What Causes AIDS?

HIV is a type of virus called a retrovirus. Like all viruses, it must invade the cells of other organisms to survive and reproduce. HIV multiplies in the human immune system's CD4+ T cells and kills vast numbers of the cells it infects. The result is disease symptoms.

About The Immune System

Our bodies use a natural defense system to protect us from bacteria, fungi, viruses, and other microscopic invaders. This system includes general, nonspecific defenses as well as weapons custom-designed against specific health threats:

• Innate, or nonspecific, immunity is the first line of defense. Our skin, tears, mucus, and saliva, as well as the swelling that occurs after an infection or injury, contain types of immune cells and chemicals that attack disease-causing agents attempting to invade the body.

• Adaptive, or specific, immunity uses specialized cells and proteins called antibodies to attack invaders that get past the first line of defense. These weapons target specific proteins called antigens, found on the surface of the invading organism. The immune system can quickly rally these custom-tailored defenses if this particular invader attacks again.

There are two types of adaptive immune responses:

• The humoral immune response involves the action of specialized antibody-producing white blood cells. The antibodies (proteins produced by the immune system to fight infectious agents such as viruses), which circulate in the blood and other body fluids, can recognize specific antigens (substances that stimulate the production of antibodies). They latch onto the viruses, bacteria, toxins, and other substances that bear these antigens, targeting them for destruction.

• The cell-mediated immune response involves the action of another group of specialized white blood cells that direct and regulate the body's immune responses or directly attack cells that are infected or cancerous.

How Do White Blood Cells Help Fight Disease?

White blood cells, particularly macrophages and B and T lymphocytes, play central roles in the immune system's defenses against viruses and other foreign invaders.

• Macrophages contribute to both nonspecific and specific immune responses. These versatile cells act as scavengers, engulfing and digesting microbes and other foreign material in a cell-eating process called phagocytosis. They also, upon encountering an invading organism, release chemical messengers that alert other cells of the immune system and summon T lymphocytes to the scene.

• B lymphocytes, or B cells, serve as the body's antibody factories. Each antibody is targeted to recognize and bind to an antigen from a specific invader. When antibodies circulating through blood and body fluids encounter this invader, they mark it for destruction.

• T lymphocytes, or T cells, are part of the cellular immune response. Some T cells, like CD4+ T cells (also called "helper" T cells), direct and regulate the body's immune responses. Others are killer cells that attack cells that are infected or cancerous.

How Does HIV Infection Become Established In The Body?

Researchers have found evidence that immune-system cells called dendritic cells may begin the process of infection. After exposure, these special cells may bind to and carry the virus from the site of infection to the lymph nodes, where other immune system cells become infected.

HIV targets cells in the immune system that display a protein called CD4 on their surface. Such cells are called CD4-positive (CD4+) cells.