Untitled - D Ank Unlimited

decomplementation 218 degranulation
is absent from the red blood cells of paroxysmal nocturnal
hemoglobulinuria patients. It facilitates dissociation of
classical complement pathway C3 convertase (C4b2a) into
C4b and C2a. It also promotes the dissociation of alternative
complement pathway C3 convertase (C3bBb) into C3b
and Bb. It inhibits C5 convertases (C4b2a3b and C3bBb3b)
on the cell surface. DAF reacts with the convertases and
destabilizes them by inducing rapid dissociation of a
catalytic subunit C2a or Bb. The inhibitory effect of DAF
is restricted to C3/C5 convertases bound to host cells; DAF
does not inhibit normal complement activation on microbial
or immune complex targets. DAF is found on selected
mucosal epithelial cells and endothelial cells. It prevents
complement cascade amplification on the surfaces of cells
to protect them from injury by autologous complement. The
physiologic function of DAF may be to protect cells from
lysis by serum. DAF competes with C2 for linkage with
C4b to block C3 convertase synthesis in the classical pathway.
The DAF molecule consists of a single chain bound
to the cell membrane by phosphatidylinositol. Paroxysmal
nocturnal hemoglobulinuria develops as a consequence of
DAF deficiency.
decomplementation
Deliberate inactivation of complement in vitro or in vivo. To
decomplement serum to remove hemolytic action, the specimen
may be heated to 56°C for 30 minutes. Other methods
for inactivation of complement include the addition of cobra
venom factor, zymosan, or other substances that take up complement
from the medium in which they are placed. Removal
of complement activity in living animals may be accomplished
through the injection of cobra venom factor or other
substances to use up or inactivate the complement system.
decorating
Term describing the reaction of tissue antigens with
monoclonal antibodies, also known as staining in the
immunoperoxidase reaction. Thus, a tissue antigen stained
with a particular antibody is said to be decorated with that
monoclonal antibody. Immunoperoxidase techniques give a
reddish-brown color to the reaction product that is read by
light microscopic observation.
defective endogenous retroviruses
Partial retroviral genomes that are integrated into host cell
DNA and carried as host genes.
defensins
Widely reactive antimicrobial cationic proteins present
in polymorphonuclear neutrophilic leukocyte granules.
They block cell transport activities and are lethal for
Gram-positive and Gram-negative microorganisms. These
peptides are rich in cystesine and are found in the skin and
in neutrophil granules that function as broad-spectrum antibiotics
that kill numerous bacteria and fungi. The inflammatory
cytokines, interleukin-1 (IL1) and tumor necrosis
factor (TNF), facilitate synthesis of defensins. Defensins
(human neutrophil proteins 1–4) are amphipathic, carbohydrate-free,
cytotoxic, membrane-active antimicrobial
molecules. Three of the defensin peptides (HP-1, HP-2,
and HP-3) are nearly identical in sequence. By contrast, the
sequences of HP-4, H-5, and HP-6 are very different. HP-1
and HP-2 are chemotactic for monocytes. High concentrations
of HP-1 to -4 (25 to 200 μg/mL) manifest antimicrobial
and/or viricidal properties in vitro. HP-4 has the
greatest defensin activity, and HP-3 the least.
deficiency of secondary granules
A rare disorder in which neutrophils are bereft of secondary
granules, a condition that has an autosomal-recessive
mode of inheritance. Affected individuals show increased
incidence of infection by pyogenic microorganisms.
degenerate binding specificity
A form of antigen-binding specificity shown by major
histocompatibility complex (MHC) class I and II molecules.
Each MHC allotype may bind numerous peptides of different
amino acid sequences.
degranulation
A mechanism whereby cytoplasmic granules in cells fuse
with cell membranes to discharge the contents from the
cells. A classic example is degranulation of mast cells or
basophils in immediate (type I) hypersensitivity. In phagocytic
cells, cytoplasmic granules combine with phagosomes
and release their contents into the phagolysosome formed
by their union. The contents of intracytoplasmic granules
are released to the outsides of cells, rendering the granules
no longer visible microscopically. The discharge may be
into a phagosome as well as into the external environment.
Degranulation is observed in selected cells that participate
in inflammatory and immunological reactions.
Macrophage
Langerhans
cell
Antigen
T lymphocyte
Lymphatic
Venule
Delayed-type hypersensitivity.
Lymphatic
T lymphocyte