PROJECT SUMMARY / ABSTRACT Anaphylaxis is a severe allergic reaction that can be rapidly progressing and fatal. In instances where the triggering allergen is not known, establishing the etiology of anaphylaxis is pivotal to long-term risk management. Our recent work has identified a novel IgE antibody response to a mammalian oligosaccharide epitope, galactose-alpha-1,3-galactose (alpha-gal), that has been associated with two distinct forms of anaphylaxis: i) immediate onset anaphylaxis during first exposure to intravenous cetuximab, and ii) delayed onset anaphylaxis 3-6 hours after ingestion of mammalian food products (e.g., beef and pork). The overarching goal for this renewal is to define whether the risk for development of an alpha-gal IgE response is intrinsic to the host (genetics) or external with a specific factor in ticks. These studies will provide insight into the factors that govern allergic responses and control IgE production more generally. The significance of investigating these reactions comes not only from the obvious importance of understanding a novel life-threatening form of food allergy, but also because of the possibility of defining a totally new mechanism for reactions related to an important food substance. Our plan of research focuses on using a cutting- edge, combined transcriptomic (single cell RNA sequencing) and surface marker expression approach to deeply immunophenotype the cells in humans with alpha-gal allergy. This approach will allow us to define the factors underpinning how tick bites transcriptionally initiate the IgE response. We believe that IgE to alpha-gal represents a novel cause of food allergy and overall, these studies are uniquely positioned to provide additional insight into a recently recognized allergic response that affects >40,000 patients in the U.S. alone and position the research to make important advances for prevention and treatment of alpha-gal syndrome (AGS).