lipoteichoic acids Components of bacterial cell walls that are recognized by Toll-like receptors.
long-term non-progressors   HIV-infected individuals who mount an immune response that controls viral loads such that they do not progress to AIDS despite the absence of antiretroviral therapy. See also elite controllers.
LPS-binding protein   Protein in blood and extracellular ﬂuid that binds bacterial lipopolysaccharide (LPS) shed from bacteria.
Ly49 receptors A family of C-type lectins expressed by mouse, but not human, NK cells. These can be either activating or inhibitory in function.
Ly49a See Ly49 receptors.
Ly49H See Ly49 receptors.
Ly108 See SLAM.
lymph The extracellular ﬂuid that accumulates in tissues and is drained by lymphatic vessels that carry it through the lymphatic system to the thoracic duct, which returns it to the blood.
lymph nodes A type of peripheral lymphoid organ present in many locations throughout the body where lymphatic vessels converge.
lymphatic system The system of lymph-carrying vessels and peripheral lymphoid tissues through which extracellular ﬂuid from tissues passes before it is returned to the blood via the thoracic duct.
lymphatic vessels, lymphatics Thin-walled vessels that carry lymph.
lymphoblast A lymphocyte that has enlarged after activation and has increased its rate of RNA and protein synthesis, but is not yet fully differentiated.
lymphocyte A class of white blood cells that bear variable cell-surface receptors for antigen and are responsible for adaptive immune responses. There are two main types—B lymphocytes (B cells) and T lymphocytes (T cells)—which mediate humoral and cell-mediated immunity, respectively. On antigen recognition, a lymphocyte enlarges to form a lymphoblast and then proliferates and differentiates into an antigen-specific effector cell.
lymphocyte receptor repertoire All the highly variable antigen receptors carried by B and T lymphocytes.
lymphoid Describes tissues composed mainly of lymphocytes.
lymphoid organs   Organized tissues characterized by very large numbers of lymphocytes interacting with a nonlymphoid stroma. The central, or primary, lymphoid organs, where lymphocytes are generated, are the thymus and bone marrow. The main peripheral, or secondary, lymphoid organs, in which adaptive immune responses are initiated, are the lymph nodes, spleen, and mucosa-associated lymphoid organs such as tonsils and Peyer’s patches.
lymphoid tissue Tissue composed of large numbers of lymphocytes.
lymphoid tissue inducer (LTi) cells   Cells of the blood lineage, which arise in the fetal liver and are carried in the blood to sites where they will form lymph nodes and other peripheral lymphoid organs.
lymphopenia Abnormally low levels of lymphocytes in the blood.
lymphopoiesis The differentiation of lymphoid cells from a common lymphoid progenitor.
lymphotoxins (LTs) Cytokines of the tumor necrosis factor (TNF) family that are directly cytotoxic for some cells. They occur as trimers of LT-α chains (LT-α3) and heterotrimers of LT-α and LT-β chains (LT-α2:β1).
lysogenic phase The phase of the viral life cycle in which the virus genome integrates into the host cell genome but remains dormant, employing mechanisms to avoid destroying its cellular host.
lysozyme Antimicrobial enzyme that degrades bacterial cell walls.