action), and opsonisation (coating pathogens to enhance phagocytosis). They can also activate the complement system to lyse bacteria.
抗体的作用方式包括凝集（将病原体聚集在一起以防止扩散）、中和（结合毒素或病毒并阻断其作用）以及调理（包裹病原体以增强吞噬作用）。它们还能激活补体系统来裂解细菌。
8. Primary and Secondary Immune Responses | 初次与二次免疫应答
When the body encounters a new antigen for the first time, the primary immune response is initiated. There is a lag phase of several days while the specific lymphocytes are activated and divide. The peak antibody level is relatively low, and it takes about 10–17 days to reach maximum concentration. The antibodies are mainly IgM, followed by IgG.
当身体首次遇到一种新抗原时，就会启动初次免疫应答。在特异性淋巴细胞被激活并分裂的期间，存在数天的滞后期。抗体峰值水平相对较低，大约需要10-17天达到最高浓度。抗体主要是IgM，随后是IgG。
If the same antigen is encountered again, memory B and memory T cells that were produced during the primary response are already present. This leads to a secondary immune response, characterised by a much shorter lag phase, a far higher peak antibody concentration, and a longer persistence of antibodies. The main antibody class is IgG.
如果再次遭遇同一抗原，初次应答中产生的记忆B细胞和记忆T细胞已经存在。这导致二次免疫应答，其特点为滞后期大大缩短，抗体峰值浓度远高于初次，且抗体持续时间更长。主要抗体类别是IgG。
This memory is the basis of vaccination. The immune system is exposed to a harmless form of the antigen, which generates memory cells without causing disease, so that a subsequent encounter with the actual pathogen elicits a rapid and effective response.
这种记忆性是疫苗接种的基础。让免疫系统接触一种无害形式的抗原，在不可起疾病的情况下产生记忆细胞，因此以后遇到真正的病原体时能激发快速有效的应答。
9. Types of Immunity: Active and Passive | 免疫类型：主动与被动
Immunity can be classified into active and passive forms, each of which can be naturally or artificially acquired.
免疫可分为主动免疫和被动免疫，各自又可通过自然或人工方式获得。
| Type | 类型 | Definition | 定义 | Example | 例子 |
|---|---|---|
| Natural active | 自然主动 | Antibodies and memory cells produced after infection | 感染后产生抗体与记忆细胞 | Recovery from measles | 麻疹康复 |
| Artificial active | 人工主动 | Antibodies and memory cells produced after vaccination | 接种疫苗后产生抗体与记忆细胞 | MMR vaccine | 麻腮风疫苗 |
| Natural passive | 自然被动 | Pre-formed antibodies transferred from mother to child | 母体预先形成的抗体传给子女 | Antibodies in breast milk / across placenta | 母乳/胎盘抗体 |
| Artificial passive | 人工被动 | Injection of pre-formed antibodies from another individual | 注射来自另一个体的预先制备的抗体 | Anti-venom or tetanus antitoxin | 抗蛇毒血清或破伤风抗毒素 |
Active immunity involves the body's own production of antibodies and memory cells; it is long-lasting. Passive immunity provides immediate protection but is temporary because the antibodies are broken down and no memory cells are produced.
主动免疫涉及身体自身产生抗体和记忆细胞，持续时间长。被动免疫提供即时保护，但只是暂时的，因为抗体会被分解，且不产生记忆细胞。
10. Vaccination and Herd Immunity | 疫苗接种与群体免疫
A vaccine typically contains antigens derived from a pathogen, which may be attenuated (weakened), inactivated, or consist only of isolated proteins or polysaccharides. Some modern vaccines contain mRNA that instructs body cells to produce the antigen themselves.
疫苗通常含有来自病原体的抗原，这些抗原可能是减毒的、灭活的，或仅由分离的蛋白质或多糖组成。一些现代疫苗含有mRNA，指示体细胞自行制造抗原。
Vaccination stimulates a primary immune response, generating memory cells and specific antibodies without causing the disease. When the vaccinated person later encounters the live pathogen, the secondary response is rapid and powerful, often preventing symptoms from developing.
疫苗接种激发初次免疫应答，产生记忆细胞和特异性抗体，而不引起疾病。当接种者后来接触活病原体时，二次应答快速且强有力，通常能阻止症状出现。
Herd immunity occurs when a sufficiently high proportion of a population is vaccinated, reducing the spread of the pathogen and protecting individuals who cannot be vaccinated (e.g. due to allergies or weakened immune systems). The threshold for herd immunity depends on the infectivity of the disease; for measles it is around 95%.
当人群中足够高比例的人接种疫苗时，就会产生群体免疫，从而减少病原体传播，保护那些无法接种疫苗的人（例如由于过敏或免疫系统较弱）。群体免疫的阈值取决于疾病的传染性；麻疹的群体免疫阈值约为95%。
11. Monoclonal Antibodies and Their Uses | 单克隆抗体及其用途
Monoclonal antibodies are identical antibodies produced by hybridoma cells, which are formed by fusing a B lymphocyte that produces a specific antibody with a myeloma (cancer) cell. The hybridoma retains the B cell's ability to produce the antibody and the cancer cell's ability to divide indefinitely.
单克隆抗体是由杂交瘤细胞产生的相同抗体，杂交瘤细胞是通过将产生特定抗体的B淋巴细胞与骨髓瘤（癌）细胞融合而成的。杂交瘤细胞保留了B细胞产生抗体的能力以及癌细胞无限分裂的能力。
These antibodies are highly specific, binding to only one epitope on an antigen. This makes them powerful tools in diagnosis and therapy. Pregnancy tests use monoclonal antibodies that bind to the hormone hCG, while some cancer treatments deliver drugs or radioactive substances attached to monoclonal antibodies that target cancer cells.
这些抗体高度特异，只与抗原上的一个表位结合。这使它们成为诊断和治疗的有力工具。验孕棒使用与hCG激素结合的单克隆抗体，而一些癌症治疗则利用附着于靶向癌细胞的单克隆抗体来递送药物或放射性物质。
Monoclonal antibodies can also be used in immunofluorescence to locate specific molecules in cells or tissues, and in ELISA tests to measure the concentration of antigens or antibodies in samples.
单克隆抗体还可用于免疫荧光，在细胞或组织中定位特定分子，以及用于ELISA检测，测定样本中抗原或抗体的浓度。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图