Biological nanoparticles, such as proteins and extracellular vesicles, are rapidly growing as nanobased drug-delivery agents due to their biocompatibility, high loading efficiency, and bioavailability. However, most of the candidates emerging preclinically hardly confirm their potential when entering clinical trials. Among other reasons, this is due to the low control of synthesis processes and the limited characterization of their potential immunoreactivity profiles. Here, we propose a combined method that allow us to fully characterize H-ferritin nanoparticles' immunoreactivity during their production, purification, endotoxin removal, and drug loading. H-Ferritin is an extremely interesting nanocage that is being under evaluation for cancer therapy due to its innate cancer tropism, favorable size, and high stability. However, being a recombinant protein, its immunoreactivity should be carefully evaluated preclinically to enable further clinical translation. Surprisingly, this aspect is often underestimated by the scientific community. By measuring proinflammatory cytokine release as a function of endotoxin content, we found that even removing all pyrogenic contaminants from the nanocage, a mild immunoreactivity was still left. When we further purified H-ferritin by loading doxorubicin through a highly standardized loading method, proinflammatory cytokine release was eliminated. This confirmed the safety of H-ferritin nanocages to be used for drug delivery in cancer therapy. Our approach demonstrated that when evaluating the safety of nanodrugs, a combined analysis of acute toxicity and immunoreactivity is necessary to guarantee the safety of newly developed products and to unveil their real translational potential.