A CD4 cell is a type of red blood cell that fights viral or bacterial infections in the body by sending signals to activate the immune system, explains Aids.gov. CD4 cells are often referred to as T-cells or T-helper cells.
HIV attacks and destroys the CD4 cells, lowering the immune system, according to WebMD. HIV binds to the surface of the CD4 cells, making more copies of HIV as the CD4 cells multiply. The virus spreads through the body by a process called HIV life cycle, according to Aids.gov.
A CD4 count measures the number of CD4 cells in a blood sample to establish the strength of the immune system, explains WebMD. The CD4 count of a healthy person is from 500 to 1500 cells per cubic milliliter of blood.