In patients with a walnut allergy double blind placebo controlled food challenge with walnut will be combined with the intake of proton pump inhibitor (PPI) or with placebo to assess the impact of PPI on threshold level and on clinical manifestation.

1. detailed case history, blood sampling and prick testing with different foods will performed.
2. double blind food provocation with with three meals containing either placebo or walnut in walnut allergic patients will be performed.
3. the provocation will be combined with the intake of drug (omeprazole) or placebo (mannitol) blinded in capsules.

Inclusion Criteria:

• signed informed consent, history of walnut allergy,minimum age 18 years

Exclusion Criteria:

• • Known hypersensitivity or allergy to Omeprazole, other proton pump inhibitors, mannitol, ingredients of placebo meal

  • Drugs not allowed during the study due to proved interaction with Omeprazole: Digoxin, Clopidogrel, Methotrexate, Antiretroviral drugs, Diazepam, Cilostazol, Phenytoin, Warfarin (R-Warfarin) and other Vitamin K antagonists, Phenytoin, Tacrolimus, Clarithromycin, voriconazole, Rifampicin and Hypericum
  • Drugs not allowed due to interference with the food challenge (or shortest interval between last treatment and food challenge)

• corticosteroids systemically (2 weeks)

• antihistamines (3 days) except hydroxyzine (10 days)

• ketotifen (2 weeks)

• betablocker (1 day)

• angiotensin converting enzyme (ACE) inhibitors (2 days)

• omalizumab (2 months)

  • Women who are pregnant or breast feeding
  • Intention to become pregnant during the course of the study
  • Lack of safe contraception, defined as: female participants of childbearing potential, not using and not willing to continue using a medically reliable method of contraception for the entire study duration, such as oral, injectable, or implantable contraceptives, or intrauterine contraceptive devices
  • Please note that female participants who are surgically sterilised / hysterectomised or post-menopausal for longer than 2 years are not considered as being of child bearing potential.
  • Uncontrolled asthma, forced expiratory volume <70% predicted value
  • Acute allergic disease
  • Chronic urticaria
  • Mastocytosis
  • Other clinically significant concomitant disease states (major organic or infectious diseases, e.g., renal failure, hepatic dysfunction, cardiovascular disease, acute febrile infection),
  • Known or suspected non-compliance, drug or alcohol abuse,
  • Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc. of the participant,
  • Participation in another study with investigational drug within the 30 days preceding and during the present study