Food allergies, intolerance and sensitivities have a lot of overlapping symptoms, but they're different conditions.
An allergy is caused by the immune system – the human body's defense against what it perceives to be harmful invaders, such as viruses. Sometimes, the immune system gets it wrong. In the case of a food allergy, the immune system interprets a particular food as a harmful invader, and it sends cells – either T cells or IgE antibodies, depending on the allergy type -- to fight it. This battle can result in symptoms like diarrhea, vomiting, stomach pain, rash, breathing difficulties, chest pain and/or, at worst, anaphylaxis [source: Woznicki]. Anaphylaxis sends the body into a state of shock, and it can be fatal [source: Mayo Clinic].
Allergic reactions occur anywhere from a few seconds to a few hours after ingesting the allergen, and a person with allergies will always have a reaction to a particular food, every time it's eaten, regardless of how much is eaten. Peanuts, eggs, soy, wheat and shellfish are common allergens [sources: Allergist, WebMD].
Food intolerance is a different animal. It's triggered by the digestive system, not the immune system. Typically, a person's digestive tract lacks the enzyme required to digest a certain food [source: Woznicki]. Symptoms can take up to two days to appear and may include cramping, bloating, vomiting, diarrhea, headaches, irritability and/or anxiety [sources: Medical News Today, WebMD]. Intolerance to lactose is common [source: Woznicki]. Celiac disease, which involves an inability to digest gluten, is technically a gluten intolerance but also has an immune-system component, which is why it's sometimes called a "gluten allergy" [sources: Celiac Disease Center, Woznicki].
Intolerance reactions within the same person can vary and often depend on how much of the food is consumed [source: Cleveland Clinic]. Intolerances can improve through the use of supplements or periodic abstinence [source: Medical News Today].
Intolerance and allergies are pretty well-understood. Sensitivities, on the other hand, are understudied and something of a mystery to medical science beyond the fact that they occasionally make people feel unwell. A person's sensitivity reaction to a food or food additive (MSG is a common one) can vary from one day to the next [sources: Kerr, IFT, Woznicki]. Aside from the fact that they're definitely not allergies, at the moment, sensitivities are anybody's guess [sources: IFT, Woznicki].
Regardless of what's behind the unpleasant feeling, if there's some evidence of a connection with eating, an elimination diet might be able to help with diagnosis. Obtaining accurate results is no picnic, though.