People with hereditary angioedema (HAE) experience episodes of soft tissue swelling. Such instances occur in the hands, feet, gastrointestinal tract, genitals, face, and throat.

During an HAE attack, one’s inherited genetic mutation results in a cascade of events that lead to swelling. The swelling is very different from an allergy attack.

Inflammation is your body’s normal response to infection, irritation, or an injury.

At some point, your body needs to be able to control the inflammation because too much can lead to problems.

There are three different types of HAE. The two most common types of HAE (types 1 and 2) are caused by mutations (errors) in a gene called SERPING1. This gene is located on chromosome 11.

This gene provides instructions for making the C1 esterase inhibitor protein (C1-INH). C1-INH helps with reducing inflammation by blocking the activity of proteins that promote inflammation.

The mutation that causes HAE can lead to a reduction in C1-INH levels in the blood (type 1). It can also result in a C1-INH that doesn’t function properly, despite a normal level of C1-INH (type 2).

At some point, your body will need C1-INH to help control inflammation. Some HAE attacks happen for no clear reason. There are also triggers that increase your body’s need for C1-INH. The triggers vary from person to person, but common triggers include:

  • repetitive
    physical activities
  • activities
    that create pressure in one area of the body
  • freezing
    weather or changes in weather
  • high
    exposure to the sun
  • insect
  • emotional
  • infections
    or other illnesses
  • surgery
  • dental
  • hormonal
  • certain
    foods, like nuts or milk
  • blood
    pressure-reducing medications, known as ACE inhibitors

If you have HAE, you don’t have enough C1-INH in your blood to control inflammation.

The next step in the chain of events leading to an HAE attack involves an enzyme in the blood known as kallikrein. C1-INH suppresses kallikrein.

Without enough C1-INH, kallikrein activity isn’t inhibited. The kallikrein then cleaves (separates) a substrate known as high-molecular-weight kininogen.

When kallikrein splits kininogen, it results in a peptide known as bradykinin. Bradykinin is a vasodilator, a compound that opens up (dilates) the lumen of blood vessels. During an HAE attack, excessive amounts of bradykinin are produced.

Bradykinin allows more fluid to pass through blood vessels into body tissues. This leakage and the blood vessel dilation it causes also results in lower blood pressure.

Without enough C1-INH to control this process, fluid builds up in the body’s subcutaneous tissues.

The excess fluid results in the episodes of severe swelling seen in people with HAE.

A third, very rare type of HAE (type 3), happens in a different matter. Type 3 is the result of a mutation in a different gene, located on chromosome 5, called F12.

This gene provides instructions for making a protein called coagulation factor XII. This protein is involved in blood clotting and also responsible for stimulating inflammation.

A mutation in the F12 gene creates a factor XII protein with increased activity. This in turn causes more bradykinin to be produced. Like types 1 and 2, the increase in bradykinin makes blood vessel walls leak uncontrollably. This leads to episodes of swelling.

Knowing what happens during an HAE attack has led to improvements in treatments.

To stop fluid from building up, people with HAE need to take medication. HAE drugs either prevent swelling or increase the amount of C1-INH in the blood.

These include:

  • a
    direct infusion of donated fresh frozen plasma (which contains C1 esterase
  • medications
    that replace C1-INH in
    the blood (these include Berinert, Ruconest, Haegarda, and Cinryze)
  • androgen
    therapy, such as a drug called danazol, which can increase the amount of C1-INH esterase inhibitor produced
    by your liver
  • ecallantide
    (Kalbitor), a medication that inhibits cleavage of kallikrein, thus preventing
    the production of bradykinin
  • icatibant
    (Firazyr), which stops bradykinin from binding to its receptor (bradykinin B2
    receptor antagonist)

As you can see, an HAE attack occurs differently from an allergic reaction. Drugs used to treat allergic reactions, like antihistamines, corticosteroids, and epinephrine, won’t work in an HAE attack.