Why does a cut turn red, hot and swollen?

A cut turns red, hot and swollen because your blood vessels are reacting on purpose. The instant skin breaks, nearby cells release chemical signals. Those signals widen the local blood vessels, so more blood flows in and the skin turns red and warm. The vessel walls also get leakier, so fluid seeps into the tissue and it swells. One of the signal chemicals makes nerves more sensitive, which is why the spot hurts. This whole reaction is called inflammation. It isn't damage, it's defense.

A Roman writer named Aulus Cornelius Celsus wrote these signs down almost two thousand years ago, in a medical book called De Medicina. He listed heat, pain, redness and swelling, a group of four now called the Celsus tetrad. A fifth sign, loss of function, was added much later by other physicians. Doctors still use this same list today, even though Celsus had no idea what was happening inside the tissue.

Inflammation belongs to the innate immune system, the body's fast, general-purpose defense that reacts the same way to almost any threat. The widened, leaky vessels let defending cells leave the blood and crowd into the injured tissue. The reaction is a balancing act: too little of it, and an infection can spread unchecked; too much of it, especially if it drags on for a long time, is linked to diseases such as atherosclerosis and osteoarthritis.