Immunologic Basis of Food Protein-Induced Enterocolitis Syndrome

Inclusion Criteria: * Provision of appropriate consent and/or assent * Age 1-60 years * Suspected or confirmed FPIES diagnosis * Reported convincing FPIES reaction (per criteria in 2017 FPIES guidelines) within: (Children \<18 years of age: The past 6-36 months) (Adults age \>18 years: The past 6 months-10 years) * Individuals of childbearing potential practicing sexual abstinence or using effective methods of contraception during study participation * English-speaking Exclusion Criteria: * Past severe FPIES defined as hospitalization due to an acute FPIES reaction with neurological compromise or requiring life support * Acute FPIES reaction in the past 6 months * Frequent gastrointestinal symptoms: nausea, abdominal pain, reflux, heartburn, emesis, diarrhea, constipation per participant or guardian report or as evidenced by FPIES Symptoms Score (FPIES-SS) * Current active eosinophilic gastrointestinal disorders, inflammatory bowel disease, gastroesophageal reflux disease, or any other chronic gastrointestinal condition * Poorly controlled atopic dermatitis at screening per PI discretion * Poorly controlled or severe asthma/wheezing at screening, defined by at least one of the following criteria: 1: History of two or more systemic corticosteroid courses within six months of screening or one course of systemic corticosteroids within three months of screening to treat asthma/wheezing; 2: Prior intubation/mechanical ventilation for asthma/wheezing; 3: One hospitalization or ED visit for asthma/wheezing within six months of screening; and 4: Inhaled corticosteroid (ICS) dosing of \>500 mcg daily fluticasone (or equivalent ICS based on National Heart, Lung, and Blood Institute (NHLBI) dosing chart). * IgE-mediated food allergies where the trigger has not been identified * Inability to discontinue prohibited medications for 7 days prior to the screening visit and lasting for the duration of study participation unless indicated for use as rescue medication * Personal or family history of prolonged QT syndrome * Personal history of arrhythmia * Current diagnosis of arterial hypertension * Current diagnosis of cardiovascular disease * Current diagnosis of any chronic autoimmune disease * Current diagnosis of liver disease * Primary or secondary immunodeficiency * Phenylketonuria (PKU) (ondansetron tablets may contain phenylalanine) * Use of systemic steroids within 30 days of screening * Chemotherapy, radiotherapy, or any systemic immunosuppressive drugs in the past 12 months * Use of biologic drugs or allergen-specific immunotherapy by any route in the past 12 months * Inability to defer routine immunizations or passive immunization with immune globulin for the duration of participation in the study * Allergy to any of the following medications: ondansetron (Zofran), dolasetron (Azemet), granisetron (Kytril), or palonosetron (Aloxi) * Pregnancy or breastfeeding * Current or past medical problems or findings from the physical examination or screening evaluation not listed above, which, in the opinion of the investigator(s), may pose additional risks from study participation, interfere with the participant's ability to comply with study requirements, and/or impact the quality or interpretation of the data obtained from the study