Heart transplantation remains the definitive treatment for end-stage heart failure unresponsive to medical therapy. This case report highlights the clinical course of a 34-year-old male with genetic dilated cardiomyopathy who presented with acute decompensated heart failure and eventually underwent successful orthotopic heart transplantation after deterioration on mechanical circulatory support. Post-transplant, the patient developed ISHLT Grade 2R acute cellular rejection, which was managed successfully with high-dose corticosteroids and optimization of immunosuppressive therapy. The pathophysiology of acute cellular rejection involves T-cell-mediated cytotoxicity via both direct and indirect pathways, leading to lymphocytic infiltration and myocyte necrosis. Diagnostic modalities such as endomyocardial biopsy remain the gold standard, although emerging biomarkers—like AlloMap gene expression profiling, donor-derived cell-free DNA (dd-cfDNA), and gene expression microarrays (MMDx)—offer promising non-invasive surveillance options. This report also outlines key elements of pre-transplant immunological assessment, emphasizing the role of HLA antibody detection, crossmatch techniques, and panel reactive antibody (PRA) scores in determining compatibility. Desensitization strategies—including therapeutic plasma exchange, IVIG, Rituximab, and proteasome inhibitors like Bortezomib—are discussed for sensitized recipients. Additionally, novel agents such as Eculizumab have shown promise in preventing antibody-mediated rejection in highly sensitized individuals. Comprehensive immunological assessment, early detection, and individualized treatment of rejection are critical for long-term graft survival and patient outcomes. This case underscores the complexity of post-transplant immunological management and the evolving landscape of biomarker-guided rejection surveillance.