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Why does inflammation cause swelling around an infection or injury?

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Immune stimulation by activated helper T cells — immune system Stimulation of immune response by activated helper T cells. Activated by complex interaction with molecules on the surface of a macrophage or some other antigen-presenting cell, a helper T cell proliferates into two general subtypes, T_H1 and T_H2. These in turn stimulate the complex pathways of the cell-mediated immune response and the humoral immune response, respectively.

Swelling, or edema, occurs during inflammation as a vital part of the body's immune system response to injury or infection.12

When tissue is damaged, cells such as mast cells release chemical mediators, most notably histamine. These chemicals trigger two primary vascular changes:34
* Vasodilation: Blood vessels in the affected area dilate, increasing blood flow, which causes the area to appear red and feel warm.124
* Increased Permeability: The walls of capillaries become more "leaky," allowing fluid, proteins, and white blood cells to migrate from the bloodstream into the surrounding tissue.124

This accumulation of fluid and immune cells at the site of injury is what produces visible swelling. While this process can cause discomfort, it is a necessary defense mechanism that delivers essential immune components to the area to eliminate pathogens, clear cellular debris, and initiate the healing process.1254