HIV infects and damages the CD4 lymphocytes. These are important cells that control the whole immune system. HIV results in a fall in the number of CD4 cells in the blood (immunosuppression) which weakens or damages the function of the immune system (immunodeficiency).
The concentration or percentage of CD4 lymphocytes is used to measure the degree of immune damage. In children the number of CD4 cells is higher than in adults and normally reduces with age. Therefore, in young children (under 5 years) the number of CD4 cells is best expressed as a percentage:
- The normal range of CD4 lymphocytes in children is 25% or above.
- With mild immunosuppression the CD4 percentage may still be normal.
- With moderate immunosuppression the range of CD4 lymphocytes is 15 to 24%.
- With severe immunosuppression the range of CD4 lymphocytes falls below 15%.
The lower the CD4 percentage the greater is the damage to the immune system and, therefore, the higher the risk of serious HIV-related infections.
A CD4 percentage below 25% in young children indicates immunosuppression.
The absolute CD4 count is used in children aged 5 years and more. Healthy HIV-negative children of 5 years or above have a CD4 count above 500 cells/μL. A CD4 count below 200 cells/μL indicates severe immune suppression.
A low total lymphocyte count suggests a low CD4 percentage or count.