CD4 T lymphocytes (CD4 cells) help coordinate the immune response by stimulating other immune cells, such as macrophages, B lymphocytes (B cells), and CD8 T lymphocytes (CD8 cells), to fight infection.
HIV weakens the immune system by destroying CD4 cells.

The CD4 cell count is a measurement of the number of CD4 cells in a sample of blood. It is one of the most useful indicators of the health of the immune system and the progression of HIV/AIDS.
A CD4 cell count is used by health care providers to determine when to begin, interrupt, or halt anti-HIV therapy; when to give preventive treatment for opportunistic infections; and to measure response to treatment.
A normal CD4 cell count is between 500 and 1,400 cells/mm.

AIDSinfo. Glossary. Internet. Accessed on August 7, 2009.