Wireless Body Area Networks (WBANs) are revolutionizing healthcare by enabling continuous, real-time monitoring of physiological signals through wearable or implantable sensors. However, the sensitive nature of medical data, resource constraints of sensor nodes, and the open wireless medium make secure data transmission a paramount challenge. This paper provides a comprehensive review of secure data transmission protocols in WBANs, analyzing their architecture, security requirements, vulnerabilities, and recent advances. We examine lightweight cryptographic schemes, mutual authentication protocols, secure routing mechanisms, and privacy-preserving techniques, highlighting their strengths, limitations, and future research directions.