The sting causes inflammation which is an immune system response involving not just redness but a complex of several actions. The redness is caused by the blood vessels and capillaries becoming distended (enlarged) to increase the flow of blood to bring immune cells to the site of injury (in this case the injury is a sting). The swelling is caused by an outpouring of fluid (plasma) from the enlarged blood vessels. Immune cells force their way out of the blood vessels into the fluid filled swollen area and scavenge for damaged cells, infected cells, and bacteria. Whilst all this is going on cells in the surrounding tissue undergo changes and start to proliferate in order to form new tissue at the site of the wound. The itching that accompanies inflammation can be caused by chemicals released during the process of inflammation or, in the case of wasp sting, irritation caused by the sting itself (or bits of sting left in the wound if the main part of the sting has been removed).