What is CD4 count?

CD4 cell count is a laboratory test that measures the number of CD4 T-cells in a sample of blood. 

CD4 cells are a type of white blood cell that helps fight infection by activating the immune system to destroy invading pathogens such as viruses, bacteria, and other microorganisms. A CD4 count is usually carried out to check the health of the immune system in a person who is infected with HIV (human immunodeficiency virus).