MACROPHAGE: A large immune cell that devours invading pathogens and other intruders. Stimulates other immune cells by presenting them with small pieces of the invader. Macrophages can harbor large quantities of HIV without being killed, acting as reservoirs of the virus.

MAGNETIC RESONANCE IMAGING (MRI): Medical imaging that uses radiofrequency radiation as its source. MRI is a noninvasive diagnostic technique that can provide information on the form and function of internal tissues and organs of the body.

MAST CELL: A granulocyte found in tissue. The contents of the mast cells, along with those of basophils, are responsible for the symptoms of allergy. See also Basophil; Granulocyte.

MEAN: The arithmetic average, or the sum of all the values divided by the number of values.

MEDIAN: The middle number in a sequence of numbers, taken as the average of the two middle numbers when the sequence has an even number of numbers (e.g., 4 is the median of 1, 3, 4, 8, 9).

MEMORY CELLS: A subset of T lymphocytes that have been exposed to specific antigens and can then proliferate (i.e., reproduce) on subsequent immune system encounters with the same antigen. See also Antigen; T Cells.

MESSENGER RNA: Also referred to as mRNA. An RNA (ribonucleic acid) that carries the genetic code for a particular protein from the nuclear DNA (i.e., the DNA in the cell's nucleus) to a ribosome in the cytoplasm and acts as a template, or pattern, for the formation of that protein. See also Cytoplasm; Ribosome.

METABOLISM: The sum of the processes by which a particular substance is handled (as by assimilation and incorporation, or by detoxification and excretion) in the living body.

METABOLITE: Any substance produced by metabolism or by a metabolic process. See also Metabolism.

METASTASIS: Transfer of a disease-producing agent (e.g., cancer cells or bacteria) from an original site of disease to another part of the body with development of a similar lesion in the new location (e.g., spread of cancer from an original site to other sites in the body).

MICROBES: Microscopic living organisms, including bacteria, protozoa and fungi.

MICROBICIDE: An agent (e.g., a chemical or antibiotic) that destroys microbes. See also Microbes.

MOLECULE: The smallest particle of a compound that has all the chemical properties of that compound. Molecules are made up of two or more atoms, either of the same element or of two or more different elements. Ionic compounds, such as common salt, are made up not of molecules, but of ions arranged in a crystalline structure. Unlike ions, molecules carry no electrical charge. Molecules differ in size and molecular weight as well as in structure.

MONOCLONAL ANTIBODIES: Antibodies produced by a hybridoma or antibody-producing cell source for a specific antigen. Monoclonal antibodies are useful as a tool for identifying specific protein molecules. See also Antibodies; Antigen; Hybridoma.

MRI: See Magnetic Resonance Imaging.

MUCOCUTANEOUS: Anything that concerns or pertains to mucous membranes and the skin (e.g., mouth, vagina, lips, anal area). See also Mucous Membrane.

MUCOSA: See Mucous Membrane.

MUCOSAL IMMUNITY: Resistance to infection across the mucous membranes. Dependent on immune cells and antibodies present in the lining of the urogenital tract, gastrointestinal tract and other parts of the body exposed to the outside world. See also Antibodies; Genitourinary Tract; Mucous Membrane.

MUCOUS MEMBRANE: A moist layer of tissue that lines body cavities or passages that have an opening to the external world (e.g., the lining of the mouth, nostrils or vagina).

MUTATION: In biology, a sudden change in a gene or unit of hereditary material that results in a new inheritable characteristic. In higher animals and many higher plants, a mutation may be transmitted to future generations only if it occurs in germ-or sex cell-tissue; body cell mutations cannot be inherited. Changes within the chemical structure of single genes may be induced by exposure to radiation, temperature extremes and certain chemicals. The term mutation may also be used to include losses or rearrangements of segments of chromosomes, the long strands of genes. Drugs such as colchicine double the normal number of chromosomes in a cell by interfering with cell division. Mutation, which can establish new traits in a population, is important in evolution. As related to HIV: HIV mutates rapidly. During the course of HIV disease, viral strains may emerge in an infected individual that differ widely in their ability to infect and kill different cell types, as well as in their rate of replication. Strains of HIV from patients with advanced disease appear to be more virulent and infect more cell types than strains obtained earlier from the same individual. See also Gene.