The symptoms of a food allergy almost always develop a few seconds or minutes after eating the food.

Some people may develop a severe allergic reaction (anaphylaxis), which can be life-threatening.

The most common type of allergic reaction to food is known as an IgE-mediated food allergy.

Symptoms include:

• tingling or itching in the mouth
• a raised, itchy red rash (urticaria) – in some cases, the skin can turn red and itchy, but without a raised rash
• swelling of the face, mouth (angioedema) or other areas of the body
• difficulty swallowing
• wheezing or shortness of breath
• feeling dizzy and lightheaded
• feeling sick (nausea) or vomiting
• abdominal pain or diarrhoea
• hay fever-like symptoms, such as sneezing or itchy eyes (allergic conjunctivitis)

Anaphylaxis

The symptoms of a severe allergic reaction (anaphylaxis) can be sudden and get worse very quickly.

Initial symptoms of anaphylaxis are often the same as those listed above and can lead to:

• increased breathing difficulties – such as wheezing and a cough
• a sudden and intense feeling of anxiety and fear
• a rapid heartbeat (tachycardia)
• a sharp and sudden drop in your blood pressure, which can make you feel light-headed and confused
• unconsciousness

Anaphylaxis is a medical emergency. Without quick treatment, it can be life-threatening. If you think you or someone you know is experiencing anaphylaxis, dial 999 and ask for an ambulance as soon as possible.

Non-IgE-mediated food allergy

Another type of allergic reaction is a non-IgE-mediated food allergy. The symptoms of this type of allergy can take much longer to develop – sometimes up to several days.

Some symptoms of a non IgE-mediated food allergy may be what you would expect to see in an allergic reaction, such as:

• redness and itchiness of the skin – although not a raised, itchy red rash (urticaria)
• the skin becomes itchy, red, dry and cracked (atopic eczema)

Other symptoms can be much less obvious and are sometimes thought of as being caused by something other than an allergy. They include:

• heartburn and indigestion that is caused by stomach acid leaking up out of the stomach (gastro-oesophageal reflux disease)
• stools (poo) becoming much more frequent or loose (though not necessarily diarrhoea)
• blood and mucus in the stools
• in babies: excessive and inconsolable crying, even though the baby is well-fed and doesn't need a nappy change (colic)
• constipation 
• redness around the anus, rectum and genitals
• unusually pale skin
• failure to grow at the expected rate

Mixed reaction

Some children can have a mixed reaction where they experience both "IgE" symptoms, such as swelling, and "non-IgE" symptoms, such as constipation.

This often happens to children who have a milk allergy.