What is Food Allergy? Why Are Some Children Intolerant to Certain Foods?

What is Food Allergy? Why Are Some children Intolerant to Certain Foods?

Food allergy originates from proteins in the food that, when consumed without being heated or cooked and not broken down by gastric acid and digestive enzymes, manage to pass through the gastrointestinal mucosa into the bloodstream, reaching target organs and causing a systemic allergic reaction. The complex digestion process affects the timing and location of allergic reactions. For example, when someone is allergic to a certain food, they may first experience itching in the mouth when consuming it. After digestion in the stomach, symptoms may appear in the abdomen, such as vomiting, diarrhea, and abdominal pain. When the allergen enters the bloodstream, it can cause a drop in blood pressure. If it reaches the skin, it can lead to hives and eczema. And if it enters the lungs, it can trigger asthma. All these reactions typically occur within minutes to an hour.

For adults, common allergenic foods include shellfish, river shrimp, (small) lobsters, crabs, fish, as well as poultry eggs, peanuts, and legumes. Some of these foods can lead to severe allergic reactions, causing a sudden drop in blood pressure that, if left untreated, can be life-threatening. For children, the most common allergenic foods are eggs, milk, and peanuts. Food allergies in children may disappear as they grow older. Milk and egg allergies generally desensitize after 3 to 6 months of avoiding consumption, but peanut and nut allergies may persist throughout life.

Cross-reactivity: If a child is allergic to river shrimp, they will likely be allergic to crabs, lobsters, and crayfish as well. This is called cross-reactivity. Some young mothers may unknowingly give shrimp as a supplementary food to their infants, preparing shrimp puree for them to eat. However, this is not advisable as the infant's intestinal tract is not yet fully developed, and their digestive function is weak. Shrimp contains a high level of proteins with small molecular sizes. If not completely broken down into amino acids, they can easily pass through the intestinal mucosa and enter the bloodstream, causing allergic reactions like eczema or hives on the skin.

Diagnosis and Differential Diagnosis: To diagnose food allergies, doctors must first identify whether the child has allergic reactions to specific foods and use tests to objectively measure allergenic foods. One such test is the skin scratch test, where diluted food extracts are applied to the forearm or back skin, and then the skin is scratched with a needle at the site of the droplet to observe local allergic reactions and swelling. A positive result indicates that the child's mast cells on the skin have produced immunoglobulin E in response to the tested food. For such patients, doctors may use radioallergosorbent testing (RAST) or enzyme-linked immunosorbent assay (ELISA) to confirm the diagnosis based on specific immunoglobulin E levels in the blood.

There are also natural substances, like histamine, present in some foods that can trigger a reaction similar to food allergies. For example, cheese, certain wines, and fish (especially tuna and mackerel) contain high levels of histamine. The histamine in fish is generally considered to be due to bacterial contamination, especially in unfrozen fish. If a child consumes foods with high histamine concentrations, they may experience strong reactions similar to food allergies. This reaction is known as histamine poisoning.

Another diagnostic method for food allergies is the double-blind, placebo-controlled food challenge (DBPCFC) test. This test is considered the gold standard for allergy testing but is only suitable for older children. The method involves placing various suspected allergenic foods into specially made opaque capsules. The patient swallows one capsule at a time, and then the medical team observes for allergic reactions. This process is repeated until all the capsules are consumed.