pls what is the meaning of CD4?

CD4 is the molecule on the surface of select immune cells. We define populations of blood cells based upon which molecules are present on their surface. Those lymphocytes that have CD4 on their surface are referred to as "CD4 cells." The relevance of this molecule and population of cells is that they are the target for HIV infection. In fact, HIV actually uses this molecule to bind to the cell and ultimately get into it to establish infection. The number of these cells declines with time during the course of infection and when they drop to below 200 an HIV-infected individual becomes at risk for developing various complicatons. In the context of HIV infection people often refer to this measure as their "CD4 count" or "T-cell count."

Best, Eric