Heart failure (HF) is a highly lethal condition, and heart transplantation represents the main treatment for end-stage HF. In this context, the treatment of acute rejection remains a significant challenge. Acute allograft rejection includes Acute cellular rejection (ACR), mediated by cytotoxic T-cell response, and Antibody-mediated rejection (AMR), which causes activation of the complement cascade, resulting in myocardial injury and microvascular damage. Endomyocardial biopsy (EMB) remains the gold standard for diagnosing AMR, based on criteria from the International Society for Heart and Lung Transplantation (ISHLT). Even though included in the essential diagnosis criteria, immunohistochemical positivity for C4d may lack specificity, being correlated to other nonimmunologic conditions, and, on the other hand, AMR can occur without complement activation (C4d-negative AMR). Therefore, AMR continues to pose significant diagnostic challenges, highlighting the need for more specific, less invasive biomarkers to improve its diagnosis and treatment.