PROJECT SUMMARY Anaphylaxis is a systemic, potentially fatal, immediate hypersensitivity reaction (IHR) that can be triggered by foods, drugs, vaccines, singing insect venom, exercise, or even idiopathically. Accurate diagnosis is essential in both the acute setting, when medical intervention can be life-saving, and during subsequent outpatient evaluation, when management depends on confirming the diagnosis and identifying the triggering substance. The diagnosis of anaphylaxis is based on symptoms, which is problematic due to the wide variability of clinical presentations and substantial overlap of allergic symptoms with other disease states. These diagnostic complexities and lack of reliable biomarkers for IHRs contribute to the misdiagnosis and subsequent mismanagement of food and drug allergies. Current known biomarkers for systemic allergic reactions are lacking in that none are both sensitive and specific. Therefore, there is a sizable unmet need to discover novel biomarkers of IHRs that are both highly-sensitive and specific for all contexts, mechanisms, and severities of anaphylactic reactions. This proposal’s central objective is to characterize and quantify circulating extracellular vesicles (EVs) as novel biomarkers of acute IHRs using patient samples obtained from ongoing clinical trials. Aim 1 will characterize the quantities, surface marker expression, and contents of plasma cell-specific EVs in adults after acute IgE-mediated IHRs to foods, and compare findings to patients’ baseline. Aim 2 will similarly characterize EVs in adults after acute non-IgE-mediated IHRs to drugs. Aim 3 will create a biobank of patient samples for analysis of alternative biomarkers using cutting edge technologies. This highly feasible and valuable clinical research project is expected to identify novel biomarkers that will aid in the accurate diagnosis of anaphylaxis as well as provide new insight into the mechanisms of certain drug IHRs. This application is in response to the program announcement for Exploratory and Developmental Research Grant Program for NIAID K-award Recipients. This award will provide crucial support and resources for the PI as she transitions to research independence.