<p>A novel electrochemical biosensor was developed for accurate and sensitive detection of CD8⁺ T cells, as a key biomarker in clinical and immunological analysis. A screen-printed electrode was modified through 20 cycles of electrodeposition using two approaches: a reduced graphene oxide–polypyrrole composite (A20) and graphene oxide reduced under UV irradiation during polymerization (B20). Characterization analysis indicated that A20 exhibited improved structural uniformity and a higher relative abundance of carboxyl groups compared to B20 sample. These features achieved through a simple and cost-effective fabrication process, significantly improved the electrochemical performance of A20, making it as an optimal sensing platform. The A20-based biosensor detected CD8⁺ T cells with limit of detection 1.19 × 10⁴ cells.mL^-1, which the results aligned with flow cytometry by 97%, confirming its precise accuracy. Further experiments using CD20⁺, CD19⁺ and CD4⁺ cells confirmed the biosensor’s selectivity. This study introduces a simple, scalable process for creating composites that greatly enhances the sensitivity of biosensors. Because it is affordable and easy to produce, this biosensor is well-suited for clinical and laboratory use, and also creates opportunities for developing sensors that detect other cellular biomarkers.</p>