LATENCY: The period when an organism (i.e., a virus or a bacterium) is in the body and not producing any ill effects. See also Clinical Latency.

LESION: A general term to describe an area of altered tissue (e.g., the infected patch or sore in a skin disease).

LIPID: Any of a group of fats and fat-like compounds, including sterols, fatty acids and many other substances.

LIPOSOMES: A spherical particle in an aqueous (watery) medium (e.g., inside a cell) formed by a lipid bilayer enclosing an aqueous compartment. See also Lipid.

LONG-TERM NON-PROGRESSORS: Individuals who are HIV-infected for seven or more years, 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 seen in other HIV-infected individuals.

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. See also Lymphatic Vessels; Lymphocyte.

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. See also Antigen; Lymphocyte.

LYMPHOCYTE: A white blood cell. Present in the blood, lymph and lymphoid tissue. See also B Lymphocytes; Lymph; T Cells.

LYMPHOMA: Cancer of the lymphoid tissues. Lymphomas are often described as being large or small cell types, cleaved or noncleaved, 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 organ 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.