Histamine and the Chemistry of Allergies

For some people, a spring breeze full of pollen means streaming eyes and endless sneezing. For others, a trace of peanut can be life-threatening. Allergies are the immune system reacting strongly to substances that are normally harmless. At the centre of the reaction is a small molecule called histamine, and the fight against allergy symptoms is a lesson in receptor chemistry.

What is an allergy?

An allergy is an immune response to a harmless substance, called an allergen. Common allergens include:

• Pollen proteins from grasses, trees and weeds
• Dust mite proteins (including digestive enzymes in their droppings)
• Pet dander — proteins in skin flakes and saliva
• Foods: peanuts, tree nuts, milk, eggs, fish, shellfish, wheat, soy, sesame
• Insect venoms, such as bee and wasp stings
• Some medicines, such as penicillin

Most allergens are proteins or glycoproteins. They’re usually small and stable enough to survive and reach the immune system, and some (like dust mite enzymes) can damage the barrier of the airways, making it easier for them to get in.

Stage 1: sensitisation

The first time a susceptible person meets an allergen, there are no symptoms. Instead, their immune system “learns” it:

1. Immune cells take up the allergen and present pieces of it to helper T cells.
2. In allergic people, these T cells steer B cells to make a particular class of antibody: immunoglobulin E (IgE) (see the immune system as chemistry).
3. IgE antibodies attach by their stems (Fc regions) to receptors on mast cells (in tissues such as the skin, airways and gut) and basophils (in blood).

The mast cells are now “armed”, covered with IgE antibodies whose binding sites point outward, ready to recognise the allergen. This can last for months or years.

Why some people make IgE against harmless proteins and others don’t is complex: genetics, early-life exposures and the environment all play a role. IgE probably evolved mainly to fight parasites such as worms.

Stage 2: the allergic reaction

When the allergen appears again, it binds to the IgE on mast cells. The key step is cross-linking: one allergen molecule, with several epitopes, binds to two or more neighbouring IgE antibodies at once, pulling their receptors together. This triggers a signalling cascade inside the mast cell.

Within seconds to minutes, the mast cell degranulates — its internal packets (granules) fuse with the cell membrane and release their contents. The most important is histamine. Mast cells also begin making other signals, such as leukotrienes and prostaglandins, which prolong and intensify the reaction over the following hours.

Meet histamine

Histamine is a small molecule, C₅H₉N₃ (molar mass 111.15 g mol⁻¹). It’s made in one step from the amino acid histidine by removing its carboxyl group as carbon dioxide — a decarboxylation, catalysed by the enzyme histidine decarboxylase (see amino acids).

Structurally, histamine has:

• an imidazole ring — a five-membered ring containing two nitrogen atoms
• a short two-carbon chain ending in an amine group (–NH₂)

At body pH, the amine group is mostly protonated (–NH₃⁺), so histamine carries a positive charge. That’s important for how it binds its receptors.

Histamine receptors

Histamine acts through four types of receptor, called H₁ to H₄. All four are G-protein-coupled receptors in cell membranes. The first two are the most important for allergies and digestion:

Receptor	Where	Effects
H₁	Blood vessels, airways, nerves, brain	Widens and leaks blood vessels, contracts airway muscle, itch, wakefulness
H₂	Stomach lining, heart	Stimulates stomach acid secretion
H₃	Brain	Controls release of neurotransmitters
H₄	Immune cells	Attracts immune cells

How histamine causes symptoms

Most classic allergy symptoms come from histamine acting on H₁ receptors:

• Leaky blood vessels: the cells lining small vessels pull apart slightly, letting fluid escape into tissues. This causes swelling, a runny nose and hives (raised, itchy welts).
• Widened blood vessels: redness and warmth; if widespread, blood pressure falls.
• Smooth muscle contraction in the airways: narrowing, wheezing and coughing.
• Nerve stimulation: itching and sneezing.

Leukotrienes, released a little later, are even more powerful at constricting airways and are important in asthma.

Antihistamines: blocking the receptor

Antihistamines are drugs that block H₁ receptors. They fit into the receptor’s binding site but don’t activate it, stopping histamine from binding. Technically, most are inverse agonists: they stabilise the receptor in its inactive shape, reducing even its small background activity (see pharmacology basics).

Like histamine, most antihistamines contain a positively charged amine that interacts with a negatively charged aspartate in the receptor pocket, plus aromatic rings that fit into hydrophobic regions.

There are two generations:

• First-generation antihistamines, such as diphenhydramine and chlorphenamine, are fat-soluble enough to cross the blood–brain barrier. In the brain, histamine acting on H₁ receptors helps keep you awake, so blocking it causes drowsiness. That’s why some are sold as sleep aids.
• Second-generation antihistamines, such as cetirizine, loratadine and fexofenadine, were designed to be more polar or to be pumped back out of the brain by transport proteins. They cause much less drowsiness. Fexofenadine is a nice case study: it’s a metabolite of an older drug, terfenadine, which had rare but dangerous effects on the heart. Its extra carboxylic acid group makes it a zwitterion, keeping it out of the brain.

H₂ blockers, such as famotidine, block histamine’s receptors in the stomach and are used to reduce stomach acid — not for allergies. They show how blocking the same messenger at different receptors gives completely different drugs.

Antihistamines work best when taken before exposure, because they prevent histamine binding rather than reversing effects already under way. They don’t block leukotrienes, which is why they’re less useful in asthma.

Anaphylaxis: when the whole body reacts

In anaphylaxis, mast cells throughout the body release their contents at once. Within minutes, blood vessels widen and leak everywhere, blood pressure plummets, airways narrow and the throat can swell. It’s a medical emergency.

The first-line treatment is an injection of adrenaline (epinephrine), usually with an auto-injector into the thigh (see adrenaline). Adrenaline works on several receptors at once, doing the opposite of histamine:

• α₁ receptors: narrows blood vessels, raising blood pressure and reducing swelling.
• β₁ receptors: strengthens the heartbeat.
• β₂ receptors: relaxes airway muscles, opening the airways.
• It also helps stabilise mast cells, reducing further release.

Antihistamines alone act too slowly and don’t treat the life-threatening parts of anaphylaxis. After using an auto-injector, emergency medical help is still needed.

Histamine in food

Some foods contain histamine themselves, produced when bacteria decarboxylate histidine. Poorly stored fish, such as tuna and mackerel, can build up high levels, causing scombroid poisoning — flushing, headache and rash that look like an allergy but aren’t immune-mediated. Aged cheeses, fermented foods and some wines also contain histamine. The body normally breaks it down using enzymes such as diamine oxidase in the gut.

Key takeaways

• An allergy is an immune response to a harmless allergen, mainly via IgE antibodies on mast cells.
• Allergens cross-link IgE, triggering degranulation and release of histamine.
• Histamine, made by decarboxylating histidine, acts on H₁ receptors to cause swelling, itching, sneezing and airway narrowing.
• Antihistamines block H₁ receptors; newer ones stay out of the brain and cause less drowsiness.
• Anaphylaxis is treated with adrenaline, which reverses histamine’s effects on several receptors.