In allergy, the immune system produces antibodies (immunoglobulin E) to foods (generally to their proteins) because it perceives them as potentially dangerous invaders. The antibodies produce a cascade reaction that can affect the skin (rashes), the respiratory system (risk of suffocation) or the digestive system (vomiting). The most common allergies tend to be to shellfish and crustaceans, peanuts and tree nuts, eggs, celery, dairy products, etc. They can be diagnosed with a skin or blood test.
Intolerances, however, are more common (affecting 20% of the population) and are not the direct and immediate result of an immune system response, but rather a reaction at the digestive level after the consumption of certain foods. They are almost always due to a deficiency of certain enzymes, but also due to malabsorption or other digestive disorders (see below). And I say that they are not a direct and immediate result of an immune response because antibodies (IgA and IgG) can appear, but they are usually rooted in intestinal permeability and are common in Crohn’s disease and other intestinal disorders. If the intestine is porous (hyperpermeable), food particles can leak out and be detected by the immune system.
Lactose intolerance due to a lack of the enzyme lactase is one of the best known. Fructose intolerance is also quite common. The most common symptoms are nausea, bloating and abdominal pain, diarrhoea, constipation, gas, belching, but also headache, mental fog, hangover feeling, sleepiness, eczema, rashes, acne, muscle and joint pain, etc.
Food sensitivity is a more recently coined term and is also closely linked to intestinal permeability: the cells of the intestinal wall allow particles (peptides, bacteria, proteins, etc.) to pass into the bloodstream, triggering an immune system response. The tricky thing is that the response and symptoms can occur several days after the food has been consumed; it is very common for them to appear within 72 hours of ingestion. The most obvious example is non-coeliac gluten sensitivity (NCGS). Many professionals still do not recognise this pathology as such, but there are several solid scientific studies (below are three) that confirm its existence.
It is clear to all of us that, when there is an allergy, the solution is to avoid that food. Unfortunately, the issue of intolerances and non-coeliac gluten sensitivity is still not taken as seriously. The problem is that as long as they are left untreated, damage to the intestinal wall continues and this leads to an immune response and systemic inflammation that can lead to more serious diseases such as arthritis, thyroiditis, multiple sclerosis and many other chronic and autoimmune diseases.