| The Immune System |
Understanding the various components of the immune system and the complex signaling that takes place between immune cells is key to understanding HIV. Both non-specific and specific lines of defense help thwart the invasion of pathogens. Non-specific defenses act quickly and indiscriminately to exclude microbes from the body or actively kill intruders. Mechanical barriers - such as the mucus, hairs, and cilia in the respiratory tract, and the flow of urine through the urinary tract - are among these non-specific defenses. Skin oils and chemicals in perspiration and gastric juices also serve as non-specific barriers. Mechanisms involving complex chemical signals such as fever and inflammation also act against a wide variety of pathogens. One non-specific defense involves phagocytes, a particular type of leukocyte (white blood cell), which act as cellular "Pac-Men," engulfing and digesting microbes or other irritants like dust and pollen.
If invaders have breached the non-specific defenses, the immune system will use a variety of leukocytes to mount directed defenses against specific invaders. Lymphocytes bind and respond to specific foreign molecules (antigens). One subset of lymphocytes, the B cells, matures into antibody-secreting cells. Another subset of lymphocytes, the T cells, includes immune cells that directly kill cancerous or virally infected cells. Some subtypes of T cells serve a regulatory function, releasing chemical signals that can stimulate or suppress a variety of immune functions. Because HIV preferentially infects one of these regulatory T cells, the so-called helper T (TH) cell, it can subvert and decimate the immune system, leading to AIDS.