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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Immunology

Broken Brakes: The Biological Mechanisms of HAE Types

At a Glance

Hereditary Angioedema (HAE) swelling is caused by an excess of bradykinin, which makes blood vessels leak. The condition is categorized into types based on the C1-inhibitor protein: Type 1 involves low protein levels, Type 2 involves defective protein, and a third type has normal protein but other genetic mutations.

Understanding the biology of Hereditary Angioedema (HAE) is about understanding a “broken brake” in your blood vessels. While the symptoms—painful swelling—are the same for most patients, the underlying reason why that “brake” fails can differ depending on which genetic type of HAE you have [1][2].

The Bradykinin “Flood”

Every HAE attack, regardless of type, is caused by an excess of a peptide called bradykinin [2][3].

Think of your blood vessels as a plumbing system and bradykinin as a chemical that makes the pipes “leaky.” Usually, your body produces bradykinin through a process called the contact system to help regulate blood flow [4]. To keep this system in check, your body uses a protein called C1-esterase inhibitor (C1-INH), which acts like a brake [5][6]. In HAE, this brake is either missing or broken, leading to a flood of bradykinin that forces fluid out of the pipes and into your tissues, causing swelling [5][7].

The Three Main Types of HAE

Doctors classify HAE based on how much C1-INH protein you have and how well it works.

Type 1: Low Level, Low Function

This is the most common form (about 85% of cases) [8]. It is caused by mutations in the SERPING1 gene, which provides instructions for making the C1-INH protein [8][9]. In Type 1, the body simply does not produce enough of the protein. When you have a blood test, both the level of the protein and its function (how well it works) will be low.

Type 2: Normal Level, Low Function

Type 2 is less common (about 15% of cases) and is also caused by mutations in the SERPING1 gene [8][10]. In this type, your body produces plenty of C1-INH protein, but the protein itself is “broken” and cannot do its job [8]. A blood test will show a normal or even high protein level, but its function will be very low.

HAE with Normal C1-INH (HAE-nC1-INH)

This type is rarer and more complex. In these patients, the C1-inhibitor is perfectly fine, but other “parts” of the plumbing system are overactive [1][11]. This was formerly called “Type 3.” Researchers have identified several genes that, when mutated, can cause this form of the disease:

  • F12 (Factor XII): The most common mutation in this group, which makes the contact system too easy to “trip” [1][12].
  • PLG (Plasminogen): A protein involved in breaking down blood clots that can also trigger bradykinin [1][13].
  • ANGPT1 (Angiopoietin-1): Affects how stable your blood vessel walls are [14].
  • KNG1 (Kininogen): The direct “parent” molecule from which bradykinin is cut [11][2].
  • MYOF, HS3ST6, and CPN1: Newer, rarer genetic variants associated with this form of swelling [1][15].

The Estrogen Connection & Pregnancy

If you are a woman with HAE, you may notice your symptoms are highly sensitive to hormones. This is especially true for the F12 mutation in HAE-nC1-INH [16][12]. Estrogen acts like “fuel” for the contact system; it increases the production of the proteins that make bradykinin [17].

Because of this, taking birth control pills containing estrogen often leads to a dramatic increase in swelling attacks [18]. Safe alternatives include progestin-only pills (the “mini-pill”), progestin implants, or non-hormonal copper IUDs.

If you are planning a family, the idea of hormones triggering attacks can be terrifying. However, rest assured that many women with HAE carry highly successful pregnancies [19]. With specialized medical coordination, highly effective and safe treatments (like plasma-derived C1-inhibitor) are available to protect you during pregnancy and delivery [19][20].

Return to Home Page

Common questions in this guide

What causes the swelling in Hereditary Angioedema?
HAE attacks are caused by an excess of a peptide called bradykinin. This chemical makes your blood vessels leaky, forcing fluid into your tissues and causing painful swelling.
What is the difference between HAE Type 1 and Type 2?
In Type 1 HAE, the body does not produce enough C1-inhibitor protein. In Type 2, the body produces normal amounts of the protein, but the protein is defective and cannot do its job properly.
Can I have HAE if my C1-inhibitor levels are normal on a blood test?
Yes, you can have a rarer form called HAE with normal C1-inhibitor. In this type, the C1-inhibitor works perfectly fine, but mutations in other genes like F12 or PLG cause the contact system to become overactive and trigger swelling.
Why do estrogen-based birth control pills trigger HAE attacks?
Estrogen acts like fuel for the contact system, increasing the production of proteins that make bradykinin. Because of this, taking birth control pills containing estrogen often leads to a dramatic increase in swelling attacks for women with HAE.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Am I classified as having Type 1, Type 2, or HAE-nC1-INH, and what specific lab values led to that conclusion?
  2. 2.If I have normal C1-INH levels but still experience symptoms, have I been tested for the F12, PLG, ANGPT1, or KNG1 genetic mutations?
  3. 3.How does my specific subtype of HAE affect the likelihood that my children will inherit the condition?
  4. 4.Since I have HAE, what are the safest non-estrogen options for birth control or managing menopausal symptoms?
  5. 5.Is my swelling driven by the contact system, and how does my treatment plan specifically target that pathway?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (20)
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    Biochemistry, molecular genetics, and clinical aspects of hereditary angioedema with and without C1 inhibitor deficiency.

    Miyata T, Horiuchi T

    Allergology international : official journal of the Japanese Society of Allergology 2023; (72(3)):375-384 doi:10.1016/j.alit.2023.04.004.

    PMID: 37169642
  2. 2

    Hereditary angioedema: Pathophysiology (HAE type I, HAE type II, and HAE nC1-INH).

    Wedner HJ

    Allergy and asthma proceedings 2020; (41(Suppl 1)):S14-S17 doi:10.2500/aap.2020.41.200081.

    PMID: 33109319
  3. 3

    Altered levels of phospholipases C, diacylglycerols, endocannabinoids, and N-acylethanolamines in patients with hereditary angioedema due to FXII mutation.

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    Allergy 2025; (80(1)):287-296 doi:10.1111/all.16197.

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    A mechanistic model of in vitro plasma activation to evaluate therapeutic kallikrein-kinin system inhibitors.

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    PLoS computational biology 2024; (20(11)):e1012552 doi:10.1371/journal.pcbi.1012552.

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    Medical algorithm: Management of C1 inhibitor hereditary angioedema.

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    Allergy 2022; (77(3)):1060-1063 doi:10.1111/all.15115.

    PMID: 34587298
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    The central role of endothelium in hereditary angioedema due to C1 inhibitor deficiency.

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    International immunopharmacology 2020; (82()):106304 doi:10.1016/j.intimp.2020.106304.

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    A novel assay of excess plasma kallikrein-kinin system activation in hereditary angioedema.

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    SERPING1 Variants and C1-INH Biological Function: A Close Relationship With C1-INH-HAE.

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    Hereditary angioedema in Austria: prevalence and regional peculiarities.

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    Recessive SERPING1 Variant Leads to Kinin-Kallikrein System Control Failure in a Consanguineous Brazilian Family with Hereditary Angioedema.

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    Expanding the Genetic and Clinical Spectrum of Hereditary Angioedema with Normal C1 Inhibitor: Novel Variants and Treatment Insights.

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    PMID: 40848077
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    Cytokine and estrogen stimulation of endothelial cells augments activation of the prekallikrein-high molecular weight kininogen complex: Implications for hereditary angioedema.

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    The Journal of allergy and clinical immunology 2017; (140(1)):170-176 doi:10.1016/j.jaci.2016.09.032.

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    Oral FXIIa inhibitor KV998086 suppresses FXIIa and single chain FXII mediated kallikrein kinin system activation.

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    Mutation of the angiopoietin-1 gene (ANGPT1) associates with a new type of hereditary angioedema.

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    Hereditary angioedema with normal C1 inhibitor associated with carboxypeptidase N deficiency.

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    Management of hereditary angioedema with normal C1Inh: a series of 163 French patients.

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This page explains the biological and genetic mechanisms of HAE for educational purposes. Always consult your immunologist or healthcare provider for an accurate diagnosis and a personalized treatment plan.

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