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What Are CD4 T-Cells?

Why Are They Important to People Living With HIV?

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Updated June 17, 2013

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

T-cells are a subset of white blood cells that play an important role in the body's immune system.

Parts of Our Immune System

There are two primary types of T-cells.

  • CD4 Cells - These cells have a glycoprotein called CD4 on their surface. These "helper" cells do not neutralize infection, but rather initiate the body's response to infections.
  • CD8 Cells - These types of T-cells have a glycoprotein called CD8 on their surface. CD8 cells, also called killer cells, are instrumental in fighting cancer and viruses. CD8 cells also produce antiviral substances (antibodies) that help fight off the foreign invader.

Understanding the Immune Response

CD4 Cells - The Key to HIV Replication

HIV is a retrovirus, meaning it needs cells from a "host" in order to make more copies of itself (replication). In the case of HIV, CD4 cells are the host cells that aid HIV in replication. HIV attaches to the CD4 cells, allowing the virus to enter and infect the CD4 cells, damaging them in the process. The fewer functioning CD4 cells, the weaker the immune system and therefore the more vulnerable a person is to infections and illnesses.

How Do We Know How Many Functioning CD4 Cells There Are?

Knowing how many functioning CD4 cells are circulating in the blood gives the HIV doctor an idea of how strong the HIV+ person's immune system really is. A simple blood test called the CD4 count estimates the number of functioning CD4 cells in the body, and therefore measures the health of the immune system. The CD4 test measures the number of CD4 cells in a cubic millimeter of blood. The normal range for CD4 counts can vary a great deal.

Normal Values - In a healthy adult, a normal CD4 count can vary a great deal but is typically 500 to 1500 cells per cubic millimeter of blood.

Less than 200 - The immune system is severely weakened, and the HIV+ person is at a much greater risk of opportunistic infections. This is classified as AIDS (or acquired immune deficiency syndrome).

200 - 500 In the U.S., treatment is strongly recommended at a CD4 count under 500, but can also be initiated at CD4 counts over 500, according to treatment guidelines from the Department of Health and Human Services.

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