Narcolepsy with Cataplexy (N/C) is a common sleep disorder, but very little is understood about the underlying mechanism that causes N/C and why symptoms are severe in some patients but not in others. N/C is caused by selective loss of the hypothalamic neurons that produce the orexin neuropeptides and is strongly associated with the Major Histocompatibility Complex (MHC) class II allele DQB1 *0602. We hypothesize that N/C is caused by CD4+ or CD8+ T cells response directed against the hypothalamic neurons producing the orexin neuropeptides, and that a more aggressive immune response results in increased severity of N/C. This hypothesis is supported by a number of publications suggesting a causal role of the cellular immune response in N/C as well as our own preliminary data. Our long term goal is to understand the cause of N/C and the determinants of disease severity. In doing so, we will also devise a much needed blood test for the early diagnosis of NC and prediction of disease severity. Our objectives are to determine whether N/C patients mount a cellular immune response to orexin peptides and whether this response is associated with more severe symptoms. The rationale is that these proposed studies will enable us to develop a blood test for early diagnosis of N/C and predictor of disease severity. To test these hypotheses, we will pursue the following set of Specific Aims: Aim 1) Correlate disease severity with the cellular immune response to orexin neuropeptides in DQB1 *0602+ Narcolepsy with Cataplexy patients. We will measure narcolepsy symptoms severity using well- validated scales and correlate symptom severity with the reactivity of CD4+ or CD8+ T cells to orexin peptides using three different assays: 1) Intracellular Cytokine Staining (ICS) assay 2) Luminex xMAP bead array on cell culture supernatant and 3) RT-qPCR for cytokine mRNA on cellular RNA . Aim 2) Identify immunodominant orexin epitopes recognized by CD4+ or CD8+ T cells in DQB1 *0602+ Narcolepsy with Cataplexy patients. We will map orexin epitopes in N/C patients using ICS and devise a diagnostic blood test and predictor of disease severity for N/C. The approach is innovative, because it departs significantly from the status quo by focusing on the cellular mechanisms leading to N/C. The proposed research is significant because it will allow us to identify the immunopathogenic mechanisms leading to N/C, develop a blood test for early diagnosis of N/C and predict disease severity. The knowledge gained from these studies will also have far reaching implications in advancing the fields of Sleep Disorders and Neuro-Immunology.