HIV viruses infecting T-lymphocytes, computer illustration. The surface of the T-cell has a lumpy appearance with large irregular surface protrusions. Smaller spherical structures on the cell surface are HIV virus particles budding away from the cell membrane. The virus has infected the T-cell, and instructed the cell to reproduce many more viruses. This viral budding causes the T-cell to die. Depletion of the number of T-cells in the blood is the main reason for the destruction of the immune system in AIDS.