How Vaccines Work (Explained Simply)

Antigens, antibodies, and the problem vaccines solve

Every pathogen, a bacterium, a virus, or another disease-causing organism, carries antigens on its surface. An antigen is a molecule your immune system recognises as foreign.

When a pathogen enters your body for the first time, your white blood cells take time to recognise its antigens and produce matching antibodies. This delay, called the primary immune response, is why you feel unwell during a first infection, the pathogen has time to multiply before your body catches up.

A vaccine solves this by showing your immune system the antigen safely, before you ever meet the real pathogen.

Two types of white blood cells carry out this response: phagocytes, which engulf pathogens, and lymphocytes, which produce antibodies that fit a specific antigen. Whether this response is triggered by an actual infection or by a vaccine, the underlying steps your body follows are the same.

What happens when you are vaccinated

1. The vaccine introduces an antigen into your body, this may be a weakened (attenuated) pathogen, an inactivated (killed) pathogen, or just a fragment of it, such as a protein from its surface.
2. Phagocytes engulf the antigen and present it to lymphocytes.
3. Lymphocytes are activated and produce antibodies that fit that specific antigen, in the same way a key fits a lock.
4. Some of the activated lymphocytes become memory cells, which remain in the body long after the vaccine antigen has been cleared.
5. No disease develops, because the vaccine does not contain a full, active pathogen capable of causing illness.

Why the second response is so much faster

This is the real purpose of the memory cells. If the actual pathogen later enters your body, the memory cells recognise its antigen immediately and multiply rapidly, producing large amounts of the matching antibody within a much shorter time than the first exposure took.

This is called the secondary immune response.

Because antibodies are produced quickly and in large numbers, the pathogen is usually destroyed before it can multiply enough to cause symptoms. This is the basis of the protection gained through vaccination.

For example, someone whose immune system has already encountered a particular antigen, whether through an earlier infection or through vaccination, usually mounts a much larger and faster antibody response if exposed to the same antigen again. In many cases, the pathogen is cleared before any symptoms have a chance to develop.

Where this fits in the SPM syllabus

Vaccination is an example of active immunity, the body makes its own antibodies in response to an antigen, whether from an actual infection or from a vaccine. This is different from passive immunity, where ready-made antibodies are given directly and no memory cells are formed, so the protection does not last.

Active immunity generally provides longer-lasting protection than passive immunity, because the memory cells formed during an active response can persist for a long time, ready to respond quickly if the same antigen appears again. In passive immunity, by contrast, the antibodies that are given are gradually broken down, and no memory cells are left behind.

Passive immunity can also occur naturally, such as when antibodies pass from a mother to her baby, or artificially, when ready-made antibodies are given directly to a person. Either way, because no memory cells are formed, the protection from passive immunity fades once the antibodies given are broken down.

Common mix-ups to avoid

What is the difference between active and passive immunity?

Vaccination is one route to active immunity, so it helps to place it beside the other forms of immunity the SPM syllabus distinguishes. Active immunity is produced when a person's own immune system meets an antigen, through infection or a vaccine, and makes its own antibodies and memory cells.

Because memory cells remain, this protection is usually long-lasting.

Passive immunity works differently: ready-made antibodies are transferred into the body rather than produced by it. This happens naturally when antibodies pass from mother to baby across the placenta or in breast milk, and artificially when antibodies are injected as an antiserum.

The protection acts immediately but is temporary, because no memory cells are formed and the transferred antibodies are gradually broken down.

• Active immunity: the body makes its own antibodies, forms memory cells, develops slowly, and lasts a long time.
• Passive immunity: antibodies come from outside the body, no memory cells form, and protection is immediate but short-lived.
• A vaccine produces active immunity artificially, using antigens that trigger the response without causing the full disease.

Why do some vaccines need booster doses?

A first dose of a vaccine triggers the primary response, producing antibodies and a first population of memory cells. Over time the level of circulating antibodies falls, and for some diseases the number of memory cells is not enough to keep the response fast years later.

A booster dose re-exposes the immune system to the antigen, triggering a secondary response that raises antibody levels again and increases the memory cell population. This is the same secondary response that makes the body react faster on a real second exposure, used deliberately to keep protection strong.

When enough people in a population are immune this way, the pathogen spreads less easily, an effect the syllabus refers to as herd immunity.

• The first dose builds an initial store of memory cells; a booster enlarges it.
• Booster timing reflects how quickly antibody levels fall for a particular disease.
• Widespread immunity slows how easily a pathogen passes between people (herd immunity).

Source:SRC-DSKP-EN

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