Cell infected with HIV. Coloured transmission electron micrograph (TEM) of HIV particles (small, round) on the membrane of a host CD4+ lymphocyte white blood cell (red). HIV (human immunodeficiency virus) attacks CD4+ T-lymphocytes (specialised white blood cells), which are crucial in the body’s immune system. It enters the cell and makes many copies of itself, which then destroy the cell as they emerge through its membrane. This severely weakens the immune system, causing AIDS (acquired immunodeficiency syndrome).