In recent years the terms "micro-/nanoplastics" (MNPLs) have caught special attention due to the increasing levels by which humans are exposed. Among MNPLs, polystyrene nanoparticles (PSNPs) are one of the most represented MNPLs in the environment. These tiny particles may enter into the human body, translocate through human barriers, interacting with blood and lymphatic immune cells, and reaching secondary organs. By using three different human leukocytic cell lines: Raji-B (B-lymphocytes), TK6 (lymphoblasts) and THP-1 (monocytes), we pursued to determine the effects of these PSNPs on the immune cell population. With this aim, the three cell lines were exposed to spherical PSNPs of about 50 nm of diameter and cytotoxicity, cellular uptake, reactive oxygen species (ROS) production, and genotoxicity were assessed at different time-points. Results show differences in all the measured endpoints, among the selected cell lines. Thus, whilst the monocytic THP-1 cells showed the highest particle internalization, no adverse effects were observed in such cells. On the other side, although Raji-B and TK6 showed lesser PSNPs uptake, mild toxicity, ROS production and genotoxicity were detected. These results highlight the importance of the cell line selection when the biological effects of PSNPs are evaluated.