An allergy is the immune system treating something harmless as a threat. The mechanism is straightforward; what makes the topic confusing is that one word covers conditions with wildly different stakes — a seasonal runny nose, a lactose intolerance that isn't an allergy at all, and a reaction that can kill within minutes.
Three different things wearing one name
Immediate allergy (IgE-mediated)
Antibodies of the IgE class recognise a specific protein and trigger mast cells to release histamine and other mediators. This is what happens within minutes: hives, swelling, sneezing, wheeze, and at the extreme, anaphylaxis.
Typical triggers: peanut and tree nuts, shellfish, egg, milk, fish, sesame, insect stings, latex, and some drugs — penicillin being the most common.
Non-IgE reactions
Some reactions are immune-mediated but not IgE-driven, and appear hours or days later. Food protein-induced enterocolitis in infants presents with delayed vomiting and lethargy rather than hives. Coeliac disease is an immune reaction to gluten that damages the small bowel over months — a genuinely different disease from wheat allergy, which is immediate.
Intolerance — not allergy at all
Lactose intolerance is a shortage of the enzyme lactase, so undigested lactose ferments in the gut and produces bloating, gas and diarrhoea. It involves no immune response, cannot cause anaphylaxis, and dose matters — a small amount is often fine. Histamine and MSG sensitivity claims sit in a murkier, less well-characterised zone.
The practical distinction: allergy is immune and can escalate; intolerance is digestive and cannot. If symptoms are only gut-related, delayed, and dose-dependent, intolerance is more likely.
A positive test is not an allergy
This is the single most important misconception in the field. Skin prick tests and specific IgE blood tests detect sensitisation — the presence of antibodies — not clinical allergy.
- People can carry specific IgE to a food and eat it without any problem
- A positive result therefore has poor positive predictive value on its own
- Result size roughly correlates with likelihood, but does not predict severity
- Diagnosis requires a convincing history of reaction plus the test, not the test alone
Wide panel testing in someone with no clear history reliably produces false labels, unnecessary elimination diets, nutritional gaps and anxiety. Interpretation is the whole job.
Separately: IgG "food intolerance" tests sold directly to consumers are not supported by evidence for diagnosing adverse food reactions. IgG is a marker of exposure, not of harm.
Seasonal and environmental allergy
Allergic rhinitis is inflammation of the nasal lining triggered by pollen, dust mite, mould or animal dander. It is genuinely common, genuinely affects sleep, concentration and work performance, and is genuinely under-treated because people accept it as normal.
- Seasonal pattern suggests pollen; year-round suggests dust mite, mould or pets
- Intranasal corticosteroid sprays are the most effective single treatment for persistent symptoms, but need days to work and must be used regularly — many people give up too early
- Non-sedating antihistamines help itch and sneeze more than blockage
- Saline rinses are cheap and useful
- Allergen avoidance — bedding covers, ventilation, keeping windows shut in high-pollen periods — helps at the margins
- Immunotherapy (tablets or injections) can modify the disease over years for selected allergens
Anaphylaxis: the part that matters
Anaphylaxis is a severe, rapidly progressing systemic reaction. Recognise it by sudden onset after exposure plus features from more than one system:
- Skin — widespread hives, flushing, swelling of lips, tongue or face
- Airway — throat tightness, hoarse voice, stridor, difficulty swallowing
- Breathing — wheeze, persistent cough, breathlessness
- Circulation — faintness, collapse, low blood pressure
- Gut — cramping, vomiting (more prominent in children)
Adrenaline (epinephrine) is the first-line treatment and must not be delayed. Antihistamines do not treat airway swelling or low blood pressure, and giving them first — or instead — is a recognised cause of worse outcomes. Call emergency services, lie the person flat with legs raised (do not let them stand up), and use an adrenaline auto-injector into the outer thigh if one is available, repeating after a few minutes if there is no improvement.
Anyone with a history of anaphylaxis should carry two auto-injectors where prescribed, and everyone around them should know where they are and how to use them.
What has actually changed: treatment, not avoidance alone
For years the advice for peanut allergy was strict avoidance plus a rescue injector. That has shifted for some patients.
- Oral immunotherapy for peanut — carefully escalating small, supervised doses — is now an established option for suitable children, and increasingly for adults. It typically induces desensitisation (tolerating more before reacting), which reduces accidental-reaction risk. It is not a cure: protection depends on continued dosing, and it carries real risks including allergic reactions to the treatment itself.
- Early introduction of peanut into the diet of infants at higher risk, around the time solid foods start, reduces the likelihood of developing the allergy. The old advice to delay has been reversed, and it is one of the clearest preventive findings in the field.
- Adrenaline first is now emphasised much more strongly than sedating antihistamines in acute management.
Practical guidance
- Describe the pattern before testing. What, how soon, which symptoms, how much was needed to trigger it, has it happened twice?
- Don't accept a diagnosis from a test result alone. Ask what the history supports.
- Be sceptical of broad panel testing and of IgG intolerance tests.
- Read labels if you have a diagnosed food allergy — "may contain" statements matter more for some people than others, and this is worth discussing with your clinician.
- Carry adrenaline if prescribed, check expiry dates, and make sure someone else knows how to use it.
- Use nasal steroid sprays consistently for persistent rhinitis — the failure is usually technique and patience, not the drug.
- Get an action plan in writing if you or your child has a serious allergy, and share it with school or workplace.
Bottom line
Allergy is an immune response, intolerance is a digestive one, and only the former can escalate. A positive skin or blood test shows sensitisation, not allergy — diagnosis needs the history. For persistent rhinitis, a nasal steroid used consistently is the workhorse treatment. And in anaphylaxis, adrenaline comes first: antihistamines are not a substitute, and delay is the main thing that turns a survivable event into a fatal one.
This article is educational and not a substitute for medical advice. If you have had a severe reaction, seek urgent assessment and a written action plan.