CD4 cells are used to respond to infections within your immune system. HIV attacks the number of CD4 cells in the body. A CD4 count is the number of CD4 cells in your blood.  Medical professionals use this count as one way to measure the health of a patient.  A low CD4 count may signify that a patient with HIV has AIDS.

For more information visit the CDC.

https://www.cdc.gov/hiv/basics/livingwithhiv/understanding-care.html