Interactions between proteins on the surface of two cells are pivotal in determining cell fate. These might be receptor–ligand pairs or adhesion molecules. Mechanistic and functional details of these interactions can be studied by optogenetics using parts of the photoreceptor phytochrome B (PhyB) of Arabidopsis thaliana. In these cases, PhyB serves as a membrane-bound ligand for engineered receptors or adhesion molecules. Here, we provide a protocol to generate cells that display PhyB as a membrane-bound protein on their surface to serve as the ligand. This is done by expressing a transmembrane SpyCatcher protein on the cells and covalently coupling purified recombinant PhyB-SpyTag to these cells. Coming from immunology, we call the PhyB-coupled cells opto-antigen-presenting cells (opto-APCs).