Illustration showing white blood cells (purple) in a blood vessel (pink tube) moving towards the site of a bacterial infection (green, top). Certain types of white blood cell, including dendritic cells (blue) and T helper cells (purple with red nuclei) can release small signal proteins collectively termed cytokines (red dots). By moving towards high concentrations of cytokines (chemotaxis), more white blood cells can travel to the site of an infection.