Mammalian and avian cell lines are increasingly used in recombinant protein production because of their capacity to produce physiologically active proteins with precise posttranslational modifications. Mammalian systems such as CHO and HEK293 are commonly utilised to produce therapeutic proteins and monoclonal antibodies; avian cell lines are gaining popularity in vaccine manufacturing due to their quick growth and effective viral replication. Advances in cell line engineering and hybrid expression platforms are increasing productivity, scalability, and biomanufacturing efficiency.