HIV Devastates the Immune System

Every day, HIV destroys billions of CD4+ T cells in a person infected with HIV, eventually overwhelming the immune system's capacity to regenerate or fight other infections. The following are different ways this may occur:

Killing Cells Directly

CD4+ T cells infected with HIV may be killed when a large amount of virus is produced and buds out from the cell surface. The budding process disrupts the cell membrane and causes the cell to die. The cell can also expire when the virus excessively uses the cell’s machinery for its own purposes, disrupting normal activities needed for the survival of the cell.

Apoptosis (cell suicide)

An HIV-infected cell undergoing apoptosis. Credit: Institute of Cell and Molecular Science

When the regulation of a cell’s machinery and functions become grossly distorted because of HIV replication, the infected cell may commit suicide by a process known as programmed cell death or apoptosis. There is evidence that apoptosis occurs most frequently in the bloodstream and lymph nodes of people infected with HIV.

The Death of Innocent Bystander Cells

Cells that are not infected with HIV may also die as a direct result of the effects of HIV infection:

• HIV may bind to the cell surface, making it appear as if the cell is infected. After antibodies attach to the virus on the cell, killer T cells, which serve to protect the immune system by killing infected cells, may mistakenly destroy the cell. This process is called antibody-dependent cellular cytotoxicity.
• CD8 T cells, also known as "killer T cells," may mistakenly destroy uninfected cells that have consumed HIV particles and display HIV fragments on their surfaces.
• Because some HIV envelope proteins bear some resemblance to certain molecules on CD4+ T cells, the body's immune responses may mistakenly damage these cells.
• Uninfected cells may undergo apoptosis. Scientists have demonstrated in laboratory experiments that the HIV envelope alone or when bound to antibodies sometimes sends an inappropriate signal to CD4+ T cells. This can cause the cells to undergo apoptosis, even if not infected by HIV.

Destruction of Immune Precursor Cells

Studies suggest that HIV also destroys precursor cells (young cells that have not yet fully developed) that later mature into cells with special immune functions. HIV can also damage the bone marrow and the thymus, which are needed for developing precursor cells. The bone marrow and thymus probably lose their ability to regenerate, further compounding the suppression of the immune system.