Say you step on a splinter. Virtually instantly, your body recognizes the need for a response. As a preparatory step, the blood vessels in the area open, or dilate. This allows more defenders to reach the action, and it leads to redness and heat in the region. More blood, more cells, more oxygen. The blood vessels go through a second change, becoming more permeable. Now other defenders can move into the tissue, along with clotting agents. These are different kinds of proteins, and as their numbers grow, the region experiences swelling. All this activity can lead to pain. In that way, the
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