Karen Binkley, MD, is a researcher and an associate professor of clinical immunology and allergy, and she discovered hereditary angioedema (HAE) with normal C1 inhibitor. She explains factor XII’s role in HAE.

Factor XII has an important role in HAE. What it does is it activates another protein, which then cleaves off a small molecule called bradykinin. And when there’s too much bradykinin, that’s what causes the swelling.

So in the case of factor XII angioedema, what happens is this factor XII is more active than it should be, and it’s cleaving off too much bradykinin, and that’s what causes the swelling.

So, there are many types of hereditary angioedema, as we’re learning.

Type 1 hereditary angioedema occurs when the C1 inhibitor levels — which is this protein in the blood that we know is either absent or not working in hereditary angioedema — are very low. There’s just not enough C1 inhibitor to put the brakes on factor XII and stop the overproduction of bradykinin.

HAE type 2 occurs when the C1 inhibitor is actually present, but it isn’t working.

And then the factor XII is, of course, a mutation in factor XII, which makes it overactive and not completely responsive to the C1 inhibitor. So it can overcome the braking actions of the C1 inhibitor from time to time.