Counting T-cells in HIV infection. A laboratory technician uses a FACSCAN to count T-lymphocyte white blood cells in HIV infection. The screen at centre shows a 2-D plot of these cells. HIV (Human Immunodeficiency Virus) is the virus which causes AIDS. The virus attacks T-lymphocyte white blood cells (T-cells) of the human immune system causing a progressive loss of immunity in AIDS patients. FACSCAN uses a fluorescent monoclonal antibody test to identify a CD4 protein on T-cells, before counting them. A normal T-cell count is about 1000 T-cells per cubic millimetre of blood. AIDS patients have a blood count of 200 or fewer T- cells per cubic millimetre of blood.