LATENCY:
The period when an infecting organism is in the body but is not producing any clinically noticeable ill effects or symptoms. In HIV disease, clinical latency is an asymptomatic period in the early years of HIV infection. The period of latency is characterized in the peripheral blood by near normal CD4 counts. Recent research indicates that HIV remains quite active in the lymph nodes during this period. Cellular latency is the period after HIV has integrated its genome into a cell's DNA but has not yet begun to replicate.
LD50:
Short for "Lethal Dose 50." In toxicology, the amount of a substance sufficient to kill one-half of the population of test subjects (e.g., mice or rats).
LENTIVIRUS:
"Slow" virus characterized by a long interval between infection and the onset of symptoms. HIV is a lentivirus as is the simian immunodeficiency virus (SIV) that infects non-human primates.
LESION:
A general term to describe an area of altered tissue (e.g., the infected patch or sore in a skin disease).
LEUKOCYTOSIS:
An abnormally high number of leukocytes in the blood. This condition can occur during many types of infection and inflammation.
LEUKOPENIA:
A decrease in the number of white blood cells. The threshold value for leukopenia is usually taken as less than 5,000 white blood cells per cubic millimeter of blood.
LIPID:
Any of a group of fats and fatlike compounds, including sterols, fatty acids, and many other substances.
LIPODYSTROPHY:
A disturbance in the way the body produces, uses, and distributes fat. Lipodystrophy is also referred to as "buffalo hump," "protease paunch," or "Crixivan potbelly." In HIV disease, lipodystrophy has come to refer to a group of symptoms that seem to be related to the use of protease inhibitor drugs. How protease inhibitors may cause or trigger lipodystrophy is not yet known. Lipodystrophy symptoms involve the loss of the thin layer of fat under the skin, making veins seem to protrude; wasting of the face and limbs; and the accumulation of fat on the abdomen (both under the skin and within the abdominal cavity) or between the shoulder blades. Women may also experience narrowing of the hips and enlargement of the breasts.
LIPOSOMES:
A spherical particle in an aqueous (watery) medium (e.g., inside a cell) formed by a lipid bilayer enclosing an aqueous compartment. Microscopic globules of lipids are manufactured to enclose medications. The fatty layer of the liposome is supposed to protect and confine the enclosed drug until the liposome binds with the outer membrane of target cells. By delivering treatments directly to
the cells needing them, drug efficacy may be increased while overall toxicity is reduced.
LIVE VECTOR VACCINE:
As pertaining to HIV, a vaccine that uses an attenuated (i.e., weakened) virus or bacterium to carry pieces of HIV into the body to directly stimulate a cell-mediated immune response.
LIVER FUNCTION TEST:
A test that measures the blood serum level of any of several enzymes (e.g., SGOT and SGPT) produced by the liver. An elevated liver function test is a sign of possible liver damage.
LOG:
Changes in viral load are often reported as logarithmic or "log changes." This mathematical term denotes a change in value of what is being measured by a factor of 10. For example, if the baseline viral load by PCR were 20,000 copies/ml plasma, then a 1-log increase equals a 10-fold (10 times) increase or 200,000 copies/ml plasma. A 2-log increase equals 2,000,000 copies/ml plasma, or a 100-fold increase.
LONG TERMINAL REPEAT SEQUENCE (LTR):
The genetic material at each end of the HIV genome. When the HIV genome is integrated into a cell's own genome, the LTR interacts with cellular and viral factors to trigger the transcription of the HIV-integrated HIV DNA genes into an RNA form that is packaged in new virus particles. Activation of LTR is a major step in triggering HIV replication.
LONG-TERM NONPROGRESSORS:
Individuals who have been living with HIV for at least 7 to 12 years (different authors use different time spans) and have stable CD4+ T cell counts of 600 or more cells per cubic millimeter of blood, no HIV-related diseases, and no previous antiretroviral therapy. Data suggest that this phenomenon is associated with the maintenance of the integrity of the lymphoid tissues and with less virus trapping in the lymph nodes than is seen in other individuals living with HIV.
LUMBAR:
Lower back region. Of, relating to, or constituting the vertebrae between the thoracic vertebrae and the sacrum region. The sacrum is the triangular bone made up of five fused vertebrae and forming the posterior section of the pelvis. The thorax is the part of the human body between the neck and the diaphragm, partially encased by the ribs and containing the heart and lungs (i.e., the chest).
LUMBAR PUNCTURE:
A procedure in which cerebrospinal fluid from the subarachnoid space in the lumbar region is tapped for examination. Also known as spinal tap.
LYMPH:
A transparent, slightly yellow fluid that carries lymphocytes. Lymph is derived from tissue fluids collected from all parts of the body and is returned to the blood via lymphatic vessels.
LYMPH NODES:
Small, bean-sized organs of the immune system, distributed widely throughout the body. Lymph fluid is filtered through the lymph nodes in which all types of lymphocytes take up temporary residence. Antigens that enter the body find their way into lymph or blood and are filtered out by the lymph nodes or spleen, respectively, for attack by the immune system.
LYMPHADENOPATHY SYNDROME (LAS):
Swollen, firm, and possibly tender lymph nodes. The cause may range from an infection such as HIV, the flu, or mononucleosis to lymphoma (cancer of the lymph nodes).
LYMPHATIC VESSELS:
A body-wide network of channels, similar to the blood vessels, that transport lymph to the immune organs and into the bloodstream.
LYMPHOID INTERSTITIAL PNEUMONITIS (LIP):
A type of pneumonia that affects 35 to 40 percent of children with AIDS, which causes hardening of the lung membranes involved in absorbing oxygen. LIP is an AIDS-defining illness in children. The etiology (cause) of LIP is not clear. There is no established therapy for LIP, but the use of corticosteroids for progressive LIP has been advocated.
LYMPHOID ORGANS:
Include tonsils, adenoids, lymph nodes, spleen, thymus, and other tissues. These organs act as the body's filtering system, trapping invaders (i.e., foreign particles, e.g., bacteria and viruses) and presenting them to squadrons of immune cells that congregate there. Within these lymphoid tissues, immune activity is concentrated in regions called germinal centers, where the thread-like tentacles of follicular dendritic cells (FDCs) form networks that trap invaders.
LYMPHOKINES:
1. Products of the lymphatic cells that stimulate the production of disease-fighting agents and the activities of other lymphatic cells. Among the lymphokines are gamma interferon and interleukin-2. 2. Non-antibody mediators of immune responses, released by activated lymphocytes.
LYMPHOMA:
Cancer of the lymphoid tissues. Lymphomas are often described as being "large cell" or "small cell" types, cleaved or non-cleaved, or diffuse or nodular. The different types often have different prognoses (i.e., prospect of survival or recovery). Some of these lymphomas are named after the physicians who first described them (e.g., Burkitt's lymphoma, Hodgkin's disease). Lymphomas can also be referred to by the organs where they are active such as CNS lymphomas, which are in the central nervous system, and GI lymphomas, which are in the gastrointestinal tract. The types of lymphomas most commonly associated with HIV infection are called non-Hodgkin's lymphomas or B cell lymphomas. In these types of cancers, certain cells of the lymphatic system grow abnormally. They divide rapidly, growing into tumors.
LYMPHOPENIA:
A relative or absolute reduction in the number of lymphocytes in the circulating blood.
LYMPHOPROLIFERATIVE RESPONSE:
A specific immune response that entails rapid T cell replication. Standard antigens, such as tetanus toxoid, that elicit this response, are used in lab tests of immunocompetency.
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