system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When a part of your body swells, like from a bee sting or as an allergic reaction, what makes the swelling and how?
gpt
ELI5: Skin acts like a wall to invaders (pathogens/foreign agents, etc.). When the wall breaks invaders enter and the body defends itself by sending its army (immune system cells) to the location causing the swelling. 

After a sting, or bite, to an area, histamine signaling molecules localize to the site resulting in increasing blood flow to the area so other immune cells can come and elicit a response to the foreign body/pathogen/sting.   If someone has a severe enough reaction (anaphylaxis) this occurs over the whole body which results in blood vessels dilating and tanking an individuals blood pressure which impairs oxygen and nutrients systemic circulation. That's why it's a life threatening issue and needs immediate intervention with drugs such as epinephrine (e.g. Epi-pens) to constrict the vessels and combat the histamine mediated vessel dilation. 

Hopefully that answers your question.