Defence against disease and vaccination – GCSE Combined Science Biology

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Every day you meet germs. They are on door handles, in the air and in your food. Yet most days you do not get ill. That is because your body has a defence system.

This matters because it explains why you get better from a cold. It also explains why vaccines protect you, and why they protect other people too.

By the end you will be able to describe how the body keeps pathogens out. You will explain what white blood cells do. You will explain how vaccination and herd immunity work.

Keeping pathogens out

A pathogen is a microorganism that causes disease. The body has barriers that stop pathogens getting in. Think of a football ground, with gates, bag checks and stewards.

The skin is a barrier, and it seals cuts with blood clots. The nose has hairs and mucus that trap particles. The trachea and bronchi have mucus and cilia, which are tiny hairs that sweep mucus up to be swallowed.

The stomach makes hydrochloric acid, which kills most pathogens in food and drink. These defences work against all pathogens, so they are called non-specific.

Phagocytes

If a pathogen gets inside the body, white blood cells attack it. There are different kinds. One kind is the phagocyte.

A phagocyte wraps around a pathogen and takes it inside. Enzymes then break the pathogen down. This is called phagocytosis. It works on many kinds of pathogen, so it is non-specific.

Think of a phagocyte as a cell that swallows an intruder whole, then breaks it down. Phagocytes act quickly, while antibodies take longer to arrive.

Lymphocytes and antibodies

Lymphocytes are another type of white blood cell. Every pathogen has antigens, which are markers on its surface. When lymphocytes detect an antigen, they make antibodies.

Antibodies are proteins that fit one antigen, like a key in a lock. They attach to the pathogen and help to destroy it. An antibody for one pathogen does not work on another.

Some lymphocytes make antitoxins. These neutralise toxins, which are poisons made by pathogens.

Memory cells and immunity

When lymphocytes meet a new antigen, they take time to make the right antibody. This is the primary response, and you may be ill.

Some of the lymphocytes become memory cells. They stay in your body. If the same pathogen enters again, they respond much faster. They make many more antibodies very quickly. This is the secondary response, and you do not get ill. You are immune.

Think of a test you have practised before. The second time is quicker. This is why you rarely get chickenpox twice.

Vaccination

A vaccine contains a small amount of a dead or weakened pathogen, or parts of one. These carry antigens. The vaccine makes lymphocytes produce antibodies, and memory cells are made.

But you do not get the disease. If you meet the live pathogen later, the secondary response is fast, so you are protected.

Think of a fire drill. You practise before a real fire. Vaccines are given in advance. They do not cure someone who is already ill.

Herd immunity, benefits and risks

If most people in a population are vaccinated, the pathogen cannot spread easily. People who are not vaccinated, such as babies or people with weak immune systems, are protected too. This is herd immunity. If vaccination rates fall, outbreaks can return.

Vaccination has wiped out smallpox and greatly reduced polio and measles. It prevents epidemics, which are many cases of a disease at once.

Some people have a mild reaction, such as a sore arm. Serious reactions are rare. A vaccine may stop working if the pathogen changes its antigens.

Recap

Skin, mucus, cilia and stomach acid are non-specific defences.

Phagocytes engulf pathogens. Lymphocytes make antibodies and memory cells.

A vaccine has a dead or weakened pathogen. It gives immunity without the disease.

Herd immunity protects people who are not vaccinated.