The terms allergies and intolerances are often used interchangeably. This article aims to demystify these terms; provides explanations for causes of, and helpful tips for managing, non-critical intolerance reactions.
It is currently Food Allergy Week in the United States established to raise awareness of this potentially life threatening condition. 32 million Americans live with a food allergy with varying degrees of severity from mild to life threatening symptoms. With such a large swathe of the population affected, it does indeed justify this promotional campaign.
However many more people experience reactions to food that are more likely to be an intolerance reaction. The terms are often used interchangeably as there is a poor understanding of what these terms mean.
→ Allergy or Intolerance?
An allergy involves the immune system. It triggers an immune response with the production of certain antibodies (IgE) in response to a food protein it sees as harmful and a threat . It tends to be an instant reaction after eating the food culprit. The immune system goes into overdrive. It may cause hives, wheezing, swollen lips, tingling in the mouth, dizziness and itching. 
The more serious life-threatening condition known as anaphylaxis can cause construction of the airways, loss of consciousness, fainting or a sudden drop in blood pressure. It requires urgent medical attention.
A food intolerance does not involve the immune system. There is an adverse reaction to a food. The symptoms are milder and certainly less serious. And the reaction is delayed. It can be delayed by hours or even days after eating the culprit food so it is not always easy to identify which food has caused the symptoms.
Most people associate food intolerances with a reaction by the digestive system. Often people may experience bloating, gas, discomfort or diarrhea and will assume they have an intolerance to a food.
The symptoms can also be systemic ie appear anywhere in the body. Such as headaches, migraine or skin conditions; even joint pain. 
→What are the common causes of a food intolerance?
1. The most common cause is digestive insufficiency. To break down food the body produces various digestive secretions including enzymes and gastric acid. If the production of these is hindered in some way then the partially broken down food can cause digestive discomfort.
A common well known food intolerance is to the milk sugar lactose. Asians and many Africans lack the enzyme lactase so cannot tolerate dairy at all.
In addition, incomplete breakdown of food can lead to partially broken down protein molecules passing into the bloodstream and may cause a systemic reaction (such as headaches or skin condition).
For most people though it is not a complete lack of the specific digestive chemicals but rather insufficient secretion at the time of eating the meal.
Helpful Tip: take the time to eat mindfully. Sit at a table without distractions like phone and TV. Concentrate on the meal. Chew food really well. Eat slowly. These approaches encourage digestive secretions.
2. Stress may be an indirect cause of food intolerance. If we are stressed the body is not primed to be digesting food.
The stress mode of the nervous system effectively turns off the digestive function. The body is primed to “fight or flight” and only when it is relaxed can it move into the “rest and digest” phase of the nervous system. So a stressed state hinders digestive secretions. It may be more noticeable that digestive discomfort is experienced at times of chronic stress.
Helpful Tip: Whilst removal of stress is not realistic, help the body shift into the “rest and digest” phase with deep breathing exercises just prior to eating. Simply breathe in slowly for a count of 4, hold for 2 and breathe out for a count of 4 – 6. Repeat several times.
3. Over consumption of common foods
Often the digestive system may be over burdened with certain food types: wheat and dairy and the most common. The consumption of these foods typically occurs every day and often at every meal. The body may cope perfectly well to digest the culprit food in small amounts but intake is too high.
Helpful Tip: Keep a food diary to assess your own food intake over the course of one week. If one food type stands out simply remove it from the diet for at least one week and keep a journal for symptom severity and reduction over this time period.
Digestive symptoms may not be related to intolerances at all. The digestive tract is a complex system with complex pathology. But these tips provide a useful starting point prior to further professional investigation.