Here, the use of nanotheranostics improves both the imaging and the treatment delivery. One of the primary concerns when using IONPs is their potential toxicity, which will vary depending on both nanoparticle structure (e.g. shape, size, surface chemistry) and dose. While IONPs are generally considered to be biocompatible, excessive accumulation or prolonged exposure may lead to adverse effects, necessitating careful evaluation of their safety profiles. Moreover, the susceptibility of IONPs to in vivo modifications (i.e. aggregation and degradation) can affect their magnetic properties and their theranostic efficacy, making it crucial to develop stable formulations for consistent performance.( ) For example, the hemolytic response of iron oxide has previously been assessed by exposing human red blood cells to IONPs. Such investigations have confirmed morphological changes that vary as a function of the nanoparticle concentration.(24) Moreover, loading efficiencies will depend on the nature of the targeted cells, as demonstrated by much previous research with IONPs in both cancer therapies and vascular thromboembolic disease management. Herein, we leverage a multiscale physico-chemical approach to assess the interaction of IONFs with the blood components. Specifically, we use flow cytometry and magnetic resonance imaging to study primary cytocompatibility and relaxation properties to study the interaction of IONFs with the biological human blood environment. Additionally, transmission electron microscopy (TEM) is used to investigate localization at the nanoscale. IONFs were chosen due to their multimodality as theranostics. Indeed, they are known to be amongst the best nanoheaters used in magnetic hyperthermia and photothermia, in addition to being excellent T2 contrast agents. In a theranostic context, these modalities are ideal for future applications in the imaging and treatment of cardiovascular diseases, as represented in Figure . Our investigations assess the interaction of IONFs with red blood cells (RBCs), white blood cells (WBCs) and platelets (PLTs) isolated from fresh human blood. The safe interaction between IONFs and human blood represents the first step of their action as nanomedicine for obstructive and thromboembolic diseases.