There are several pathways that can culminate in angioedema and urticaria.
Activation of mast cells by IgE is the most well known mechanism. When IgE binds to receptors on mast cells, several things happen. The mast cells release histamine. This in turn causes dilation of the nearby vessels and causes fluid to leak from the bloodstream into the tissues. This causes nerve cells to activate and release substance P, which also contributes to vasodilation and causes mast cells to release more histamine. In response to activation by IgE, mast cells will also produce PGD2 and leukotrienes C4 and D4.
The complement system is one of the ways our body identifies infectious agents and triggers the immune system to kill them. Complement proteins are in the blood all the time, and they can be activated by three distinct pathways, all of which are triggered by pathogens: the classical pathway, the alternative pathway and the lectin pathway. Regardless of which pathway activates the complement system, the molecules C3a, C4a and C5a are produced. These molecules bind to receptors on mast cells and induce histamine release.
Following initial dilation of local vessels, proteins that normally are found in the plasma move into the skin. This activates the kinin system, which produces bradykinin through a series of steps. Bradykinin is a very powerful vasodilator and contributes significantly to loss of volume from the blood stream to the tissues.
C3a, C5a, PGD2, and leukotrienes C4 and D4 all draw other inflammatory cells to the site of activated mast cells. These cells release further molecules to stimulate histamine release. This mechanism perpetuates inflammation beyond the original insult.
Bradykinin levels are normally controlled by the enzyme ACE. When patients take ACE inhibitor medications (like Lisinopril, etc), this interferes with bradykinin degradation and cause urticarial and angioedema.
C1 esterase inhibitor regulates complement and kinin pathways. In patients who are deficient in C1 esterase inhibitor, bradykinin may be overproduced.
Many autoimmune conditions cause the formation of IgG1 and IgG3 antibodies. These molecules can interfere with the complement system and cause production of fragments that activate mast cells, like C3a.
NSAIDs are well characterized in their ability to cause angioedema and urticaria. While the mechanism is not fully understood, it is thought that since NSAIDs stop production of prostaglandins, the mast cells overproduce leukotrienes, which contribute to the angioedema and urticaria.
There are several non-immunologic methods that can result in angioedema and urticaria. Heat or pressure on the skin; radiocontrast dyes; alcohol; vancomycin; opioids; and foods like shellfish and strawberries have been linked to these conditions.
Jonathan A. Bernstein, et al. The diagnosis and management of acute and chronic urticaria: 2014 update. J Allergy Clin Immunol Volume 133, Number 5.
Usmani N,Wilkinson SM. Allergic skin disease: investigation of both immediate and delayed-type hypersensitivity is essential. Clin Exp Allergy 2007;37:1541-6.
Zuberbier T, Maurer M. Urticaria: current opinions about etiology, diagnosis and therapy. Acta Derm Venereol 2007;87:196-205.
Ferdman, Ronald M. Urticaria and angioedema. Clin Ped Emerg Med2007; 8:72-80.