CD4+ cells are part of the immune system and are a type of white blood cell. White blood cells protect the body against infection. CD4+ cells are also called T-lymphocytes, T-cells, or T-helper cells.

HIV invades and destroys CD4+ cells. But the body continues to produce new CD4+ cells to fight the HIV infection. If the infection is not treated with medications, the body gradually loses the ability to produce enough CD4+ cells to replace the number that are being destroyed by HIV. As the number of CD4+ cells in the blood drops, it becomes more difficult for the immune system to fight infections.

CD4+ counts are measured every 3 to 4 months in people who are infected with HIV. The CD4+ count is an important measurement of how HIV is affecting your immune system and can help you decide when to begin treatment for HIV or when you need to try a different combination of medicines.