免疫

Your immune system 免疫系统 is the set of cells that defends the body against pathogens 病原体. The first cells to act are phagocytes 吞噬细胞, a group of white blood cells that includes macrophages 巨噬细胞 and neutrophils 中性粒细胞.

A phagocyte destroys a pathogen by phagocytosis 吞噬作用: it surrounds the pathogen, takes it inside in a vesicle, and digests it with enzymes. After this, a macrophage displays parts of the pathogen on its own surface, ready to alert other immune cells.

This false-coloured electron micrograph captures the moment in real life: a phagocyte reaching out to grab and engulf rod-shaped bacteria.

An antigen 抗原 is a molecule (usually a protein) on a cell surface that the immune system can recognise.

• self antigens 自身抗原 are the body's own markers. The immune system learns to ignore them.
• non-self antigens 非自身抗原 are foreign, for example the antigens on a pathogen. These trigger an immune response.

The first time a new pathogen enters, the body makes a slow immune response 免疫反应. The main steps are:

1. a macrophage engulfs the pathogen and displays its antigen.
2. T-helper cells 辅助性T细胞 recognise that antigen and become active. They release chemicals that switch on other cells.
3. B-lymphocytes 淋巴细胞 with a matching shape are selected. They divide to form plasma cells 浆细胞, which pour out antibodies 抗体, and memory cells 记忆细胞.
4. T-killer cells 杀伤性T细胞 destroy the body's own cells that have been infected.

These steps take several days, because the few B-lymphocytes that match the antigen must first be found and then divide many times. This delay is the lag at the start of the primary response, and it is why you feel ill during a first infection.

Memory cells stay in the body for years after the infection is over. If the same pathogen enters again, the memory cells start a secondary immune response that is much faster and larger than the first. The pathogen is destroyed before it can make you ill. This is what we mean by long-term immunity.

An antibody is a Y-shaped protein made by plasma cells. The two tips of the Y are antigen-binding sites 抗原结合位点. Each site has a special shape, called the variable region 可变区, that fits one antigen only, like a lock and key.

Antibodies help in several ways: they stick to antigens, clump pathogens together so they are easier to deal with, mark pathogens so phagocytes find them, and block harmful toxins.

A monoclonal antibody 单克隆抗体 is a single type of antibody, all identical. They are made by the hybridoma 杂交瘤 method:

1. an animal is given an antigen, so it makes B-lymphocytes that produce the wanted antibody.
2. these B-lymphocytes are fused with tumour cells, which divide endlessly.
3. the fused cell (the hybridoma) both makes the antibody and divides without stopping, producing large amounts of one identical antibody.

Monoclonal antibodies are used in the diagnosis 诊断 of disease (to detect a specific molecule, as in a pregnancy test) and in treatment (to carry drugs to specific target cells, such as cancer cells).

Immunity can be active or passive, and natural or artificial.

• active immunity 主动免疫 — your own body meets an antigen and makes its own antibodies and memory cells. It is slow to start but long-lasting.
• passive immunity 被动免疫 — ready-made antibodies are given to you from outside. It works at once but does not last, because there are no memory cells.
• natural immunity 天然免疫 — gained in a natural way (active: after an infection; passive: antibodies passed from mother to baby).
• artificial immunity 人工免疫 — gained on purpose (active: your body is made to respond to a safe dose of antigen; passive: you are injected with ready-made antibodies).

A vaccine 疫苗 contains antigens — often a dead or weakened pathogen, or part of one. The antigens trigger a primary immune response and make memory cells, so you gain long-term immunity without becoming ill.

Vaccination 疫苗接种 programmes can control the spread of a disease across a population. If enough people are vaccinated, the pathogen cannot pass easily from person to person. This protects even the people who are not vaccinated, an effect called herd immunity 群体免疫.

Worked example. Classify each as active or passive, natural or artificial: (a) a baby receives antibodies in breast milk; (b) a child is vaccinated against measles; (c) someone recovers from chickenpox; (d) a patient bitten by a snake is given antivenom. Ask two questions each time. Did the person make the antibodies themselves? If yes it is active; if they were handed ready-made ones, it is passive. Did it happen by chance or deliberately? Naturally, or artificially. So (a) is natural passive - ready-made antibodies by a natural route; (b) is artificial active - the vaccine's antigens make the child produce their own; (c) is natural active - the infection made them produce their own; (d) is artificial passive - ready-made antibodies given deliberately. Only active immunity makes memory cells, which is exactly why passive immunity acts immediately but is short-lived.

Common mistakes.

• Saying the body makes antibodies "for" a pathogen after it arrives. The B-lymphocyte with the matching receptor already exists; the antigen selects it and it is then cloned.
• Saying memory cells make antibodies. Memory cells divide fast into plasma cells when the antigen returns; the plasma cells make the antibodies.
• Saying antibodies kill pathogens. Antibodies bind antigens: they clump pathogens together, block toxins and mark pathogens for phagocytes, which do the killing.
• Saying a vaccine contains antibodies. A vaccine contains antigens; your own cells then make the antibodies and memory cells. Ready-made antibodies give passive immunity.
• Saying the secondary response is faster "because the body remembers". Name the cells: there are many more memory cells than the first matching B-lymphocytes, so there is almost no lag and far more antibody is made.
• Calling a monoclonal antibody "a mixture of antibodies". Every molecule is identical, made by one clone of hybridoma cells, so it binds one antigen only.

• Distinguish phagocytes (engulf, non-specific) from lymphocytes: B cells → antibodies (humoral), T cells (helper/killer, cell-mediated).
• Label an antibody (variable region, antigen-binding site) and explain agglutination and neutralisation.
• Compare the primary and secondary response on a graph — memory cells make the secondary faster and larger.
• Distinguish active vs passive and natural vs artificial immunity with an example of each; explain vaccination and herd immunity.