How does histamine from mast cells cause swelling around an insect bite?

When an insect bite occurs, the body’s immune system responds to the introduction of foreign substances, such as venom or saliva, by triggering mast cells located in the connective tissue near the site of the injury.123

Upon stimulation, these mast cells undergo a process called degranulation, during which they release stored chemical mediators, most notably histamine. Histamine acts on the surrounding blood vessels by causing them to dilate and increasing their permeability. This increased vascular permeability allows fluid, plasma proteins, and immune cells to leak out of the bloodstream and into the surrounding tissue. The accumulation of this fluid in the tissue is what results in the localized swelling, often observed as a wheal or bump at the site of the bite.4253