Seasonal allergies, often known as hay fever or allergic rhinitis, involve the immune system reacting to typically harmless substances in the environment. These reactions occur during specific times of the year, usually when airborne particles like pollen from trees, grasses, and weeds are prevalent. The immune system mistakenly identifies these allergens as threats, leading to symptoms such as sneezing, an itchy or runny nose, and watery eyes.
Common Ages for Allergy Onset
Seasonal allergies typically begin in childhood, with the most common age of onset falling between 2 and 5 years old. Symptoms tend to be most pronounced in school-age children, teenagers, and young adults. While seasonal allergies can develop at almost any age, they are rare in infants under two years old because it usually takes at least two seasons of exposure to allergens for the immune system to become sensitive. For children who experience chronic nasal symptoms under the age of two, the cause is often related to recurrent colds, enlarged adenoids, or even an allergy to cow’s milk, rather than seasonal pollen. Most people who experience seasonal allergies will have developed them by the time they reach 20 years of age.
Factors Influencing Allergy Development
The development of seasonal allergies is influenced by a combination of genetic predisposition and environmental exposures. Individuals with a family history of allergic conditions, such as asthma, eczema, or allergic rhinitis, have an increased likelihood of developing seasonal allergies themselves. If one parent has allergies, their children have a 30-50% chance of developing them, and this rises to 60-80% if both parents are affected. This suggests a genetic component makes the immune system more sensitive to specific allergens.
Environmental elements also play a significant role in allergy development. Factors such as the concentration of allergens in the air, climate changes, and exposure to pollutants can influence both the onset and severity of allergic reactions. For instance, warmer temperatures can cause plants to blossom earlier and produce pollen for longer durations, extending the allergy season. Additionally, the “hygiene hypothesis” suggests that overly sanitary environments might suppress the natural development of the immune system, making it more prone to overreacting to harmless substances.
The Sensitization Process
Seasonal allergies arise from sensitization, where the immune system learns to react to a harmless substance as a threat. This begins when the body first encounters an allergen, such as pollen. Specialized immune cells play a role in recognizing these foreign proteins. During this initial exposure, the immune system produces specific immunoglobulin E (IgE) antibodies for that allergen.
These IgE antibodies then attach themselves to the surface of mast cells, which are abundant in tissues like the nasal mucosa. Upon subsequent re-exposure to the same allergen, the allergen binds to the IgE on the mast cells. This binding triggers the mast cells to release chemicals, including histamine, leukotrienes, and prostaglandins, into the bloodstream.
Allergies in Adulthood
New allergies can develop at any point in life, not just during childhood. Approximately one-quarter of adults experience seasonal allergies. Changes in the immune system that occur with aging can make it more sensitive to allergens that were previously tolerated.
Adult-onset allergies can also be triggered by new or increased exposure to allergens. For example, moving to a new geographic location with different types of plants or higher concentrations of allergens can initiate new sensitivities. Starting new hobbies or activities that increase exposure to specific allergens, such as gardening, may also lead to the development of symptoms. Hormonal shifts and a history of other allergic conditions, like eczema in childhood, can also increase the likelihood of developing new allergies later in life.