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secretory piece 642 selective IgA deficiency
bile may prevent bacterial growth in the small gut. Intestinal
peristalsis also guards against overgrowth of microorganisms
in blind loops.
secretory piece
A 75-kDa polypeptide chain synthesized by epithelial cells
of the gut for linkage to immunoglobulin A (IgA) dimers
present in body secretions. Secretory component facilitates
IgA transport across epithelial cells and protects secretory
IgA released into the lumen of the gut from proteolytic
digestion by enzymes in the secretions. It is not formed by
plasma cells in the lamina propria of the gut that synthesize
the IgA molecules with which it combines. Secretory component
has a special affinity for mucus, thereby facilitating
the attachment of IgA to the mucous membranes. Also
called secretory component.
sedimentation coefficient
The rate at which a macromolecule or particle sediments
that is equivalent to the velocity per unit centrifugal field:
s = (dx/dt)/w 2 x, where s is the sedimentation coefficient,
dx/dt is the velocity, w is the angular velocity, and x is the
distance from the axis of the centrifuge rotor. The size,
shape, and weight of the macromolecule in question and the
concentration and temperature of the solutions (but not the
centrifuge speed) determine the sedimentation coefficient.
Measurement is in Svedberg units.
sedimentation pattern
The configuration of red blood cells on a test tube or plastic
plate bottom at the conclusion of a hemagglutination test.
The formation of a covering mat on the curved bottom of
a tube or well signifies that agglutination has taken place.
The formation of a round button where the red blood cells
settled to the midpoint of the bottom of the tube or well and
were not retained on the curvature constitutes a negative
reaction with no agglutination.
Sedormid purpura (historical)
A form of thrombocytopenic purpura occurring in patients
who received Sedormid (allyl-isopropyl-acetyl carbamide).
The drug served as a hapten complexing with blood platelets.
The resulting platelet–drug complex was recognized as
foreign by the immune system. Antibodies formed against it
lysed blood platelets in the presence of complement, leading
to thrombocytopenia followed by bleeding and purpura
manifested on the skin. This was a type II hypersensitivity
reaction. Sedormid is no longer used, but the principle of
the hypersensitivity it induced is useful for understanding
autoimmunity induced by certain drugs.
segmental exchange
Refer to gene conversion.
selectins
A group of cell adhesion molecules (CAMs) that are glycoproteins
and play an important role in the relationship
of circulating cells to the endothelium. The members of
this surface molecule family have three separate structural
motifs: a single N terminal (extracellular) lectin motif
preceding a single epidermal growth factor repeat and
various short consensus repeat homology units. Selectins
are involved in lymphocyte migration. These carbohydratebinding
proteins facilitate adhesion of leukocytes to
endothelial cells. A single chain transmembrane glycoprotein
located in each selectin molecule has a similar modular
structure including the extracellular calcium-dependent
lectin domain. The three groups of selectins include
Mel-14 (LAM-1)
ELAM-1
Lectin-like domain
EGF-like domain
CD62
Short consensus repeat
Selectins.
L-selectin (CD62L) expressed on leukocytes; P-selectin
(CD62P) expressed on platelets and activated endothelium;
and E-selectin (CD62E) expressed on activated endothelium.
Under shear forces, the characteristic structural motif
is comprised of an N terminal lectin domain, a domain with
homology to epidermal growth factor (EGF), and various
complement regulatory protein repeat sequences. Refer also
to E-selectin, L-selectin, P-selectin, and CD62.
selective IgA and IgG deficiency
This disease that affects both sexes is either X-linked,
autosomal-recessive, or acquired later in life. A genetic
defect in the switch mechanism may cause immunoglobulin-producing
cells to change from immunoglobulin M
(IgM) to IgG or IgA synthesis. Respiratory infections with
pyogenic microorganisms or autoimmune states that include
hemolytic anemia, thrombocytopenia, and neutropenia may
occur. Numerous IgM-synthesizing plasma cells are demonstrable
in lymph nodes and spleens of affected individuals.
selective IgA and IgM deficiency
A concomitant reduction in both immunoglobulin A (IgA)
and IgM concentrations with normal IgG levels. The IgG
produced in many individuals may not be protective and
recurrent infections may result. The response to many
immunogens is inadequate in this disease, which occurs in
four males for every female affected.
selective IgA deficiency
The most frequent immunodeficiency disorder affecting
approximately 1 in 600 individuals. It is characterized by nearly
absent serum and secretory immunoglobulin A (IgA). The IgA
level is less than 5 mg/dL, whereas the remaining immunoglobulin
class levels are normal or elevated. The disorder is familial
or may be acquired in association with measles, other types of
virus infection, or toxoplasmosis. Patients may appear normal