White blood cells protect the immune system by fighting infections, but human immunodeficiency virus targets and destroys these helpful cells, according to WebMD. Monitoring white blood cells is crucial to treating patients infected with HIV because a low count makes their weakened immune systems highly vulnerable to viruses.
HIV is contracted from human body fluids during sexual contact, drug use, blood transfusions or occupational exposure to blood, according to the U.S. Department of Health and Human Services. Mothers can also transmit the disease during and after pregnancy. HIV symptoms appear within four weeks of infection, causing a flu-like condition known as acute retroviral syndrome, according to the U.S. Centers for Disease Control and Prevention. HIV is impossible to cure because the immune system cannot eliminate it, and the infection eventually progresses to acquired immunodeficiency syndrome.
A special type of white blood cell, known as CD4 or T-helper, normally prevents harmful bacteria from overwhelming the body. The spleen, thymus gland and lymph nodes produce CD4 cells, and every cubic millimeter of blood usually contains between 500 and 1,500 of these cells, according to WebMD. HIV reduces CD4 count by binding to the cells, which inadvertently spread the infection by replicating.
CD4 cells become outnumbered and gradually dwindle, so doctors recommend antiretroviral therapy for patients with a count below 200 even where there are no symptoms, WebMD states. Physicians may also start preventive treatment when the count is between 350 and 500.