The complement system is an essential component of the human immune system, enhancing immune response. The proteins that are part of the complement system, i.e. the complement factors, destroy intruding pathogens such as bacteria (known as antigens). They adhere to antigens to mark them and cause other components of the immune system to ingest and dissolve them ( phagocytosis). The complement system also triggers inflammatory reactions that reinforce the immune response. If the complement cascade is not properly regulated, angioedema may be the result.

Angioedema means the swelling (edema) of the vascular system (Greek angio - vessel). As these swellings affect the vascular system, they are non-specific, and the time or location of their appearance is not predictable. Skin, mucous membranes or inner organs may be affected. Edema of the skin can be quite disfiguring and is often found in the face, but also in the limbs or genitalia. The affected skin is red and painful, but does not always itch. Edema in the intestinal tract is a more serious matter, associated with severe pain and spasms which can be confused with colic or appendicitis. Edema in the upper airways can become life-threatening when larynx, nose or tongue is affected.

These episodic swellings may be caused by an allergy and the associated release of histamine. An allergen leads to the activation of IgE, a specific antibody for allergic responses, which causes the mast cells within the immune system to secrete histamine. Histamine is a cell hormone that increases the permeability of blood vessels in the skin. Fluid leaks into surrounding tissue, which swells and itches in response.
Angioedema may also be caused by bradykinin, which has a similar action to histamine. It, too, renders blood vessels more permeable, leading to swollen, red tissue, spasms in smooth muscle and pain.

The response resembles the response to a bee sting. In fact, one of the active components in bee toxin is bradykinin. The purpose of higher permeability in blood vessels is to give components of the immune system easier access to the inflammation site. Bradykinin is a component of the coagulation (blood-clotting) system that forms when factor XII, an enzyme involved in coagulation, is activated through cell debris or pathogens. Bradykinin release can also be caused by medication such as ACE inhibitors or by a deficiency in C1-INH, a regulator of the complement system.