Urticaria and Angioedema: Quieting the Mast Cell

One cell, two rashes

The mast cell is a landmine of pre-loaded chemicals, and histamine is the biggest charge. Mast cells sit in the skin and mucosa, packed with granules of pre-formed mediators — above all histamine. When the cell is triggered it degranulates: it dumps its cargo into the surrounding tissue in seconds. Histamine acts on H1 receptors on small blood vessels and nerves, and three things follow — vessels leak (swelling), vessels dilate (redness), and sensory nerves fire (itch). Where the leak is superficial, in the upper dermis, you get a wheal: the classic hive, raised and itchy, gone within 24 hours. Where the same process happens deeper, in the lower dermis and subcutis, you get angioedema — a soft, poorly demarcated swelling of lips, eyelids, tongue or hands that stings rather than itches and lasts longer. Same cell, same mediators, different depth. That is why urticaria and angioedema are two faces of one disease and so often travel together.

Acute versus chronic — and the hunt for a trigger

Urticaria is called acute if it settles within six weeks and chronic if the daily or near-daily hives outlast that. Acute urticaria is common and usually self-limiting — a viral infection, a food or drug reaction, an insect sting. Here it is worth looking for a trigger, because removing it ends the problem. Chronic urticaria is where students expect to find the cause and are surprised to fail: in the great majority it is chronic spontaneous urticaria, with no external allergen at all. Much of it is autoimmune — the patient has autoantibodies that cross-link the mast cell's own IgE receptor and make it fire without any allergen present. This is a crucial reframing: chronic urticaria is usually not an allergy, and endless allergy testing and elimination diets mostly waste time and add anxiety. The task is not to find a food to avoid; it is to quiet the mast cell.

Step one: the antihistamine, and why it's an inverse agonist

First-line therapy is a second-generation, non-sedating H1-antihistamine — and it does more than "block" histamine. The foundation of urticaria treatment is the H1-antihistamine. Pharmacologically these are not simple blockers but inverse agonists: the H1 receptor has baseline activity even without histamine bound, and the drug pushes the receptor toward its inactive state, actively damping the signal rather than merely occupying the site. The modern first-line agents are the second-generation antihistamines — cetirizine, loratadine, fexofenadine, and bilastine. They are preferred because they are large, poorly lipophilic molecules that barely cross the blood–brain barrier, so they achieve strong peripheral H1 blockade with little sedation. This builds directly on the histamine-receptor pharmacology taught in the Autonomic and Principles section, where the first-versus-second-generation split is introduced.

The first-generation antihistamines — diphenhydramine, chlorphenamine, hydroxyzine — still block H1, but they are small and lipophilic, cross into the brain, and cause sedation. Worse, they are dirty at other receptors: they carry an anticholinergic burden (dry mouth, blurred vision, urinary retention, constipation, confusion in the elderly). For a chronic itchy disease that a patient may treat for months or years, that daytime drowsiness and anticholinergic load are unacceptable, and modern guidelines specifically advise against using them as routine first-line therapy. Their sedation is not a therapeutic bonus — it is a side effect being mistaken for one. The idea of anticholinergic burden is developed further in the Autonomic and Principles section.

If a licensed dose doesn't control the hives, the next move is not a new drug — it's more of the same one. The single most important prescribing point in chronic urticaria is up-dosing. Guidelines endorse increasing a second-generation antihistamine up to four times the standard licensed dose before adding anything else. Because these drugs have such a wide safety margin, quadrupling the dose is well tolerated and often succeeds where the standard dose failed. A common real-world error is to give the licensed dose once, see partial benefit, and jump straight to steroids — skipping the cheap, safe, effective step of simply using more antihistamine. Only when a patient remains symptomatic on a properly up-dosed second-generation antihistamine do you escalate.

Climbing the ladder: add-ons, then biologics

When an up-dosed antihistamine is not enough, two modest add-ons are traditional. An H2-antihistamine (such as ranitidine, historically, or famotidine) can be added because a minority of the skin's histamine receptors are H2, so blocking both subtypes occasionally adds a little; the effect is small. A leukotriene receptor antagonist — montelukast — targets a different mast-cell mediator (the leukotrienes) and helps a subset of patients, again modestly. These are worth trying because they are safe and oral, but neither is a reliable answer for genuinely refractory disease. Note what is deliberately absent from the maintenance ladder: long-term oral corticosteroids. They work, but the cost of chronic steroids — weight gain, bone loss, diabetes, hypertension — is unacceptable for a benign, long-lasting condition. Steroids have a role only as a short rescue course for a severe acute flare, never as ongoing control.

For antihistamine-refractory chronic urticaria, the game-changer targets IgE itself. The transformative third-line drug is omalizumab, a monoclonal antibody against IgE. It binds free IgE and lowers the amount available, which in turn down-regulates the IgE receptors on mast cells and makes them far harder to trigger. In effect it quiets the mast cell one step upstream of histamine release rather than mopping up histamine downstream — and it works impressively well in chronic spontaneous urticaria that has defeated antihistamines, often clearing hives that had persisted for years. Its safety profile is reassuringly good; it is given as a periodic subcutaneous injection. The same anti-IgE strategy is used in allergic asthma, as covered in the Respiratory section, which is where omalizumab was first established. The field is moving quickly: newer agents include next-generation anti-IgE antibodies (such as ligelizumab) and oral BTK inhibitors that dampen mast-cell signalling from inside the cell, several of which are in advanced trials for chronic urticaria.

Beyond omalizumab, for the small number of patients with severe, genuinely refractory disease, ciclosporin is the recognized option. As a calcineurin inhibitor it broadly suppresses T-cell activation and mast-cell mediator release, and it is effective — but it demands monitoring of blood pressure and renal function and carries the immunosuppressant risks familiar from transplant medicine, so it is reserved for the resistant minority and used for limited periods. The overall shape of the ladder is worth memorizing: up-dosed second-generation antihistamine → add-on (H2 blocker / montelukast) → omalizumab → ciclosporin, with short steroid courses reserved for acute flares only.

The trap: when the swelling isn't histamine at all

Angioedema comes in two mechanistic flavours, and they need completely different drugs. Most angioedema that accompanies hives is histaminergic — mast-cell, histamine-driven — and it responds to the same antihistamines, and, when part of anaphylaxis, to adrenaline. But there is a second, entirely separate pathway: bradykinin-mediated angioedema. Here the swelling is driven not by histamine but by bradykinin, a vasoactive peptide, and this is the single most dangerous concept in the topic — because bradykinin-mediated swelling does not respond to antihistamines, corticosteroids, or adrenaline. Give those drugs and the airway keeps swelling while you watch. Two causes matter. The first is ACE-inhibitor-induced angioedema: ACE normally degrades bradykinin, so an ACE inhibitor lets bradykinin accumulate, and a patient on, say, ramipril or lisinopril can develop lip, tongue or laryngeal swelling — sometimes after months or years on the drug. The treatment is to stop the ACE inhibitor permanently (and never re-challenge). This connects directly to the Cardiovascular section, where ACE inhibitors and the bradykinin cough are taught.

The second bradykinin cause is hereditary angioedema, caused by a deficiency (or dysfunction) of C1-esterase inhibitor. Without that brake, the complement and contact pathways run unchecked and bradykinin is overproduced, giving recurrent attacks of swelling — face, limbs, gut (causing severe abdominal pain), and dangerously the larynx — typically with no urticarial wheals and no itch, often from childhood and sometimes with a family history. The critical exam point is that hereditary angioedema does not respond to antihistamines, steroids, or adrenaline; treating it as an allergy is a fatal error. It needs mechanism-specific therapy: C1-inhibitor concentrate (replacing the missing protein) or icatibant, a bradykinin B2-receptor antagonist that blocks bradykinin at its target receptor. Newer prophylactic options include the monoclonal antibody lanadelumab, which inhibits plasma kallikrein upstream of bradykinin. The pattern — deep swelling without hives, unresponsive to antihistamines — is the flag that should make you think bradykinin, not histamine.

And when the swelling is part of a whole-body allergic collapse, one drug comes first. When mast-cell degranulation is massive and systemic — angioedema plus airway compromise, wheeze, hypotension and shock — that is anaphylaxis, and the life-saving drug is adrenaline (epinephrine), given intramuscularly without delay. Antihistamines and steroids are, at most, adjuncts that treat the skin and may reduce late-phase symptoms; they do not reverse the airway and circulatory collapse and must never delay adrenaline. Anaphylaxis management is covered in full in the Respiratory and emergency sections; the single point to carry from here is that in the swelling patient who is also hypotensive or wheezing, adrenaline comes first and everything else second.