In a communication system using aggregation of component carriers, a transmitter comprises a transmitting entity configured to transmit data messages on multiple component carriers. The transmission of the data messages is accomplished on the basis of a retransmission protocol. The retransmission protocol requires sending of an acknowledgement message in response to verifying correct receipt of a data message. A receiver comprises a receiving entity configured to receive the data messages on the basis of the retransmission protocol. In response to a decision to deactivate transmission on one of the aggregated component carriers, the retransmission protocol is triggered to retransmit those data messages for which no acknowledgement message of the retransmission protocol has been received. The retransmission is accomplished on at least one other of the component carriers. A further retransmission protocol may be used on each of the component carriers, with respective transmitting entities in the transmitter and respective receiving entities in the receiver.