CD4 count

medicine

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

July 15, 2026

Quick Summary

CD4 refers to a protein found on the surface of certain immune cells, primarily helper T cells, also known as CD4+ T cells or T4 cells. These cells are crucial for a healthy immune system, as they help activate other immune cells to fight infections.123

The human immunodeficiency virus (HIV), which causes AIDS, targets these CD4+ T cells. HIV uses the CD4 protein to attach to and enter the T cells, where it replicates and destroys them. As HIV progresses, it depletes the body's CD4+ T cells, severely weakening the immune system.3

Doctors monitor the number of CD4+ T cells in a person's blood, a measurement called the CD4 count, to assess the health of their immune system and the progression of HIV disease.3

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

3 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

AIDS
Life cycle of HIV
That measurement, called the CD4 count, provides a good indication of the status of the immune system. Physicians also measure the amount of virus in the bloodstream—i.e., the viral load—which provides an indication of how fast the virus is replicating and destroying helper T cells.
Read More
That measurement, called the CD4 count, provides a good indication of the status of the immune system. Physicians also measure the amount of virus in the bloodstream—i.e., the viral load—which provides an indication of how fast the virus is replicating and destroying helper T cells.
Read More