Last updated: 30.11.22

How Do You Know if You’re Having an Allergic Reaction to Food?

Food allergies are surprisingly common, with 1 in 20 people having at least one. They can range from being fairly minor irritations that you might not even be aware of, to the triggers of life-threatening illnesses. (Source)

One of the more interesting points about allergies is that, while more common in children and infants, they can develop at any age. It’s also entirely possible to become allergic to a food that you’ve eaten all your life, including things like shellfish, milk, eggs and nuts. 

As a result, it’s important for people to have a basic understanding of allergies and how they can manifest. Being able to recognise an allergic reaction means that you can quickly seek medical advice, whether for a minor concern or in the event that you believe you’re suffering from anaphylaxis. 

In this article, we explain the different types of food allergies and their symptoms, the types of ingredients that you might be allergic to, and what to do if you have a food allergic reaction. 

Types of Food Allergy

It’s helpful to know that there are two types of food allergies; IgE-mediated food allergies and non-IgE-mediated food allergies. The former is more commonly known, and the two types present themselves in different ways. It’s also possible to have a mixture of the two, which is more prevalent in children.

IgE-Mediated Food Allergies

The most common form of food allergy is IgE-mediated. These are allergies which show symptoms after seconds or minutes of eating and are triggered because the immune system produces an antibody called immunoglobulin E (IgE).

Non-IgE-Mediated Food Allergies

Non-IgE-mediated food allergies are much harder to identify and diagnose because they generally don’t have immediate symptoms. In some cases, it can actually be days between coming into contact with or eating a food you’re allergic to and the symptoms presenting themselves.

What are the Main Food Allergens?

It’s possible to have an allergic reaction to any ingredient, but people generally tend to be allergic to one of fourteen main allergens. Being aware of these allergens can make it easier to know if you’re having an allergic reaction to food, as if you start experiencing symptoms you can quickly determine if a common allergen might have caused this.

Here are the fourteen most common allergens that might cause an allergic reaction.

  • Cereals containing gluten

Gluten is an ingredient that many people are intolerant of, but it can cause allergic reactions as well. Cereals containing gluten include wheat, rye, barley and oats.

  • Eggs

Eggs are an ingredient in plenty of different dishes, so it’s an allergen that can be tricky to avoid if you know you are allergic to it. It’s particularly common in baked goods like cake, but is also used in sauces and condiments.

  • Milk

Milk is a very common allergy, particularly in children and babies, but some people continue to have a milk allergy into adulthood. Milk and milk powder are used in a wide range of different food products, from obvious choices like cheese and chocolate to products like biscuits, cake or even crisps.

  • Nuts

Nut allergies are pretty common, again particularly amongst children, with some people reacting severely even when being in the same room as the ingredient. Tree nuts are all included in this allergen category, including cashews, almonds and hazelnuts.

  • Peanuts

Peanuts fall under a different category from tree nuts as they grow differently, but are still one of the most common food allergies. They’re often found in desserts, but may also make an appearance in peanut oil which can be used to cook a wide range of things.

  • Fish

Any type of fish can trigger an allergic reaction in someone with this allergy, although some people may have more of a sensitivity to certain types than others. Fish is also an ingredient in plenty of sauces and is found in Worcestershire sauce, so can be tricky to avoid when you’re eating certain kinds of cuisine.

  • Soya

Soya comes from soybean legume and is a kind of plant-based protein often used as a meat alternative or in many kinds of Asian cooking. It appears in ingredients like tofu and soy milk and isn’t as prominent an allergy as others as it’s not used that often in mainstream cooking.

  • Crustaceans

A crustacean is an animal that has a hard shell or a crust, like a lobster, crab or prawn. People with a crustacean allergy should avoid seafood dishes and any meal that is made using a crustacean-based sauce, like shrimp paste.

  • Molluscs

The most common types of molluscs that people discover they are allergic to are mussels or squid. These types of ingredients can be eaten whole, but similar to crustaceans may also be found as an ingredient in certain kinds of sauces.

  • Celery

Celery is a vegetable that can be used as a whole ingredient in salads and cooked dishes, but is also ground up and used as a seasoning. Celeriac also falls under this category of allergen, but is less common as an ingredient.

  • Mustard

Mustard is best known for being a popular condiment, but may also be found in things like marinades, curries, sauces and salad dressings. A small amount of the product is usually used, but if you have a severe allergy then this can be enough to trigger a dangerous allergic reaction.

  • Sesame seeds

You’ll often find sesame seeds sprinkled on top of bread or used alongside other seeds in salads or as a topping for things like soup. Sesame oil is also used a lot in cooking, so any product cooked with this can become contaminated and trigger a reaction.

  • Sulphites

Most people aren’t aware that sulphites can cause an allergic reaction, but they’re actually one of the most common ingredients that people are allergic to. You can find sulphites in dried fruit, soft drinks, alcohol and even meat products, so it’s an ingredient that can be difficult to obviously identify.

What are the Signs of An Allergic Reaction to Food?

In order to know whether you’re having an allergic reaction to food, you need to understand the symptoms of food allergies. Here are some of the most common for both IgE-mediated and non-IgE-mediated allergies.

Symptoms of IgE-mediated food allergies include the following:

  • Itching or tingling in the mouth
  • Rashes on the skin, itchiness, or redness
  • Swelling, often in the face or mouth
  • Dizziness and nausea
  • Difficulty swallowing or breathing
  • Vomiting
  • Sneezing and itching in the eyes
  • Stomach pains and diarrhoea

Symptoms of non-IgE-mediated food allergies include the following:

  • Eczema or an itchy rash
  • Vomiting
  • Diarrhoea
  • Constipation
  • Abdominal pain and cramping

You should note that not everybody experiencing an allergic reaction will have all of these symptoms. In fact, it’s likely that you’ll only get a couple, especially if your reaction is quite mild.

It’s important to be aware of all the potential symptoms of an allergic reaction to food, as this means you can react quicker in realising that you might be having one. In many cases, you’ll likely only need to take an antihistamine, but a severe allergic reaction to food may lead to anaphylaxis.


Anaphylaxis is generally the result of an IgE-mediated food allergy, and is a medical emergency with potentially life-threatening implications. It is essentially a very severe reaction to a food allergen, again involving the body’s own immune system. 

Should you or anyone else present with symptoms of anaphylaxis, it’s essential to call 999 and seek emergency medical help immediately.

Symptoms of anaphylaxis include the following:

  • Swelling in the mouth, particularly the tongue
  • Severe breathing difficulties
  • A tight chest
  • Dizziness and fainting
  • Difficulty swallowing
  • Collapsing

It should be noted that anaphylaxis is often erroneously called anaphylactic shock. The two conditions are not the same; anaphylactic shock is a type of shock caused by anaphylaxis. Shock is a medical emergency whereby a person’s blood pressure drops so low that oxygen is not properly supplied to the body.

What to Do If You Have a Food Allergic Reaction

If you’ve never had an allergic reaction to food before, starting to experience the symptoms can be scary. It’s important to stay calm if this situation does arise and follow the below advice so that you stay safe and remain healthy.

If you’re only having a mild allergic reaction to food, you can manage the symptoms by taking an antihistamine tablet. These are available over the counter from most pharmacies and many people already have them at home.

If you start to experience more severe symptoms that indicate anaphylaxis, you should seek emergency medical treatment. Official guidance from the NHS says that you should ring 999 if you suspect someone is having a severe and life-threatening allergic reaction, with symptoms such as struggling to swallow, struggling to breathe and feeling faint.

If you are aware of your food allergy you may have been given an auto-injector pen by your doctor. If you feel yourself starting to have a serious allergic reaction, you should ask someone to call for an ambulance and administer this yourself, or get someone to help if you are too unwell to do so.


What happens when you have an allergic reaction to food?

When you have an allergic reaction to food, your body’s immune system reacts to a protein in an ingredient like it is a harmful pathogen. This triggers the immune system to try and attack the protein to protect the body, which leads to symptoms like itching, wheezing, rashes, stomach pain and difficulty breathing.

How long does an allergic reaction to food last?

An allergic reaction to food can last anywhere from a few minutes to several days. Most allergic reactions are only bad for a short while and will then die down to be manageable, but some people can still feel the impact of a severe allergic reaction to food days after they were exposed to an allergen.

How quickly does an allergic reaction to food happen?

An allergic reaction to food can happen instantly or it can take place a long time after the person has been exposed to an allergen. In severe cases, the signs of an allergic reaction to food might appear seconds after coming into contact with a product, whilst some people might not start to feel unwell until several hours later and have a delayed allergic reaction to food.


Food allergy awareness is a very important part of training for those that work with food, whether in manufacturing, retail or catering. If you’re regularly exposed to lots of different ingredients, it can be very useful to know the signs of an allergic reaction so that you can identify if you are having one and need emergency medical attention. Acting quickly saves lives, and knowing the signs and how to respond can make a huge difference.

If you’d like to learn more about what causes an allergic reaction and how to help if someone is having one, we cover these topics in our ‘Food Allergy Awareness’ online course, which is particularly suitable for those that work with food.