CD4 cells (also known as CD4+ T cells) are white blood cells that fight infection. CD4 cell count is an indicator of immune function and disease progression and one of the key determinants for the need of opportunistic infection (OI) prophylaxis. CD4 cell counts are obtained from bloodwork as part of laboratory monitoring for HIV infection.

Studies have shown that starting HIV medicines soon after you are diagnosed, and ideally when your CD4 cell count is high, will greatly help your health and will reduce the risk of OIs. Regularly checking your CD4 cell count will allow you to begin necessary prophylactic medications to reduce your risk of opportunistic infections.