CD4 cells, also known as helper T cells or T cells, are a type of white blood cell that fight off infections and keep you from getting sick. They are called helper cells because one of their main roles is to send signals to other types of immune cells, including CD8 killer cells, which then destroy the infectious particle. CD4 cells are found on the surface of immune cells such as T helper cells, monocytes, macrophages, and dendritic cells. CD4 is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR) and assists the latter in communicating with antigen-presenting cells.

A CD4 count is a blood test that measures the number of CD4 cells in a sample of your blood. CD4 counts can help monitor your risk for developing opportunistic infections or certain cancers. If your risk increases, your provider may give you treatment to help prevent infections. CD4 results are usually given as a number of cells per cubic millimeter of blood. A healthy adult has between 500-1600 CD4 cells. If CD4 cells become depleted, for example in untreated HIV infection, or following immune suppression prior to a transplant, the body is left vulnerable to a wide range of infections that it would otherwise have been able to fight.