Inclusion Criteria:
* Written informed consent from adult participants or parent/guardian for children. Written assent from participants above the age of 12 years.
* Male or female participants aged 5 through 40 years, in a good general health condition according to the participant's medical history and the clinical examination and no chronic diseases, except:
* Allergic rhinitis
* Asthma controlled with therapy steps 1-3 according to Global Initiative for Asthma (GINA) 2017.
* Well-controlled atopic dermatitis.
* SPT to tree nut's extract and/or the natural tree nut (Prick-to-Prick) ≥ 3 mm (the largest diameter of the wheal) and serum IgE to tree nut of ≥ 0.35 kUA/L.
* sIgE \> 0.1 kUA/L to at least one seed storage protein of the offending tree nut, except for pistachio, where Ana o 3 will serve as a sensitization marker.
* Positive baseline OFC, at any dose but the final, conducted following PRACTALL (Practical Issues in Allergology, Joint United States/European Union Initiative) / iFAAM (Integrated Approaches to Food Allergen and Allergy Risk Management) guidelines.
* Exclusion of active co-allergy to walnut, almond or pistachio, unless they are introduced to the diet simultaneously, by a negative open OFC or a recent (within the last two months) history of uneventful digestion, except for pistachio introduction in participants who are co-allergic to cashew.
* Forced Expiratory Volume at 1 second (FEV1) or Peak Expiratory Flow Rate (PEFR) ≥ 80% of predictive.
* The participant and/or the legal custodian should agree to comply with the study procedures.
Exclusion Criteria:
* History of systematic anaphylaxis GRADE 5 according to Sampson (loss of bowel control, respiratory arrest, severe bradycardia and/or hypotension or cardiac arrest, loss of consciousness) or/and during OFC.
* Recent (within 12 months) initiation of immunotherapy to any allergen.
* Past or present eosinophilic esophagitis (EoE).
* Chronic urticaria.
* Severe or uncontrolled atopic dermatitis.
* Uncontrolled asthma of any severity or asthma controlled with therapy steps 4-5 according to GINA 2017.
* Concurrent or within 6 months therapy with omalizumab or another monoclonal antibody.
* Systematically used corticosteroids:
* Daily for more than a month during the last year,
* Courses of corticosteroids during the last 3 months.
* \>2 Courses of corticosteroids lasting ≥1 week during the last year.
* Inability to discontinue antihistamines 4 days before skin prick testing or OFC.
* Severe immune disease, severe cardiovascular disease, malignancy, chronic infection, poor compliance, and severe psychiatric disorders.
* Any important clinical condition that can affect the safety of the participant or the results of the study, including but not limited to cardiovascular diseases, malignancies, diseases of the liver and/or the kidneys, hematologic diseases, physiologic, immunologic and endocrinologic diseases.
* Use of beta-blockers, angiotensin-converting enzyme (ACE) inhibitors or angiotensin-receptor blockers (ARB).
* Pregnancy or lactation.
* History of alcohol or drug abuse.
