C1 inhibitor (C1-INH) is a protein in your blood that acts as a brake on inflammation. It controls several interconnected defense systems, preventing them from activating when they shouldn’t. When C1-INH levels are too low or the protein doesn’t work properly, the result is uncontrolled swelling episodes that can affect the skin, gut, and airways, a condition known as hereditary angioedema.
What C1 Inhibitor Does in the Body
C1-INH belongs to a family of proteins called serine protease inhibitors, or serpins. These proteins work by latching onto enzymes that trigger inflammatory cascades and physically blocking their active sites. Think of C1-INH as a master switch that keeps several alarm systems from firing at once.
The protein was originally named for its role in the complement system, one of the body’s first-response defense networks against infection. C1-INH shuts down the first step of complement activation by binding to enzymes called C1r and C1s, forming a complex that prevents the cascade from proceeding. It also regulates a second branch of complement called the lectin pathway and helps dampen a third branch, the alternative pathway, by preventing key molecules from assembling.
But complement control is only part of the picture. C1-INH also regulates the contact activation system (part of blood clotting), the system that generates a molecule called bradykinin, and the system that breaks down blood clots. In all of these, its job is the same: prevent overactivation. This makes C1-INH one of the most broadly important anti-inflammatory proteins in the bloodstream.
The Bradykinin Connection
The symptoms of C1-INH deficiency come primarily from one molecule: bradykinin. Bradykinin is a small peptide that tells blood vessels to become more permeable, allowing fluid to leak into surrounding tissues. In normal amounts, this is part of the healing process. In excess, it causes dramatic swelling.
C1-INH keeps bradykinin production in check at two points. First, it blocks an activated clotting factor (Factor XIIa) from converting a precursor enzyme into its active form, kallikrein. Second, it directly inhibits kallikrein itself. When C1-INH is absent or dysfunctional, both of these checkpoints fail, kallikrein runs unchecked, and bradykinin floods the local tissue. This is why the swelling in C1-INH deficiency looks and behaves differently from allergic reactions: it doesn’t respond to antihistamines, steroids, or epinephrine, because the mechanism has nothing to do with histamine.
What Happens When C1-INH Is Deficient
Low or dysfunctional C1-INH causes episodes of deep swelling beneath the skin and mucous membranes. The swelling is distinct from typical allergic reactions. It doesn’t itch, doesn’t produce hives, and doesn’t leave pitting when pressed. It most commonly appears in the arms and legs, but it can occur almost anywhere on the body.
Attacks typically build gradually over several hours, peak at 12 to 36 hours, and resolve over two to five days without treatment. Swelling in the throat, tongue, or larynx is the most dangerous presentation because it can obstruct the airway. Abdominal attacks are also common and can mimic a surgical emergency, with sudden, severe pain caused by swelling in the intestinal wall, sometimes without any visible external swelling at all.
Hereditary vs. Acquired Deficiency
C1-INH deficiency comes in two broad categories: inherited and acquired.
Hereditary angioedema (HAE) is caused by mutations in the SERPING1 gene, which provides the instructions for making C1-INH. Hundreds of different mutations have been identified, and they produce three recognized types:
- Type 1 HAE: The most common form. Mutations cause the body to produce reduced amounts of C1-INH protein. Both the protein level and its functional activity are low on blood tests.
- Type 2 HAE: The body produces normal or even elevated amounts of C1-INH, but the protein is structurally defective and doesn’t work properly. Blood tests show normal protein quantity but reduced functional activity. These mutations typically cluster in a specific region of the gene called exon 8, near the protein’s active site.
- Type 3 HAE (HAE with normal C1-INH): All standard lab tests come back normal. This form appears to involve gain-of-function mutations in Factor XII of the clotting pathway or other poorly understood mechanisms that lead to excess bradykinin through alternative routes.
Acquired C1-INH deficiency looks clinically similar but develops later in life rather than being present from birth. The most common underlying causes are autoimmune conditions and B-cell cancers such as non-Hodgkin lymphoma. In some cases, the angioedema symptoms appear before the lymphoma is diagnosed, so a new diagnosis of acquired C1-INH deficiency warrants screening for underlying blood cancers.
Estrogen exposure can also play a role in triggering attacks, particularly in certain subtypes. Oral contraceptives, hormone replacement therapy, and pregnancy have all been associated with worsening symptoms.
How C1-INH Deficiency Is Diagnosed
Diagnosis starts with a clinical pattern: recurrent episodes of swelling without hives that don’t respond to standard allergy treatments. From there, blood tests confirm the problem.
The key lab tests measure two things: the quantity of C1-INH protein in the blood (normal range is 15 to 35 mg/dL) and its functional activity (normal range is 70 to 130%). In Type 1 HAE, both numbers are low. In Type 2, the quantity looks normal but the functional activity is reduced. A complement protein called C4 is also commonly checked as a screening tool because it runs low when C1-INH isn’t doing its job.
A 2025 international consensus paper outlines a stepwise approach: clinical history, lab testing to rule out C1-INH deficiency, therapeutic trials when initial workup is unclear, and genetic testing for cases where standard labs are normal but the clinical picture is suggestive. Referral to an angioedema specialist is recommended when the diagnosis remains uncertain after initial testing.
Treatment for C1-INH Deficiency
Because the swelling is driven by bradykinin rather than histamine, standard allergy medications are ineffective. Treatment instead targets the bradykinin pathway directly or replaces the missing protein.
C1-INH replacement therapy delivers the functional protein through an IV infusion. Both plasma-derived and lab-made (recombinant) versions are available and are equally effective for treating acute attacks. For people with frequent episodes, regular infusions every three to four days can prevent attacks from occurring in the first place.
Other treatment options work further along the pathway. Some block the bradykinin receptor on blood vessel walls, preventing the molecule from causing leakage even when it’s present in excess. Others inhibit plasma kallikrein, the enzyme that produces bradykinin. The choice between these approaches depends on whether the goal is treating an active attack, preventing future attacks, or both, and the best fit varies from person to person based on attack frequency and severity.