HIV infected cell. Coloured scanning electron micrograph (SEM) of a 293T cell infected with the human immunodeficiency virus (HIV, blue dots). Small spherical virus particles, visible on the surface, are in the process of budding from the cell membrane. Any non highlighted vesicles of uneven shape are exosomes, thought to be involved in cell communication and transmission of disease, and under investigation as a means of drug delivery. Because 293T cells have lost their ability to protect themselves from viral infection, something that all cells are normally very good at, 293T are easily transfected, or infected, and can be used to produce large amounts of virus. This makes these cells an extraordinarily valuable tool in medicine and research. Magnification: x6600 at 10cm wide. Specimen courtesy of Greg Towers, University College London, UK.