Placebo: Mind over Matter in Modern Medicine. Dylan Evans

your bones is a serious condition called osteomyelitis; this was what eventually killed the girl who knelt on the radiator.

      Next time you are gripped by an intense pain, then, you might pause to consider how lucky you are. Evolution has endowed you with a vital defence mechanism, and without it your life expectancy would be considerably shorter. In a world where minor injuries such as scratches, burns, and bruises are common fare, it really is good to be able to feel bad.

      SWELLING

      The same logic applies to all the other aspects of the acute phase response. Swelling, for example, is also a defensive process, caused by the leakage of plasma and the migration of immune cells into the area of damaged tissue. All bodily damage, whether caused by injury or infection, consists of broken cells, and when the walls of a cell rupture, an array of molecules, which would not otherwise be released, spill out into the surrounding tissue. Some of these molecules trigger the sensory nerves to produce the ongoing, second type of pain just described. The sensory nerves also react by causing the blood vessels to widen, increasing local blood flow, and making the walls of the blood vessels more permeable. With greater blood flow, more white blood cells – the infantry of the immune system – can be carried to the site of the injury. The greater permeability of the blood-vessel walls enables the white blood cells to flow out of the arteries and veins into the surrounding tissue to defend against possible bacterial invaders. If no bacteria have found their way into the wound, particular white blood cells known as macrophages clear up the debris of the shattered cells by engulfing and digesting it. If bacteria have gained a foothold and started to multiply, the white cells form a barrier to create a pus-filled abscess in which the blood fluid – the serum – plays a key role in healing.

      Besides clearing up debris and attacking bacteria themselves, the macrophages also release a number of chemical messengers. These signalling molecules, or cytokines, play a vital role in co-ordinating the acute phase response by facilitating both short-distance communication among the immune cells themselves, and long-distance communication between the immune cells at the injured site and the brain.