A subset of white blood cells (leucocytes) called ‘T (helper) lymphocyte’ cells that have a protein molecule called ‘CD4’ on the surface of these cells. CD4 ‘helper’ cells initiate the body’s reaction to infections by regulating the immune response by sending out chemical signals (release of cytokines) that interact with and stimulate the growth and activity of other immune system cells, such as B cells and CD8 cells. CD4 cells are like the ‘commanders’ of the immune system alerting the entire immune system (the ‘army’ of other cells) to an ‘invader’. CD4 is also present on the surface of other immune system cells called macrophages and dendritic cells. HIV has an affinity with (is attracted to) to the CD4 cell surface, and binds with CD4 allowing HIV to attach then enter and infect the human CD4 T cells. This inactivates the function of the CD4 cells to ‘command’ the immune system, since CD4 T cells are the primary target of HIV. HIV destroys the immune system by invading and attacking CD4 cells, then hiding in them (unable to seen by other immune system cells) until it re-emerges as a new virus particle from within the human cell (killing that cell in the process), then the new virus particle goes on to infect other CD4 T cells. The gradual loss of CD4 T cells then impairs the function of the immune system. HIV antiretroviral (ARV) treatment stops the destruction of CD4 T cells by slowing HIV replication.