Question

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

Answer 1

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

Answer 2

reddening and experience of pain during insect 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 .When you are bitten by an insect or get bruised there is a break/opening in your skin through which infectious microorganisms enter your body. If these microbes are not eliminated at this location , they might enter the blood circulation and spread to other body organs thus causing systemic damage. Cells of the immune system which help in fighting microbes/infection like neutrophils and macrophages are present in the blood circulation but they dont know the exact location of the microbes ( which are located at the injury site). Therefore to guide these cells to the injury site some signals have to be released telling them the exact location. These signals ,called inflammatory mediators are released by the dendritic cells which are immune cells located under the SKIN. These dendritic cells detect the microbes through specific receptors present on their cells and in turn release the inflammatory mediators . These inflammatory mediators perform different functions like
1) they cause the blood vessels to dilate (vasodilation) so that greater number of immune cells present in the blood can reach the injury site. Increased blood flow means increased number of RBCs This causes reddening and since blood also carries heat you experience a burning sensation.
2) The immune cells present in the circulation have to enter the tissue to fight off infection. A few inflammatory mediators increase the permeability of the blood vessels
allowing the immune cells to cross blood vessels (extravasation) and enter the tissue. The increased permeability also leads to leakage of plasma into the tissue which causes swelling .
3) one of the inflammatory mediators called bradykinin causes pain sensation.

This is the detailed explanation of why you experience swelling , reddening , heat and pain when injured or bitten by an insect