Lungs
The invaginated respiratory surfaces of terrestrial vertebrates, land snails, and spiders that
connect to the atmosphere by narrow tubes.
Leucoplasts
A colorless plastid in the cytoplasm of plant cells that makes and stores starch.
Luteinizing hormone (LH)
A protein hormone secreted by the anterior pituitary that stimulates ovulation in females
and androgen production in males.
Lymph
The colorless fluid, derived from interstitial fluid, in the lymphatic system of vertebrate
animals.
Lymph node
A mass of spongy tissues, separated into compartments; located throughout the lymphatic
system, lymph nodes remove dead cells, debris, and foreign particles from the circulation;
also are sites at which foreign antigens are displayed to immunologically active cells.
Lymphatic system
A system of vessels and lymph nodes, separate from the circulatory system, that returns
fluid and protein to the blood.
Lymphocyte
A white blood cell. The lymphocytes that complete their development in the bone marrow
are called B cells, and those that mature in the thymus are called T cells.
Lymphokine
A chemical, released by an activated cytotoxic T cell, that attracts macrophages and
stimulates phagocytosis.
Lysis
Disintegration of a cell by rupture of its plasma membrane.
Lysogenic bacteria
Bacteria carrying a bacteriophage integrated into the bacterial chromosome. The virus may
subsequently set up an active cycle of infection, causing lysis of the bacterial cells.
lysogenic cycle
A type of phage replication cycle in which the viral genome becomes incorporated into the
bacterial host chromosome as a prophage.
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