The main impediment to efficient delivery of nanoparticles is the immune system, as it readily identifies nanoparticles as foreign and removes them before they can reach their target. This project developed and used a murine model to understand and quantify nanoparticle-immune cell interactions in various tissues. The model identified that substantial improvement in delivery efficiency can be achieved by reducing nanoparticle associations with immune cells. This work advances the field of knowledge on the cellular interactions of nanoparticles in the body, and contributes towards advancing drug delivery efficiencies of future therapeutics.