Why do scraped knees and insect bites become red painful or itchy

When we feel pain, such as when we touch a hot stove, sensory receptors in our skin send a message via nerve fibres (A-delta fibres and C fibres) to the spinal cord and brainstem and then onto the brain where the sensation of pain is registered, the information is processed and the pain is perceived.

When knee is scrapped and wound is acquired, insects bites to create a insicion on the skin. The blood capillaries of that part if the bodies get distorted in shape or even get damaged. So there is a reaction of immune system by sending much blood at that wound which introduces antibodies so reaction of antibodies with antigens causes allergic reaction.

Reddening and experience of pain during insects bite or injury are classical signs of a complex immunological process called inflammation. This is a mechanism by which the body tries to curtail infection and simultaneously promote tissue repair.