The immune response.

Step 1 of 4

A first meeting

The body fights pathogens on two lines. Innate immunity acts at once and against any invader: the skin, the mucus, stomach acid, and white blood cells that swallow anything that looks foreign. Adaptive immunity learns: it recognises one particular pathogen, and it takes time.

In the Scene a virus lands on the airway lining of someone who has never met it. Vaccine is fixed at None, and Days since infection stands at day two. The upper chart shows the virus on a logarithmic scale: each gridline is a factor of ten.

Step 2 of 4

Antibodies

Everyone has millions of kinds of B cell, each carrying an antibody of a different shape on its surface. When a virus arrives, only the few whose antibody fits it bind to it. With help from helper T cells they divide again and again, and some become plasma cells that pour out antibodies. Days since infection stands at day fourteen.

Step 3 of 4

The race

At a first meeting there is a race: the virus multiplies, and the immune system searches for and multiplies the matching cells. Start from the day the virus lands, and think about who will lead in the first week.

Step 4 of 4

Days ill

You feel ill while the virus is above a certain level, mostly because of the inflammation the immune system itself stirs up: fever, tiredness, aches. The upper chart marks that level with a dashed line. Days since infection stands at Three weeks on.