MAC:
See Mycobacterium Avium Complex.
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.
MACROPHAGE-TROPIC VIRUS:
HIV strains that preferentially infect macrophages in cell culture experiments. They readily fuse with cells that have both CD4 and CCR5 molecules on their surfaces, whereas the same viral isolates fail to fuse with cells expressing only CD4. These isolates are the main strains found in patients during the symptom-free stage of HIV disease.
MAGNETIC RESONANCE IMAGING (MRI):
A noninvasive, non-x-ray diagnostic technique -- also called nuclear magnetic resonance or NMR -- based on the magnetic fields of hydrogen atoms in the body. MRI provides computer-generated images of the body's internal tissues and organs.
MAI:
See Mycobacterium Avium Complex.
MAJOR HISTOCOMPATIBILITY COMPLEX (MHC):
Two classes of molecules on cell surfaces. MHC class I molecules exist on all cells, and hold and present foreign antigens to CD8 cytotoxic T lymphocytes if the cell is infected by a virus or other microbe. MHC class II molecules are the billboards of the immune system. Peptides derived from foreign proteins are inserted into MHC's binding groove and displayed on the surface of antigen-presenting cells. These peptides are then recognized by T lymphocytes so that the immune system is alerted to the presence of foreign material. See Histocompatibility Testing.
MALABSORPTION SYNDROME:
Decreased intestinal absorption resulting in loss of appetite, muscle pain, and weight loss. See AIDS Wasting Syndrome.
MALAISE:
A generalized, nonspecific feeling of discomfort.
MALIGNANT:
Refers to cells or tumors growing in an uncontrolled fashion. Such growths may spread to and disrupt nearby normal tissue, or reach distant sites via the bloodstream. By definition, cancers are always malignant, and the term "malignancy" implies cancer. See Metastasis.
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.
MEMORY T 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.
MENINGES:
Membranes surrounding the brain or spinal cord. Part of the so-called "blood-brain barrier." See Meningitis.
MENINGITIS:
An inflammation of the meninges (membranes surrounding the brain or spinal cord), which may be caused by a bacterium, fungus, or virus. See Cryptococcal Meningitis; Central Nervous System Damage.
MESSENGER RNA:
Also referred to as mRNA. An RNA (ribonucleic acid) that carries the genetic code for a particular protein from 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.
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.
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).
MHC:
See Major Histocompatibility Complex.
MICROBES:
Microscopic living organisms, including bacteria, protozoa, viruses, and fungi.
MICROBICIDE:
An agent (e.g., a chemical or antibiotic) that destroys microbes. New research is being carried out to evaluate the use of rectal and vaginal microbicides to inhibit the transmission of sexually transmitted diseases, including HIV.
MICROENCAPSULATED:
Surrounded by a thin layer of protection. A means of protecting a drug or vaccine from rapid breakdown.
MICROSPORIDIOSIS:
An intestinal infection that causes diarrhea and wasting in persons with HIV. It results from two different species of microsporidia, a protozoal parasite. See Pathogen; Protozoa; AIDS Wasting Syndrome.
MITOCHONDRIA:
Organelles (particles of a living substance) within the cytoplasm of the cells, mitochondria have their own independent DNA, and serve as a source of energy for the cell.
MN:
A strain of HIV used in vaccine development.
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.
MOLLUSCUM CONTAGIOSUM:
A disease of the skin and mucous membranes caused by a poxvirus (molluscum contagiosum virus, MCV) infection. It is characterized by small dome-shaped papules (bumps) on the face, upper trunk, or extremities. The disease most frequently occurs in children and adults with impaired immune response. It is transmitted from person to person by direct contact. It is also autoinoculable (i.e., a secondary infection produced by contact with a lesion on one's own body). In persons living with HIV, molluscum contagiosum is often a progressive disease, resistant to treatment. When CD4+ cells fall below 200, the lesions tend to proliferate and spread.
MONOCLONAL ANTIBODIES:
Antibodies produced in the laboratory by a hybridoma or antibody-producing cell source for a specific antigen. Monoclonal antibodies are useful as tools for identifying specific protein molecules.
MONOCYTE:
A large white blood cell that ingests microbes or other cells and foreign particles. When a monocyte enters tissues, it develops into a macrophage.
MONONEURITIS MULTIPLEX (MM):
A rare type of neuropathy that has been described with HIV infection. It may fall into two different settings. One type occurs during the early period of the infection and has a more benign outcome. The second form occurs later and is more aggressive, leading to progressive paralysis and death in some patients. It has been suggested that MM is related to multifocal cytomegalovirus (CMV) infection.
MONOVALENT VACCINE:
A vaccine that is specific for only one antigen.
MORBIDITY:
The condition of being diseased or sick; also the incidence of disease or rate of sickness.
MRI:
See Magnetic Resonance Imaging.
mRNA:
See Messenger RNA.
MUCOCUTANEOUS:
Anything that concerns or pertains to mucous membranes and the skin (e.g., mouth, eyes, vagina, lips, or anal area).
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.
MUCOUS MEMBRANE:
Moist layer of tissue lining the digestive, respiratory, urinary, and reproductive tracts -- all the body cavities with openings to the outside world except the ears.
MULTIPLE DRUG RESISTANT TUBERCULOSIS (MDR-TB):
A strain of TB (see tuberculosis) that does not respond to two or more standard anti-TB drugs. MDR-TB usually occurs when treatment is interrupted, thus allowing organisms, in which mutations for drug resistance have occurred, to proliferate.
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. Mutation, which can establish new traits in a population, is important in evolution. As related to HIV: During the course of HIV disease, HIV 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. Of course, HIV does not mutate into another type of virus.
MYALGIA:
Diffuse muscle pain, usually accompanied by malaise (vague feeling of discomfort or weakness).
MYCOBACTERIUM:
Any bacterium of the genus Mycobacterium or a closely related genus.
MYCOBACTERIUM AVIUM COMPLEX (MAC):
1. A common opportunistic infection caused by two very similar mycobacterial organisms, Mycobacterium avium and Mycobacterium intracellulare (MAI), found in soil and dust particles. 2. A bacterial infection that can be localized (limited to a specific organ or area of the body) or disseminated throughout the body. It is a life-threatening disease, although new therapies offer promise for both prevention and treatment. MAC disease is extremely rare in persons who are not infected with HIV.
MYCOPLASMA:
1. Smallest free-living organisms known to infect humans. Mycoplasma cause a variety of illnesses, especially of the lungs and sexual organs. 2. Any microorganism of the genus Mycoplasma, also called pleuropneumonia-like organism.
MYCOSIS:
Any disease caused by a fungus.
MYELIN:
A substance that sheathes nerve cells, acting as an electric insulator that facilitates the conduction of nerve impulses. See Chronic Idiopathic Demyelinating Polyneuropathy.
MYELOPATHY:
Any disease of the spinal cord.
MYELOSUPPRESSION:
Suppression of bone marrow activity, causing decreased production of red blood cells (see anemia), white blood cells (see leukopenia), or platelets (see thrombocytopenia). Myelosuppression is an effect of some drugs, such as AZT.
MYELOTOXIC:
Destructive to bone marrow.
MYOCARDIAL:
Refers to the heart's muscle mass.
MYOPATHY:
Progressive muscle weakness. Myopathy may arise as a toxic reaction to AZT or as a consequence of the HIV infection itself.
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