Single-center, cross-sectional, longitudinal, observational, population based cohort study with stratified sample (by age group, gender, and residential area).

This study will involve schoolchildren (aged 6-18 years), population cohort based on differential exposure and susceptibility to allergy(paediatric population, allergic vs. healthy subjects). participants will be selected based on proven sensitization to at least one food allergen (preferentially but not only to seafood) with a diagnosis of at least one allergic disease (i.e. allergic asthma, allergic rhinitis (AR), atopic dermatitis (AD) and/or food allergy) for at least a year. Subjects will be recruited from three distinct and different geographical regions in Croatia: 2 continental regions (Zagreb, urban) and Slavonia (urban and rural) and 1 Mediterranean region (urban and rural population), differing in the levels of exposure (allergens, pollutants), consumption of water (main water source- bottled water, public water source, well water), food consumption (food packed in plastic material, use of fertilizers in agriculture), nutrition (food from marine environments, industrially processed food, homegrown food) as well as different environmental protection measures and human activities.In each region and subgroup, we will recruit 70 allergic children and 140 healthy (non-allergic) children in a cross-sectionalstudy protocol, summing up to al total of 630 participants.

The study will include visits:

1. Initial (screening) visit (V0 +3-6 months): written consent, assessment of inclusion and exclusion criteria, spirometry, Fractional exhaled nitric oxide (FeNO), questionnaires…
2. V2 (V0+6-9 months), questionnaires, environmental skin prick test (APT), sIgE (+ISAC), spirometry, FeNO, blood collection stool,
3. V2-V5 (V0+months 6-30 months)- regular visits (assessment) every 6 months, stool samples (second sampling)
4. Visit 4 (V0+18-24 months): environmental assessments, skin prick test (APT), sIgE (+ISAC), spirometry, FeNO, blood collection
5. V5 (last visit, V0+24-30 months)- final assessment of the study outcome measures, check and document consumption, adherence, preventive measures, level of control, therapy correction

Participants in the control group will be involved in collaboration with other outpatient clinics (eg. from surgical outpatient clinics who have had minor surgical procedures, from the Otorhinolaryngology- Ear, Nose, Throat (ENT) outpatient clinics at least 7 days after acute ear inflammation or laryngitis, from emergency ambulatory patients at least 7 days after acute respiratory illness, etc.). All participants who will be included in the control group should have a negative history of allergic diseases, as well as other severe chronic diseases, including malignant, autoimmune, or mental illnesses. They will need to fulfil the ISAAC questionnaire for children perform spirometry and prick test with a standard palette of inhalational and nutritional allergens to exclude allergic diseases.

No intervention tests or methods will be involved in this study. Patients who will be involved in the study will continue to take their regular therapy (according to guidelines) i.e. conduct treatment and all other non-therapeutic measures as if they were not involved in the study. Any deterioration or emergency intervention will be specifically evaluated and recorded as additional visits.

Inclusion Criteria:

• Proven sensitization to at least one food allergen with a diagnosis of at least one allergic disease for at least one year
• Clinical diagnosis of allergic rhinitis (AR) for at least 1 year with symptoms of the nose, induced after exposure to allergens via IgE-mediated hypersensitivity reactions (symptoms of watery rhinorrhea, nasal obstruction, nasal itching and sneezing).
• Diagnosis of atopic dermatitis (AD) with AD symptoms lasting for more than 3 months, with or without food allergy

Additional inclusion criteria:

• Significant clinical allergy to indoor and outdoor allergens, an food allergens with positive skin prick test (SPT) and specific IgE levels (>0.35kUA/L)

Exclusion Criteria:

• Known inborn or perinatal pulmonary disease;
• Pulmonary malformation
• Oxygen therapy after birth with a duration of more than 24 h
• Ventilator support or mechanical ventilation after birth
• Diagnosis of cystic fibrosis
• Primary ciliary dyskinesia
• Heart failure diagnosed after birth affecting pulmonary circulation
• Major respiratory diseases such as e.g. interstitial lung disease and other congenital and serious chronic disease
• Fever of at least 38.5 °C during the last two weeks prior to the planned visit

Participants in the control group will be involved in collaboration with other outpatient clinics (eg. from surgical outpatient clinics who have had minor surgical procedures, from the ENT outpatient clinics at least 7 days after acute ear inflammation or laryngitis, from emergency ambulatory patients at least 7 days after acute respiratory illness, etc.). All participants who will be included in the control group should have a negative history of allergic diseases, as well as other severe chronic diseases, including malignant, autoimmune, or mental illnesses.