There are two related concepts that are often interchanged: food allergy and food intolerance.
Allergies occur when our body mistakenly identifies an inert substance as harmful, and triggers an immunological response. In the most common form of allergy, the allergenic substance is recognized by a blood protein called immunoglobulin E, or IgE, which binds it. The IgE molecule bound to the allergenic can then interact with different cells, which release the chemicals induced in the immunological response. The response is different depending on the allergy, but can include things such as asthma, rhinitis, skin rashes and so on. While in theory any food can induce an allergic response, some are more prone than others. These include peanuts, milk, eggs and seafood.
On the other hand, food intolerance is a different problem. When someone is intolerant to a certain food, it means that it lacks the ability to metabolize it, that is, to transform it into a substance that can be assimilated by the body. For instances, people that are “lactose intolerant” can’t metabolize the sugar lactose, which is present in the milk, because they don’t produce enough amount of a protein called “lactase”. Instead, the lactose is transformed by the intestinal flora through a different reaction, producing a number of by-products that end up causing diarrhea and bloating.
So as you can see, some foods like milk can produce adverse reactions through different mechanisms: some people are allergic to milk (i.e. something in the milk is perceived by the body as harmful and triggers an immunological response), while some are intolerant (i.e. they can’t metabolize the lactose in the milk).
But while these examples are pretty clear-cut, reality is often much more complex. Let’s see for example the case of gluten.
Gluten is a mix of proteins found in many cereals, such as wheat, barley or oat. Many people feel an adverse reaction when they ingest bread or other derivatives of these cereals. However, there can be a wide variety of causes.
The most famous cause is celiac disease. People with this disease can’t eat bread because gluten induces a strong immunological response. However, celiac disease is usually not considered a food allergy because of its particularities. First, unlike most food allergies, is not mediated by IgE. Instead, what happens is that the presence of gluten makes the immunological system attack the cells of the small intestine. Hence, celiac disease is usually classified as an autoimmune disease, rather than a food allergy.
Then there is wheat allergy, which is a regular food allergy mediated by IgE. People with this allergy have also to give up bread and watch out for other foods, as many contain traces of wheat as a cryptic contaminant.
A third condition, which is less understood, is called “gluten sensitivity”. Its diagnosis is usually made after confirming that the person rejects gluten, and discarding celiac disease and wheat allergy as the causes.
So as you can see, biology can be pretty complex and there are far more conditions that words to name them!