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Why does inflamed skin become red?
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inflammation
The main function of complement proteins is to aid in the destruction of pathogens by piercing their outer membranes (cell lysis) or by making them more attractive to phagocytic cells such as macrophages (a process known as opsonization). Some complement components also promote inflammation by stimulating cells to release histamine and by attracting phagocytic cells to the site of infection.
Inflamed skin becomes red primarily due to the dilation of blood vessels (vasodilation) in the area of injury.12
Following an initial, transient constriction, the small blood vessels in the damaged tissue dilate, which significantly increases blood flow to the site. Because the skin's color is influenced by the state of the cutaneous circulation, this surge in blood flow—and the resulting engorgement of the dilated vessels—causes the skin to appear bright red. This process is a fundamental component of the acute inflammatory response, often accompanied by other cardinal signs such as heat, swelling, and pain.321