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Why can an insect bite produce a swollen welt?

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Pathways of complement activation — inflammation The main function of complement proteins is to aid in the destruction of pathogens by piercing their outer membranes (cell lysis) or by making them more attractive to phagocytic cells such as macrophages (a process known as opsonization). Some complement components also promote inflammation by stimulating cells to release histamine and by attracting phagocytic cells to the site of infection.

An insect bite produces a swollen welt primarily due to the body's allergic reaction to substances introduced into the skin, such as saliva, venom, or excretory products.1

When these foreign substances enter the body, they trigger mast cells to release histamine. This release is a key component of the inflammation process, which serves as the body's local response to injury. Histamine causes nearby blood vessels to dilate and become more permeable. This increased permeability allows fluid and immune system cells—such as white blood cells—to leak from the bloodstream into the surrounding tissue to fight the perceived threat and begin the healing process. The accumulation of this fluid in the tissue is what manifests visibly as a swollen, often itchy, welt or wheal.21