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S value (Svedberg unit) 673 switch site
SV40. Resolution = 3.1 Å.
in African green monkey cell cultures. Inoculation into
newborn hamsters leads to the development of sarcomas.
SV40 has 5243 base pairs in its genome. It may follow
either of two patterns of life cycle according to the host cell.
In permissive cells such as those from African green monkeys,
the virus-infected cells are lysed, causing the escape
of multiple viral particles. Lysis does not occur in nonpermissive
cells infected with the virus. By contrast, they may
undergo oncogenic transformation in which SV40 DNA
sequences become integrated into the genome of the host
cell. Cells that are transformed have characteristic morphological
features and growth properties. SV40 may serve as
a cloning vector. It is a diminutive icosahedral papovavirus
that contains double-stranded DNA. It may induce progressive
multifocal leukoencephalopathy and is useful for the in
vitro transformation of cells as a type of “permissive” infection
ultimately resulting in lysis of infected host cells.
S value (Svedberg unit)
The sedimentation coefficient of a protein ascertained by
analytical ultracentrifugation.
Svedberg unit (S)
A sedimentation coefficient unit equal to 10 –13 seconds.
Whereas most immunoglobulin molecules such as IgG sediment
at 7S, the pentameric IgM molecule sediments at 19S.
Sweet’s syndrome (acute febrile neutrophilic dermatosis)
A syndrome with neutrophilia, fever, and erythematous and
painful skin plaques with pronounced dermal neutrophilic
inflammation. About 10 to 15% of the cases may have an
underlying malignant disease such as a myeloid proliferative
disorder, acute myelogenous leukemia, or other tumor.
The cutaneous lesions may become vesicular, and pustular
skin lesions resemble those in bowel bypass syndrome.
Patients may develop arthritis, myalgia, conjunctivitis, and
proteinuria. Immunofluorescence reveals IgG, IgM, and
C3 in some lesions. Systemic steroid treatment has proven
effective in improving skin lesions.
Swiss agammaglobulinemia
A type of severe combined immunodeficiency (SCID) that
has an autosomal-recessive mode of inheritance. Patients
usually die during infancy as consequences of severe
diarrhea, villous atrophy, and malabsorption with disaccharidase
deficiency. Because of severely impaired humoral
and cellular immune defense mechanisms, patients face
increased susceptibility to various opportunistic infections
such as those induced by Pneumocystis carinii, Candida
albicans, measles, varicella, and cytomegalovirus. Patients
are also at increased risk of developing graft-vs.-host disease
following blood transfusion. A stem cell defect leads to
diminished numbers of T and B lymphocytes. Patients have
elevated liver enzymes, lymphopenia, anemia, and diarrhea,
causing an electrolyte imbalance. They are usually treated
with antibiotics, γ globulin, and HLA-6 antigen match bone
marrow transplants.
Swiss-type immunodeficiency
Refer to Swiss agammaglobulinemia.
Swiss type of severe combined immunodeficiency
A condition resulting from a defect at lymphocytic
stem cell level that produces cellular abnormalities that
affect both T and B cell limbs of the immune response,
culminating in impaired cell-mediated immunity and
humoral antibody responsiveness following challenge
by appropriate immunogens. The mode of inheritance
is autosomal-recessive. Refer also to severe combined
immunodeficiency syndrome.
switch
The change within an immunologically competent B
lymphocyte from synthesizing one isotype of heavy polypeptide
chain such as μ to another isotype such as γ during
differentiation. The switch signal comes from T cells.
Isotype switching does not alter the antigen-binding variable
region of the chain at the N terminus.
switch cells
A subset of T lymphocytes that governs isotype differentiation
of B lymphocytes exiting the Peyer’s patches to ensure
that they become immunoglobulin A (IgA)-producing
plasma cells when they home back to the lamina propria of
the intestine from the systemic circulation.
switch defect disease
Refer to hyperimmunoglobulin M syndrome.
switch region
The amino acid sequence between the constant and variable
portions of light and heavy polypeptide immunoglobulin
chains. This segment is encoded by D and J genes and
controls recombination associated with immunoglobulin
class switching. Specific switch region sequences are critical
for switching from one immunoglobulin isotype (class)
to another. During isotype switching, the active heavy chain
V region exon undergoes somatic recombination with a 3′
constant region gene at a switch region of DNA.
switch recombination
Immunoglobulin isotype switching mechanism. Based on
pairing of switch regions located 5′ of each C H exon (other
than Cs). The C H switch region, an original component of
the V(D)J-C gene, couples with the downstream C H exon’s
switch region, leading to excision of the intervening C H
exons. This is followed by union of the V(D)J exon with
the new C H exon. Selected cytokines including TGFβ,
INFγ, and IL4 and germline transcripts govern C H exon
selection.
switch site
Breakage points on a chromosome where gene segments
unite during gene rearrangement. In immunology, a switch
site is an abbreviated DNA sequence 5′ to each gene encoding
a heavy chain C region. It serves as an identification site
S