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V(D)J recombinase 739 veno-occlusive disease (VOD)
Victor Clarence Vaughn.
V(D)J recombinase
Enzymes required to recombine VDJ segments. An enzyme
that can identify and splice the V (variable), J (joining), and
in some cases D (diversity) gene segments that confer antibody
diversity. This collection of enzymes makes possible
the somatic recombination events that produce functional
antigen receptor genes in developing T and B lymphocytes.
Some, including recombination-activating genes (RAG)-1
and RAG-2, are present only in developing lymphocytes;
others are ubiquitous DNA repair enzymes.
V(D)J recombination
The formation of unique variable (V) exons by site-specific
recombination at the DNA level of pre-existing Ig and TCR
loci of V,D, and J gene segments. Accomplished by the recognition,
cutting and rejoining of heptamer–nonamer recombination
signal sequences (RSSs) that flank these gene segments.
When three segments are involved, the D and J gene segments
are linked first, followed by the joining of V to the DJ unit.
V(D)J recombination class switching
A mechanism to generate multiple-binding specificities by
developing lymphocytes through exon recombination from
a conservative number of gene segments known as variable
(V), joining (J), and diversity (D) at seven different loci that
include μ, κ, and λ for B cell immunoglobulin genes and α,
β, γ, and δ genes for T cell receptors.
VDRL (Venereal Disease Research Laboratory) test
Reaginic screening for syphilis. VDRL antigen is combined
with a patient’s heat-inactivated serum and the combination
is observed for flocculation by light microscopy after 4 minutes.
Reaginic assays are helpful to screen for early syphilis
and are usually positive in secondary syphilis, although
the results are more variable in tertiary syphilis. VDRL is
negative in approximately half of the neurosyphilis cases.
Reaginic tests for syphilis may be biologically false positive
in cases of malaria, lupus erythematosus, and acute
infections. Biologic false-positive VDRL tests may also be
seen in some cases of hepatitis, infectious mononucleosis,
rheumatoid arthritis, and even pregnancy.
V domain
Refer to variable region.
V gene segment
Refer to variable (V) gene segment.
Veiled cell.
Veiled cell
Venoocclusive disease (VOD) accompanying graft-vs.-host disease of the
liver. Left: Early VOD showing concentric subendothelial widening and
sublobular central venules with degeneration of surrounding pericentral
hepatocytes. No depositions of fibrin and factor VIII are apparent. Right:
Late lesion of VOD showing fibrous obliteration of central venules and
sinusoids by combination of types 3, 1, and even type 4 collagens.
vector
A DNA segment employed for cloning a foreign DNA fragment.
A vector should be able to reproduce autonomously
in a host cell, possess one or more selectable markers,
and have sites for restriction endonucleases in nonessential
regions that permit DNA insertion into the vector
or replacement of a segment of the vector. Plasmids and
bacteriophages may serve as cloning vectors.
veiled cell
A mononuclear phagocytic cell that serves as an antigenpresenting
cell. It is found in the afferent lymphatics and
marginal sinuses. It may manifest interleukin-2 (IL2) receptors
in the presence of granulocyte–macrophage colonystimulating
factor (GM-CSF).
veno-occlusive disease (VOD)
Serious liver complication after marrow transplantation.
Histopathology of early VOD reveals concentric subendothelial
widening and sublobular central venules with
degeneration of surrounding pericentral hepacytes along
with depositions of fibrin and factor VIII. Late lesions show
fibrous obliteration of central venules and sinusoids by combinations
of type 3, 1, and even type 4 collagen. The clinical
diagnosis is reasonably accurate based on the combination
of jaundice, ascites, hepatomegaly, and encephalopathy in
V