Abstract: The present invention relates to artificial Antigen Presenting Cells (aAPCs) comprising artificial Antigen Presenting Molecules (aAPMs) and, in particular, comprising dimers of the aAPMs as well as to methods for producing aAPCs. The invention further relates to compositions comprising the aAPCs and to vectors encoding the aAPMs of the aAPCs. Embodiments of the invention have been particularly developed for use in assays for determining an antigen-specific T cell response or a plurality of antigen-specific T cell responses and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.