A subset of white blood cells (leucocytes) called “T (cytotoxic – ‘killer’) lymphocyte” cells that have a protein molecule called “CD8” on the surface of these cells. CD8 functions to detect virus-infected cells (and cancer cells) and ‘kill’ cells that are infected, rather than killing the virus directly, and so they prevent viral replication. CD8 cells are usually activated by CD4 cells, but CD8 cells are unable to kill CD4 cells because HIV ‘hides’ in CD4 cells (see CD4 cells). As HIV infection progresses (without HIV ARV treatment) CD8 cell function declines due to decrease in the number of CD4 cells. However, in primary (early) HIV infection, the CD8 cell count can increase to twice the normal range. This is a sign that the body is responding to the presence of HIV (an invading pathogen), although this immune inflammatory response does not last in absence of HIV ARV treatment – which supresses HIV virus from ongoing replication and preserves or restores the immune system.