PRINCIPLES OF TOXICOLOGY

GLOSSARY 565
lipoprotein Any of a group of conjugated proteins in which at least one component is a lipid.
Lipoproteins, classified according to their densities and chemical qualities, are the principal means
by which lipids are transported in the blood.
locus of action (site of action) The part of the body (organ, tissue, or cell) where a chemical acts to
initiate the chain of events leading to a particular effect.
luteinizing hormone A gonadotropic hormone of the anterior pituitary, which acts with the folliclestimulating
hormone to cause ovulation of mature follicles and secretion of estrogen by thecal and
granulosa cells.
lymphocyte A mononuclear leukocyte with a deep-staining nucleus containing dense chromatin and
a pale-blue-staining cytoplasm. Chiefly a product of lymphoid tissue. Participates in humoral and
cell-mediated immunity. See also B cell; T cell.
lymphokine Any of various substances released by T cells that have been activated by antigens. They
function in the immune response through a variety of actions, including stimulating the production
of nonsensitized lymphocytes and activating macrophages.
macrophage Any of the large phagocytic cells of the reticuloendothelial system.
makeup air In workplace ventilation, air introduced into an area to replace the air that has been
removed.
malignant tumor Relatively autonomous growth of cells or tissue. Each type of malignant tumor
has a different etiology and arises from a different origin. The condition tends to become
progressively worse and to result ultimately in death. There are many common properties of
malignant tumors, but the invasion of surrounding tissue and the ability to metastasize are
considered the most characteristic.
margin of safety The magnitude of the range of doses involved in progressing from a noneffective
dose to a lethal dose. Consequently, the slope of the dose–response curve is an index of the margin
of safety of a compound.
megakaryocyte A giant cell found in bone marrow, containing a greatly lobulated nucleus from
which mature blood platelets originate.
mesenchymal cells (tissue) The meshwork of embryonic connective cells or tissue in the mesoderm
from which are formed the connective tissues of the body, the blood vessels, and the lymphatic
vessels.
mesothelioma A tumor developed from the mesothelial tissue—the simple squamous-celled layer
of the epithelium, which covers the surface of all true serous membranes (lining the abdominal
cavity, covering the heart, and enveloping the lungs).
metabolism The biochemical reactions that take place within an organism. It involves two processes:
anabolism (assimilation or constructive processes) and catabolism (disintegration or destructive
processes). All metabolic processes involve energy transfer.
metallothionein An inducible metal-binding protein involved in trafficking and detoxification
mechanisms for various heavy metals.
metaplasia The transformation of cells from a normal to an abnormal state.
metastasis The establishment of a secondary growth site, distant from the primary site. One of the
primary characteristics of a malignant tumor.
methemoglobin A compound formed from hemoglobin by oxidation of iron in the ferrous state to
the ferric state. Methemoglobin does not combine with oxygen.
methemoglobinemia Presence of methemoglobin in the blood, resulting in cyanosis.
microsomes The fragments of the smooth reticular endothelium. This is the source of the microsomal
enzymes that are capable of catalyzing a variety of biotransformation reactions, including hydroxylation,
dealkylation, deamination, alkyl side-chain oxidation, hydrolysis, and reduction.
miosis Contraction of the pupil of the eye.