The present invention provides biodegradable, biomimetic particles for interacting with cells, including immune cells. In various embodiments, the particles comprise a polymer blend comprising a polyester, such as poly(lactic-co-glycolic acid) (PLGA) and a polyamine, such as poly(beta-amino ester) (PBAE). The particles further comprise, on their surface, one or more ligands for one or more cell surface receptor(s) or cell surface molecule(s). In some embodiments, the cell surface receptor or cell surface molecule is on an immune cell, such as a lymphocyte (T cell or B cell), natural killer cell, dendritic cell, or other cell of the immune system or tumor microenvironment.