Fluorescent light micrograph showing the lymph node of a person with untreated HIV (human immunodeficiency virus). HIV is the cause of AIDS (acquired immune deficiency syndrome). The virus infects the immune system's T-lymphocytes, ultimately killing them, leading to a very weak immune system. Abnormal proliferation of B cells (green), a type of white blood cells that produce antibodies, are seen outside of germinal centres. Surrounding the B cells, are cells producing transcription factor T-bet (magenta) are seen.