the mixing of electronic states. MCD is frequently
used in combination with absorption and CD studies
toaffect electronic assignments. The three contribu-
tions to the MCD spectrum are the A-term, due to
Zeeman splitting of the ground and/or excited degen-
erate states; the B-term, due to field-induced mixing of
states; and the C-term, due to a change in the popula-
tion of molecules over the Zeeman sublevels of a
paramagnetic ground state. The C-term is observed
only for molecules with ground-state paramagnetism,
and becomes intense at low temperatures; its variation
with field and temperature can be analyzed to provide
magnetic parameters of the ground state, such as spin,
g-factor, and zero-field splitting. Variable-temperature
MCD is particularly effective in identifying and
assigning electronic transitions originating from para-
magnetic CHROMOPHOREs.
magnetic resonance imaging(MRI) The visualiza-
tion of the distribution of nuclear spins (usually water)
in a body by using a magnetic field gradient (NMR
IMAGING). A similar technique, but less widely used, is
to visualize the distribution of paramagnetic centers
(EPR imaging).
magnetic susceptibility For paramagnetic materials,
the magnetic susceptibility can be measured experimen-
tally and used to give information on the molecular
magnetic dipole moment, and hence on the electronic
structure of the molecules in the material. The para-
magnetic contribution to the molar magnetic suscepti-
bility of a material, χ, is related to the molecular
magnetic dipole moment mby the Curie relation: χ=
constant m^2 /T.
magnetotactic Ability to orient in a magnetic field.
major histocompatibility complex (MHC) A large
cluster of genes on chromosome 6 in humans, encoding
cell-surface proteins that play several roles in the
immune system. Several classes of protein such as
MHC class I and II proteins areencoded in this region.
In humans, these are known as human leukocyte anti-
gens (HLA). Class I protein molecules are designated
HLAA, B, or C. Class II molecules are designated DP,
DQ, or DR.- MHC class I molecule. A molecule encoded to genes
ofthe MHC that participates in antigen presentation
to cytotoxic T (CD8+) cells. - MHC class II molecule. A molecule encoded by genes
of the MHC that participates in antigen presentation
to helper T (CD4+) cells.
The ability of T lymphocytes to respond only when
they “see” the appropriate antigen in association with
“self” MHC class I or class II proteins on the antigen-
presenting cells is called MHC restriction.malaria Atropical disease caused by a protozoa of
the genus Plasmodium(Plasmodium falciparum) and
transmitted to humans by the bite of mosquitoes of the
genus Anopheles.malignant Term used to designate a cancerous
condition.
See alsoCANCER.Mallophaga One of the insect orders, known as
chewing lice, with three families, Menoponidae,
Philopteridae, and Trichodectidae, and made up of the
chewing lice, characterized by flattened, wingless bod-
ies, chewing mouthparts, and gradual metamorphosis.
Commonly found on dogs and cats.Malpighian tubule An excretory organ in arthro-
pods (insects) that correspond functionally to the kid-
neys of vertebrates. The tubules maintain internal salt
and water balance and remove wastes such as urea;
uric acid; urates of sodium, calcium, and ammonia;
leucin; and various salts of calcium and potassium
(oxalates, carbonates, and phosphates). Most of the
nitrogen excreted is in the form of uric acid. Opens
into the posterior section of the alimentary canal (gut).
In some insects, such as Mermeleon formicarius,
Chrysopa perla,and Euplectrus bicolor,the Malpighi-
an tubules produce a substance just before the time of
pupation that is spun out of the anus in the form of silk
threads, with which the cocoon is woven.212 magnetic resonance imaging