These non-specific defences work against any pathogen:

• skin: a tough barrier
• nose: hairs and mucus trap particles
• trachea and bronchi: mucus traps pathogens and cilia sweep it up to the throat
• stomach: hydrochloric acid kills pathogens in food
• eyes: tears contain lysozyme, an enzyme that breaks down bacteria

If a pathogen gets in, white blood cells attack it in three ways:

• phagocytosis: a phagocyte engulfs the pathogen and digests it
• antibodies: lymphocytes make antibodies that bind to the pathogen's antigens, the molecules on its surface
• antitoxins: these neutralise the toxins that bacteria release

Each antibody has a shape that fits one antigen only, so antibodies against measles do nothing against flu.

Antibodies are made by lymphocytes. They are proteins, not cells, and they do not digest pathogens themselves.

The first time, lymphocytes take days to make enough of the right antibody, so the person is ill. Some become memory cells. The second time, memory cells make the antibody fast and in large amounts: the pathogen is destroyed before symptoms appear. The person is immune.

1. First peak 8 units, second peak 56 units
2. 56 ÷ 8 = 7 times greater