Bradykinin is a vasoactive nonapeptide that is formed as an end product of the kallikrein-kinin system. This system helps to counterbalance the vasoconstrictive properties of the renin-angiotensin-aldosterone system. Bradykinin is thought to be the primary mediator of most nonallergic forms of angioedema. In this system, activated factor XII from the coagulation cascade converts prekallikrein to kallikrein in a positive feedback loop.
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