Allergic reactions work the same in children as they do in adults. An allergic reaction is the immune system’s attempt to defend the body from a perceived threat that, in reality, is harmless. During this process, a foreign substance called an antigen comes into contact with the body. The immune system responds to the perceived threat of the antigen by producing protective proteins called antibodies.  

Each antibody is specifically designed to recognize a particular antigen. The antibody that responds to allergens is called immunoglobulin E (IgE). The antibody is attached to a mast cell, which contains granules of histamines and other chemicals. When the antibody binds to the allergen, the mast cell releases histamines and other chemicals, which trigger an allergic reaction. This produces symptoms such as itchy eyes, runny nose and sneezing. In some cases, the reaction is more extreme, resulting in hives, eczema (skin inflammation) or breathing difficulties.

While all humans have stores of IgE, allergic individuals produce the antibody in much greater amounts. To make matters worse, increased exposure to the allergen causes higher production of the IgE antibody in the individual, causing greater sensitization to the allergen with each additional exposure. This sets the stage for more frequent and increasingly severe future reactions to the allergen.

Childhood allergies provide the foundation for many of the allergic problems that people will experience throughout their lifetime. While some people outgrow their childhood allergies, many will see their symptoms escalate into adulthood.

Some allergies tend to afflict children more than adults. For example, children are more likely to have food allergies than adults. Conversely, children are less likely to have seasonal allergies because they have not yet lived long enough to be sensitized to many of these allergens. Over time, however, they can develop many of these allergies with increased exposures.

According to the American Academy of Allergy, Asthma and Immunology (AAAAI), allergy symptoms result in about two million missed school days each year. Children with allergies may have trouble sleeping due to nasal congestion. This may cause drowsiness that impacts their ability to learn and play. Studies also have shown that children with allergies are more likely to become aggressive, depressed and irritable.

Untreated, severe nasal congestion can – in some cases – lead to more serious complications. A child who must breathe constantly through the mouth can develop a facial pattern called adenoidal face, in which the face becomes elongated due to the impact force of air on the mouth's soft bones. This may cause the teeth to grow in at an improper angle, as well as causing an overbite.

Allergy symptoms can also cause fluid to build up within the ear, affecting hearing and leading to frequent ear infections. This can impact speech development, and may require the insertion of ear tubes.

Many of the same allergens that trigger allergies also provoke symptoms in those who have asthma, a chronic inflammation of the airways that makes breathing difficult. These include pollens, molds, pet dander, perfumes and more. Therefore, allergy management is an important part of asthma management in children.