AQA GCSE Biology immune system explained: antigens, antibodies, and vaccination

A pathogen is a microorganism that causes disease. AQA wants four types: bacteria, viruses, fungi, and protists. Bacteria can reproduce rapidly and may produce toxins that make you feel ill. Viruses reproduce inside your cells and damage them as they burst out.

Diseases caused by pathogens are communicable, meaning they can spread between people. The spread can be reduced by hygiene, destroying vectors, isolating infected individuals, and vaccination.

Before your white blood cells get involved, your body tries to stop pathogens entering at all. These are non-specific defences — they work against anything, not one particular pathogen.

• Skin: a physical barrier that also produces antimicrobial secretions, and scabs over when cut.
• Nose: hairs and mucus trap particles and pathogens in the air you breathe in.
• Trachea and bronchi: lined with mucus that traps pathogens and cilia that waft it back up away from the lungs.
• Stomach: hydrochloric acid kills most pathogens in food and drink before they get further.

If a pathogen does get in, white blood cells take over. AQA wants three distinct roles, and it is worth keeping them separate because questions often test exactly one.

• Phagocytosis: a white blood cell engulfs the pathogen and digests it.
• Producing antibodies: lymphocytes produce antibodies that lock onto the pathogen and lead to its destruction.
• Producing antitoxins: these neutralise the toxins released by bacteria.

Here is the distinction worth nailing. An antigen is a marker on the surface of a pathogen — every pathogen has its own unique antigens, like a name badge. An antibody is the protein your lymphocytes make to match that specific antigen, like a key cut for one lock.

The first time you meet a pathogen, it takes a few days to produce the right antibody, which is why you feel ill. But your body keeps memory cells afterwards. If the same pathogen returns, those memory cells produce the correct antibody quickly and in large amounts, destroying the pathogen before you get ill. This is immunity.

This is why you only tend to get measles once. After the first infection, the memory cells for the measles virus stay in your body, so a second exposure is dealt with before it can take hold.

A vaccine introduces a small amount of a dead or inactive form of a pathogen, carrying its antigens. Your white blood cells respond by producing antibodies, and you are left with memory cells — without having to suffer the disease first.

• If the real pathogen infects you later, the memory cells produce antibodies rapidly and you do not become ill.
• If a large proportion of the population is vaccinated, the pathogen struggles to spread. This protects people who cannot be vaccinated.
• A common exam slip: a vaccine contains antigens, not antibodies. Your own body makes the antibodies in response.