NRN1 (neuritin 1) is a neurotrophic factor that functions beyond its classical role in neuronal development. While best characterized for promoting neurite outgrowth and branching in hippocampal and cortical neurons, NRN1 has emerged as a multifunctional immune and neural protein with significant disease relevance. In the immune system, NRN1 is produced by follicular regulatory T cells and acts as a critical suppressor of B cell-mediated autoimmunity 1. It is internalized by B cells, where it suppresses plasma cell differentiation, reduces IgE class switching, and downregulates BLIMP-1 while upregulating BCL6 1. This immunoregulatory function explains how NRN1 prevents autoantibodies and allergic responses. In neuropsychiatric contexts, NRN1 variants associate with schizophrenia risk, age at onset, and cognitive performance 23. Specific haplotypes correlate with early-onset schizophrenia and inefficient dorsolateral prefrontal cortex activity during working memory tasks 2. NRN1 also shows epistatic interactions with BDNF in schizophrenia and bipolar disorder susceptibility 3. Emerging clinical applications include NRN1 as a circulating biomarker: peripheral blood NRN1 expression is elevated in gastric cancer patients and decreases post-resection, suggesting utility for cancer detection and monitoring 4. Additionally, NRN1 appears dysregulated in Alzheimer's disease proteomes 5. These findings position NRN1 as a pleiotropic protein linking immune regulation, neural function, and disease pathology.