eli5 why the body makes an area of infection swell up ?
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When various cells of the immune system recognize common elements of pathogens, they set off a cascading alert signal that is also sensed by non-immune cells.
In particular, the cells that make up the walls of blood vessels change their shape so that the vessel becomes slightly leaky, which causes blood (carrying complement proteins, antibodies and other soluble factors) to enter the tissue. The leaky vessel wall also gains a sort of velcro-like adhesion capability which allows immune cells passing by in the blood to catch onto the wall and squeeze themselves through; this is necessary because they’re too big to just fit through the leaky cracks.
So now you’ve got a lot of fluid entering the tissue. You normally get some fluid entering anyway, but that’s matched by a draining system via the lymphatic vessels. Right now that drainage capacity is way exceeded, so the tissue starts to swell. Plus, because the blood is red and warm (core body temperature ~ 37°C), and it’s right beneath the skin, the area itself becomes reddish and warm to the touch.
Collectively, this process of signaling and its results are called **inflammation**.
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