Reliability stands as the first important factor when dealing with medical data within the context of a wireless body area network (WBAN). The sensor nodes on body send their data to the coordinator based on a beacon-enabled access mode defined in IEEE 802.15.4 medium access control (MAC) specifications. This paper studies an effective backoff resolution into the carrier sense multiple access with collision avoidance (CSMA/CA) procedure of the beacon-enabled access mode of IEEE 802.15.4. Whilst the standard introduces the backoff as a resolution to less probability of identical backoffs, it does not address the efficiency of the generated backoff time in non-identical backoff situations. This phenomenon degrades the reliability of the received data at the coordinator device. A dynamic and reliable MAC algorithm for WBAN is presented in which length of the backoff period for each node gets decided based on its past successful trials in accessing the channel and also its data rate. This encourages a fair access to the medium among all the sensor nodes as moderate backoff values are assigned to each node. The primary contributions in this paper are less delay endured and higher data reliability while making no changes to the level of energy efficiency.