What is being tested?
This test measures the number of CD4 cells (also known as T-helper cells) in your blood and assesses the status of your immune system. CD4 cells are a type of white blood cell that fight infection, and they play an important role in your immune system. They help to identify, attack, and destroy specific bacteria, fungi, and other germs that affect the body. CD4 cells are made in the spleen, lymph nodes, and thymus gland, and they circulate throughout the body in the bloodstream. CD4 cells are a major target for HIV, which binds to the surface of CD4 cells, enters them, and either reproduces immediately, killing them in the process, or remains in a resting state, reproducing later. As the HIV virus gets into the cell and replicates, the number of CD4 cells in the blood gradually declines. As HIV disease progresses, the CD4 count will go down and as treatment reduces the progression, the CD4 count will go back up.

How is the sample collected for testing?
Blood is drawn by needle from a vein in your arm.