T-Cells

The healthy T cell is one type of immune system cell.

WHAT ARE T-CELLS?
T-cells are a type of lymphocyte (white blood cell). They are an important part of the immune system. There are two main types of T-cells. T-4 cells, also called CD4+, are "helper" cells. They lead the attack against infections. T-8 cells, (CD8+), are "suppressor" cells that end the immune response. CD8+ cells can also be "killer" cells that kill cancer cells and cells infected with a virus.

Researchers can tell the T-cells apart by specific proteins on the cell surface. A T-4 cell is a T-cell with CD4 molecules on its surface. This type of T-cell is also called "CD4 positive", or CD4+.

WHY ARE T-CELLS IMPORTANT IN HIV?
When HIV infects humans, the cells it infects most often are CD4+ cells. The virus becomes part of the cells, and when they multiply to fight an infection, they also make more copies of HIV.

When someone is infected with HIV for a long time, the number of CD4+ cells they have (their T-cell count) goes down. This is a sign that the immune system is being weakened. The lower the T-cell count, the more likely the person will get sick.

There are millions of different families of T-cells. Each family is designed to fight a specific type of germ. When HIV reduces the number of T-cells, some of these families can be totally wiped out. You can lose the ability to fight off the particular germs those families were designed for. If this happens, you might develop an opportunistic infection.

WHAT DO THE NUMBERS MEAN?

The CD4+ cell count is a key measure of the health of the immune system. The lower the count, the greater damage HIV has done. Anyone who has less than 200 CD4+ cells, or a CD4+ percentage less than 14%, is considered to have AIDS according to the US Centers for Disease Control.

CD4+ counts are used together with the viral load to estimate how long someone will stay healthy.

CD4+ counts are also used to indicate when to start certain types of drug therapy:

from aidsinfonet.org