Seasonal Allergies

Seasonal Allergies (Seasonal Allergic Rhinitis)

Definition and Overview

People with seasonal allergies — also called hay fever or allergic rhinitis — react to pollen from plants. Allergic rhinitis (AR) is a heterogeneous disorder that, despite its high prevalence, is often undiagnosed; it is characterized by one or more symptoms including sneezing, itching, nasal congestion, and rhinorrhea. Seasonal allergic rhinitis (SAR) is fairly easy to identify because of the rapid and reproducible onset and offset of symptoms in association with pollen exposure.

Historically, AR was thought to be a disease process of the nasal airway alone, but the development of the unified airway theory has classified AR as a component of systemic allergic response, with other associated conditions such as asthma and atopic dermatitis sharing an underlying systemic pathology. Atopic allergic sensitization is defined by the production of an immunoglobulin E (IgE)-mediated immune response toward allergens.

Epidemiology and Prevalence

Allergic rhinitis represents a global health concern, affecting approximately 400 million people worldwide. In the United States, approximately one-quarter of adults (25.2%) have a diagnosed seasonal allergy. Women are more likely to have a seasonal allergy (29.5%) compared with men (20.7%). The prevalence of seasonal allergies increases from 24.3% in adults ages 18–44 to 27.7% in those 45–64, and then declines to 25.5% in those 65–74 and 21.7% in those age 75 and older.

White non-Hispanic adults are more likely to have a seasonal allergy (28.5%) compared with Black non-Hispanic (24.4%), Asian non-Hispanic (17.2%), and Hispanic (16.5%) adults. The percentage of adults with a diagnosed seasonal allergy is higher among those living in nonmetropolitan areas than those in metropolitan areas.

AR is known to peak in the second to fourth decades of life and then gradually decline; the incidence in the pediatric population is also quite high, making it one of the most common chronic pediatric disorders. The prevalence of AR has increased over the years, with increased urbanization and environmental pollutants thought to be some of the leading causes of the disease.

Symptoms and Clinical Presentation

Allergic rhinitis is an atopic disease characterized by symptoms of nasal congestion, clear rhinorrhea, sneezing, postnasal drip, and nasal pruritis. Symptoms may include sneezing, coughing, a runny or stuffy nose, and itching in the eyes, nose, mouth, and throat. AR can lead to sleep disturbance, fatigue, depressed mood, and compromised cognitive function, which can impair the quality of life and productivity in many people.

SAR can result in hyperresponsiveness to allergens such as cigarette smoke, even once the pollen season is over. As a result of the development of mucosal inflammation, the nose becomes primed to allergen and reacts more vigorously to subsequent allergen exposure, but also becomes hyperresponsive to irritants and to changes in atmospheric conditions.

Body Systems Involved

The Immune System: IgE-Mediated Sensitization

Allergic rhinitis involves an early phase, largely mediated through mast cells, and a late phase which involves cellular infiltration and mediator release. In the early phase, mast cells release mediators as a result of antigen cross-linking adjacent immunoglobulin E molecules bound to mast cell surfaces. This results in an accumulation of histamine, which gives rise to the characteristic symptoms of rhinitis — sneezing, itching, rhinorrhea, and congestion.

The late phase of the allergic response, occurring hours after challenge, involves infiltration of the nasal epithelium by eosinophils, basophils, monocytes, and T-lymphocytes, which release leukotrienes, kinins, histamine, and a host of other mediators. The most important part of the late-phase response is probably mediated via the production of cytokines (IL-4, IL-5, IL-6, IL-8, GM-CSF, and RANTES) by mast cells, Th2 lymphocytes, or epithelial cells.

In sensitized subjects, allergen exposure activates immune cells, including Th2 lymphocytes, dendritic cells, mononuclear cells, mast cells, and others, both within the nose and in nasal-associated lymphatic tissues. Some of these Th2 cells migrate to the bone marrow, where they stimulate the bone marrow to produce and recruit inflammatory cells, including basophils, eosinophils, and mast cell precursors, to the inflamed target tissues.

The Nasal Mucosa and Upper Airway

The infiltration of tissues by cells normally present only in the blood is brought about by the production of adhesion molecules, such as VCAM-1 and E-selectin, which cause circulating eosinophils, basophils, and T-lymphocytes to adhere to endothelial cells before moving through the endothelium into the tissue (diapedesis).

The Lower Airway and Systemic Effects

In individuals with seasonal allergic rhinitis without co-existent asthma, nasal allergen provocation produces increased adhesion molecule expression, eosinophil infiltration in both the upper and lower airways, and increased bronchial hyper-reactivity — demonstrating that an allergic nasal reaction produces systemic inflammatory changes.

The Gut–Immune Axis

Based on evidence that more than 70% of the cells of the immune system are located in the gut, and that the microbiota plays a leading role in modulating the inflammatory process and the immune response, the gut microbiome has increasingly been considered relevant to allergic rhinitis pathophysiology. Probiotics are known to exert anti-allergic properties and influence respiratory allergies through the gut–lung axis by modulating intestinal dysbiosis, an event related to immune tolerance toward allergens.

Contributing and Associated Factors

Genetic Factors

There is clear evidence to support the concept that allergic diseases are influenced by genetic predisposition and environmental exposure. Polymorphisms of candidate genes have been associated with clinical expression of these diseases; however, characterization of these susceptibility markers in discriminating an "allergic individual" from the general population has not yet been achieved. There is a strong genetic component to the allergic response, which is driven through mucosal infiltration and action on plasma cells, mast cells, and eosinophils.

Environmental and Lifestyle Factors

The clinical expression of allergic disease has been reported in relation to factors such as changes in lifestyle, modification in diet, geographic variations, climate, socioeconomic conditions, family structure or history, infant feeding, excessive allergen exposure especially during early life, and cigarette smoking.

Many epidemiological studies have shown that environmental exposure to bacterial products (i.e., endotoxin) may have a crucial role in the development of tolerance to ubiquitous allergens found in natural environments, making the hygiene hypothesis particularly attractive, since it is closely linked to modernization of lifestyles and improvement in living conditions.

A number of epidemiological studies have supported the "hygiene hypothesis," which is based on the observations that Th1 responses induced by microbial stimulation can counterbalance allergen-induced Th2 responses.

Air Pollution

Epidemiological investigations have revealed that exposure to airborne fine particulate matter (PM₂.₅) is associated with heightened rates of clinic visits for allergic rhinitis; animal studies demonstrate that PM₂.₅ exposure exacerbates nasal allergy symptoms in sensitized murine models. PM₂.₅ has the capacity to induce cellular oxidative stress and inflammatory responses, particularly in inflamed nasal mucosal epithelial cells, and PM₂.₅-induced oxidative stress is postulated as a mechanism exacerbating allergic rhinitis symptoms.

Climate and Seasonal Patterns

The allergic response has been shown to be less intense in a hot, humid environment and more marked in a cold, dry environment, possibly due to changes in osmolality of the nasal surface fluid.

Associated Comorbidities

Under the unified airway theory, AR is classified as a component of systemic allergic response, with other associated conditions such as asthma and atopic dermatitis sharing an underlying systemic pathology. The severity of allergy symptoms can range from itchy, watery eyes and hives to life-threatening anaphylaxis, and the presence of allergies can negatively impact an individual's quality of life.

Nutrients Studied in Relation to Seasonal Allergies

Vitamin D

Scientific Evidence

Various studies have implicated the association between low serum vitamin D levels and allergic diseases. The active form of vitamin D3 has been shown to have immunoregulatory effects on both innate and adaptive immunity, which can significantly affect the outcome of allergic responses in allergic rhinitis. The active form 1,25(OH)₂D3 inhibits the proliferation and induces apoptosis of activated B cells, and inhibits plasma cell differentiation and immunoglobulin secretion, including IgE secretion.

Multiple studies have shown that vitamin D deficiency is correlated with allergic rhinitis classification, and a significant proportion of allergic rhinitis patients show severe vitamin D deficiency; several researchers have concluded that supplementation of vitamin D in allergic rhinitis patients alters the natural course of AR toward significant clinical improvement.

Although the results are inconsistent, several studies have suggested a slight tendency for serum vitamin D levels to be inversely associated with the risk of allergic rhinitis; however, at least one recent study has reported no significant association between vitamin D levels and allergic rhinitis, indicating the need for further investigation. Various therapeutic studies on allergic diseases including allergic rhinitis have demonstrated that the use of vitamin D supplements can decrease the severity of allergic diseases, but results are still controversial. Overall, the evidence is preliminary and based on observational and small interventional studies; large, well-controlled randomized trials are needed before firm conclusions can be drawn.

Quercetin

Scientific Evidence

Quercetin inhibits mast cell degranulation, reduces the production of histamine and pro-inflammatory cytokines, and restores homeostasis of the immune system by modulating the Th1/Th2 and Treg/Th17 balances; its antioxidant properties also help to dampen oxidative stress, a critical factor in the pathophysiology of allergic diseases.

In vitro studies have consistently demonstrated quercetin's ability to suppress allergic reactions; in vivo studies, particularly in murine models of allergic rhinitis, have confirmed its efficacy in relieving symptoms such as nasal itching, sneezing, rhinorrhea, and congestion, and dampening type 2 mucosal inflammation.

However, the human evidence base is narrow. Human studies are still scarce, as only two clinical trials have investigated quercetin as a monotherapy; both reported promising results, including symptom reduction and improved quality of life, though larger, randomized trials are needed to validate these findings. A 2025 systematic review (PRISMA 2020 guidelines, covering PubMed and related databases, 2000–2024) identified only 18 studies meeting inclusion criteria, comprising 14 animal studies and just four human clinical trials. Preclinical evidence consistently demonstrated quercetin's ability to reduce oxidative stress markers, and human studies showed superior symptom improvement when quercetin-containing supplements were added to standard therapy compared with conventional treatment alone. While most evidence derives from animal studies, quercetin shows promise as a safe adjuvant therapy; large-scale human clinical trials using high-bioavailability formulations are needed to establish standardized clinical protocols. Evidence strength is therefore rated as preliminary to moderate for human use.

Spirulina (Arthrospira platensis)

Scientific Evidence

Spirulina has the ability to modulate immune functions and exhibits anti-inflammatory properties by inhibiting the release of histamine by mast cells; a few randomized controlled trials and systematic reviews suggest that this alga may improve several symptoms and may have antiallergic effects.

Spirulina is a blue-green alga produced and commercialized as a dietary supplement for modulating immune functions; a double-blind, placebo-controlled study evaluated the effectiveness and tolerability of spirulina for treating patients with allergic rhinitis and found that spirulina consumption significantly improved symptoms and physical findings compared with placebo (P < 0.001), including nasal discharge, sneezing, nasal congestion, and itching.

A review of two clinical trials totaling 215 patients found that the first study described a significant reduction in runny nose, nasal congestion, and itching (p < 0.001), and in the second study, the prevalence of rhinorrhea, nasal congestion, and decreased smell were significantly less in the experimental group; however, the level of evidence is very low and limited, requiring caution due to the small number of clinical trials and participants. Evidence strength is preliminary; larger trials with standardized methodology are needed.

Herbs and Botanicals Studied in Relation to Seasonal Allergies

Butterbur (Petasites hybridus)

Traditional Use

Butterbur has been used for health purposes for many centuries; in the Middle Ages, the odor and smoke from burning butterbur roots were thought to fight plague. Historically, it was also used in European herbal traditions for respiratory conditions and as an antispasmodic.

Scientific Evidence

Several laboratories have characterized petasins (petasin, isopetasin, and neopetasin) isolated from extracts of butterbur as pharmacologically active components that inhibit leukotriene synthesis in leukocytes; in vitro studies have revealed that petasins may have several intracellular targets. In an open clinical trial in patients suffering from allergic rhinitis, a reduction of leukotriene and histamine levels in nasal fluids was associated with butterbur extract administration.

A key randomized, double-blind, parallel-group trial compared butterbur (CO₂ extract tablets ZE 339) with cetirizine in 125 patients with seasonal allergic rhinitis. Improvement in SF-36 score was similar in the two treatment groups for all items tested hierarchically; butterbur and cetirizine were also similarly effective with regard to global improvement scores on the clinical global impression scale. A post-marketing surveillance study enrolling 580 patients found that symptoms of seasonal allergic rhinitis improved in 90% of patients; differences observed before and after therapy were significant and clinically relevant for all symptoms, and efficacy, tolerability, and improvement in quality of life were positively rated by 80%, 92%, and 80% of patients, respectively.

At the systematic review level, findings from a 2007 systematic review of six randomized controlled trials suggest that butterbur is superior to placebo or similarly effective compared with non-sedative antihistamines for intermittent allergic rhinitis; however, firm conclusions could not be drawn because three of the large trials reviewed received financial support from a manufacturer of butterbur, representing a conflict of interest. The NCCIH notes that studies of a butterbur leaf extract suggest that it may be helpful for symptoms of allergic rhinitis when taken orally. However, the American Academy of Neurology stopped recommending it in 2015 because of serious concerns about its safety. Evidence strength: moderate but with notable safety concerns and conflict-of-interest limitations.

Stinging Nettle (Urtica dioica)

Traditional Use

Urtica dioica (stinging nettle) has a long history of use in European and Middle Eastern herbal medicine for inflammatory conditions of the respiratory tract and musculoskeletal system. Leaves were historically prepared as infusions (teas) or freeze-dried extracts for inflammatory and allergic complaints.

Scientific Evidence

A randomized, double-blind, placebo-controlled clinical trial enrolled 74 patients with confirmed allergic rhinitis and a positive skin prick test, assigning them to receive Urtica dioica root extract (150 mg) or placebo for one month. Based on the Sino-Nasal Outcome Test (SNOT-22), a significant improvement in clinical symptom severity was observed in both groups (P < .001); a statistically significant reduction in mean nasal smear eosinophil count was observed after treatment with nettle (P < .01); however, mean IgE and IL-4 and IL-5 levels in the study group before and after treatment with nettle showed no significant changes. Critically, the current study showed certain positive effects of nettle in managing allergic rhinitis on controlling symptoms based on the SNOT-22, but similar effects were demonstrated by placebo as well, underscoring the need for larger, longer-term studies. An earlier trial used freeze-dried nettle leaf (600 mg); Mittman et al. reported that while the freeze-dried extract of nettle leaves reduced allergy symptoms based on global assessment at the end of the double-blind clinical trial, only a small difference was observed between the herb and placebo on daily response diaries, leaving insufficient proof to support or refute its use. Evidence strength: weak/inconclusive in human trials to date.

Probiotics and the Gut Microbiome

Scientific Evidence

Probiotics are live microorganisms that help the normal state of the intestine, and if prescribed correctly, they can stimulate the mucosal immune system to prevent inflammatory symptoms of allergy and atopy. The clinical benefit of probiotic therapy depends on numerous factors, such as the type of bacterium, route of administration, dosing, regimen, and other underlying host factors; selection of the most beneficial probiotic strain and the timing of supplementation still need to be determined.

A 2016 systematic review and meta-analysis identified 23 studies with 1,919 patients, including 21 double-blind randomized controlled trials and 2 randomized crossover studies. Multiple probiotic strains, study populations, and outcome measures were utilized in individual trials; seventeen studies showed a significant clinical benefit from the use of probiotics in at least one outcome measure. However, evidence for their use has yet to be firmly established.

An exploratory randomized controlled trial found that a multispecies probiotic formulation (including L. acidophilus, L. rhamnosus, B. breve, and B. longum) elicited anti-allergic and anti-inflammatory properties that were beneficial in improving symptoms and quality of life in AR patients; further research on larger sample sizes was recommended to enhance the reliability and validity of the findings. Conversely, a smaller randomized clinical trial (28 participants) found that the use of probiotics had no significant effect on the outcome of patients with AR when added as adjuvant therapy. Evidence strength: mixed; promising but inconsistent across strains and populations.

Dietary Factors Discussed in the Literature

The Mediterranean Diet Pattern

The Mediterranean diet (MD), which includes a lot of fruits and vegetables, whole grains, legumes, nuts, olive oil, and fish, has been linked to a variety of health benefits, including a lower risk of chronic and allergic disease. A cross-sectional study in 690 children aged 7–18 years in rural Crete found that a high level of adherence to the Mediterranean diet was protective for allergic rhinitis (OR 0.34; 95% CI 0.18 to 0.64), while more modest protection was observed for wheezing and atopy. A high consumption of nuts was found to be inversely associated with wheezing (OR 0.46), whereas margarine increased the risk of both wheeze (OR 2.19) and allergic rhinitis (OR 2.10).

A cross-sectional study of 1,476 Mexican children (ages 6–7) likewise found that adherence to a Mediterranean dietary pattern was inversely associated with rhinitis ever (OR = 0.41, 95% CI = 0.22–0.77) and current sneezing (OR 0.71). However, the evidence is not unequivocal: two systematic reviews found that children's adherence to the Mediterranean diet appears to have a protective effect on asthma but not necessarily on allergic rhinitis, eczema, or atopy symptoms specifically, and both highlight the need for further research due to remarkable heterogeneity among studies. Studies on the association between the Mediterranean diet and rhinitis in school-aged children have not obtained conclusive results, with some populations showing the protective association and others not.

Antioxidant Nutrients: Vitamins A, C, E, and Zinc

Two systematic reviews and meta-analyses confirmed a weakly positive effect of vitamins A, D, C, E, zinc, fruits, vegetables, and the Mediterranean diet on asthma outcomes; however, the evidence for a particular role of antioxidants in asthma and other allergic diseases remains inconclusive, and the association between diet and asthma is still far from clear.

Dietary Fat Composition and Omega-6/Omega-3 Balance

A hypothesis advanced in the scientific literature proposes that in developed countries in the second half of the 20th century, a fall in consumption of saturated fat and increase in consumption of margarine and vegetable oils rich in n-6 polyunsaturated fat, particularly linoleic acid, may contribute to the rise in allergic disease. This "lipid hypothesis" connects rising dietary n-6:n-3 polyunsaturated fatty acid ratios with pro-inflammatory immune skewing, and supports dietary investigation of omega-3-rich foods (oily fish, flaxseed) in atopic conditions — though clinical trial evidence specifically for seasonal allergic rhinitis remains limited.

Dairy and Other Dietary Triggers

Dietary history should be evaluated, as nutritional factors such as dairy intake may contribute to the development or exacerbation of allergies. Mediterranean-style diets may be protective due to higher intake of antioxidant-rich vegetables, fruits, legumes, and whole grains, and limited meat and refined grain intake.

Breastfeeding and Early Nutrition

Limited evidence suggests that longer duration of breastfeeding and avoidance of early introduction of potentially allergenic foods may reduce the likelihood of allergic sensitization.

Lifestyle Factors Discussed in the Literature

Saline Nasal Irrigation

Nasal irrigation is an effective, safe, low-cost strategy for treating and preventing upper respiratory tract diseases; high-volume, low-pressure saline irrigations are the most efficient method for removing infectious agents, allergens, and inflammatory mediators. Nasal irrigation improves symptoms, reduces recurrence, enhances the efficacy of topical drugs, and decreases the need for antibiotics and decongestants. This is among the most consistently supported non-pharmacological physical interventions for allergic rhinitis.

Sun Exposure and Vitamin D Status

Vitamin D deficiency is widespread, partly due to westernization and sedentary lifestyle leading to less sun exposure and less cutaneous vitamin D production. Epidemiological evidence has associated lower sun exposure with both vitamin D deficiency and higher rates of atopic disease, though the causal direction remains under investigation.

Urbanization and Hygiene

An increasing worldwide prevalence of allergic diseases such as allergic rhinitis and asthma has been observed over the last decades; the reasons for this increase are still incompletely understood. The hygiene hypothesis, and its later extension to include gut flora diversity, remains a central framework for understanding why urbanized populations have higher rates of allergic sensitization than those in rural or farming environments.

Allergen Avoidance and Environmental Control

Environmental control measures — including monitoring pollen counts, keeping windows closed during peak pollen seasons, showering after outdoor activity, and using air filtration — are widely recommended in the AR literature as adjuncts to any nutritional or pharmacological approach. These reduce the total allergen burden to which the mucosal immune system is exposed.

Summary of Evidence Quality

• Butterbur (Petasites hybridus): The most clinically tested botanical for seasonal AR; a 2007 systematic review of six RCTs suggests benefit comparable to non-sedating antihistamines, but conflicts of interest and safety concerns limit conclusions.
• Probiotics: A large body of RCT evidence (23 trials, 1,919 patients) shows benefit in a majority of trials, but results are inconsistent across strains, populations, and outcome measures; evidence is promising but not definitive.
• Vitamin D: Observational and small interventional studies associate deficiency with worse AR and suggest supplementation may improve symptom scores, particularly in deficient individuals; evidence is preliminary and results are mixed.
• Quercetin: Strongly supported mechanistically and in preclinical (animal/in vitro) studies; only two monotherapy human clinical trials exist as of 2025; evidence is preliminary.
• Spirulina: Two small RCTs showed significant symptom improvement versus placebo; the overall level of evidence is rated very low due to small sample sizes and limited replication.
• Stinging Nettle (Urtica dioica): One RCT showed eosinophil reduction but no significant change over placebo in symptom scores; evidence is weak and inconclusive.
• Mediterranean dietary pattern: Epidemiological evidence from multiple countries suggests a protective association, but it is not conclusive for rhinitis specifically, and causation is not established.
• Saline nasal irrigation: Well-supported as a safe adjunctive physical intervention with consistent evidence for symptom relief and allergen removal.

References

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