Abstract: Compositions, systems, and methods for activating dendritic cells are disclosed. Also disclosed are compositions, systems, and methods for reducing viability of cancer cells and treating cancer, as well as preventing an increase in volume of a tumor present in a body of a living subject, along with methods of treating other diseases and infections. The systems and methods involve application of an alternating electric field to dendritic cell(s) in vivo or ex vivo and/or to a subject or cancer cells isolated therefrom. The systems and methods may further include administration of activated dendritic cells to a subject. The compositions comprise populations of isolated dendritic cells activated by exposure to an alternating electric field.