American Medical Association Family Medical Guide - American Medical Association [621]
The organs of the immune system
Lymph node
Lymph nodes play a major role in the body’s immune response. Lymph nodes contain infection-fighting white blood cells called lymphocytes, which circulate throughout the body among the blood, lymph nodes, and lymphatic vessels and are always on guard against potentially harmful microorganisms. The lymphatic vessels carry lymphocytes from the lymph nodes into the bloodstream and deliver lymphocytes into the lymph nodes from the bloodstream.
HIV Infection and AIDS
The term AIDS—acquired immunodeficiency syndrome—refers to the most advanced stages of infection with HIV, the human immunodeficiency virus. By destroying cells that are part of the body’s immune system, HIV progressively disables the body’s ability to fight infection and some types of cancer. People infected with HIV often develop life-threatening infections that are called opportunistic because they seize the chance presented by the impaired immune system to take hold in the body. Such infections usually do not make healthy people sick.
HIV is a retrovirus, which means that, unlike other viruses that carry DNA as their hereditary material, HIV carries RNA. Retroviruses reproduce by turning their RNA into DNA using an enzyme called reverse transcriptase. When a retrovirus such as HIV infects a cell, it inserts its RNA into the cell along with reverse transcriptase. The resulting DNA contains the genetic instructions that allow the virus to replicate. HIV belongs to a subgroup of retroviruses called “slow” viruses. Infection by a slow virus is characterized by a long period—sometimes decades—between initial infection and the appearance of symptoms. Worldwide, roughly 40 million people are living with HIV; about 48 percent of adults infected with HIV are women.
HIV
HIV is a microscopic spherical retrovirus. Its RNA, which contains its genetic information, resides inside a protective coating made up of proteins. HIV uses an enzyme called reverse transcriptase to convert its RNA into DNA inside the host cell it invades and uses the host DNA to make copies of itself.
HIV infection gradually impairs the functioning of the immune system by disabling and killing specialized immune system cells called CD4-positive T cells, or helper T cells, that signal other cells in the immune system to perform their special functions. Healthy people have 800 to 1,200 helper T cells per cubic millimeter (mm3) of blood. After HIV infection, the number of helper T cells gradually decreases. When the number of helper T cells falls below 200 cells per mm3, the affected person becomes especially vulnerable to opportunistic infections, such as some specific types of pneumonia and cancer.
How HIV Infects a Helper T Cell
When a particle of HIV encounters a CD4-positive T cell, it recognizes it by a molecule called a CD4 receptor on the cell’s surface. The HIV particle binds with the CD4-positive T cell, which enables the HIV particle to enter the cell. Inside the cell, HIV converts its RNA into DNA. The newly made viral DNA enters the cell’s nucleus, where it is spliced into the host cell’s DNA. This processed HIV then moves out of the nucleus into the cytoplasm of the cell, where it uses the cell’s protein-making machinery to make copies of itself. Once these new viral particles leave the cell, they become infectious and search for other CD4-positive T cells to invade.
People can become infected with HIV through contact with infected blood or other body fluids, usually by having unprotected sex with an infected partner or by sharing contaminated needles. Infected women can spread HIV to their babies during pregnancy or childbirth. Infected mothers can also transmit HIV to their babies in breast milk. Having a sexually transmitted disease (see page 477)—such as syphilis,