Why does the skin get red and swollen?

Our immune system keeps us healthy. If it finds something that seems to be a threat, or a dead cell that the body doesn't accept, immune system cells attack in a network of interactions that can cause swelling and redness. First, scouting cells attract the attention of immune system cells that travel around in our blood. Then our blood vessels allow these cells to pass through. The area swells up to make space for the “battle field” between the intruder and the immune system cells, and this can stimulate nerve endings. Finally, the immune system cells eat up the foreign object or damaged cells, and then die off. Their remains is yellow, and you find in spots or bruises.